Free thyroxine is an independent predictor of subcutaneous fat in euthyroid individuals

Relationship between vitamin B6 intake and thyroid function in US adults: NHANES 2007-2012 results

BACKGROUND

Existing studies have focused on the relationship between vitamin B6 and thyroid disease. However, there is a lack of large cross-sectional studies reporting on the relationship between vitamin B6 and thyroid function. Therefore, the present study aimed to assess the association between vitamin B6 intake and thyroid function in a population of US adults aged 20 years and older, using data from the National Health and Nutrition Examination Survey (NHANES) between 2007 and 2012.

METHODS

Demographic, dietary, thyroid function, and relevant data from NHANES 2007-2012 were collected. The relationship between vitamin B6 intake and thyroid function was analysed using weighted multiple regression and restricted cubic spline analysis, including subgroup and interaction analysis.

RESULTS

The study included 6954 participants with a weighted mean age of 47.39  ± 16.60 years, mean vitamin B6 intake of 2.07 ± 1.15 mg, and mean TT4 level of 7.88 ± 1.61 μg/dL. A statistically significant negative correlation was noted between vitamin B6 intake and TT4 levels (β  =  -0.05, 95% CI  =  -0.10 to 0.00, P  =  0.033). In addition to this, subgroup analyses we found: In the gender subgroup, a significant negative correlation was found between vitamin B6 intake and TT4 levels in the male population (β = -0.06, 95% CI  =  -0.11 to -0.01, P  =  0.028); in the age subgroup, a significant negative correlation was found between vitamin B6 intake and TT4 levels in older people aged 60-80 years (β  =  -0. 13, 95% CI  =  -0.23 to -0.04, P  =  0.008); in the BMI subgroup, we found a significant negative correlation between vitamin B6 intake and TT4 levels in overweight people (BMI: 25-29.9 kg/m2) (β  =  -0.13, 95% CI  =  -0.20 to -0.06, P < 0. 001); in the iodine content subgroup, we found a significant negative correlation between vitamin B6 intake and TT4 levels in people with a normal iodine intake (100-299 ug/L) (β  =  -0.07, 95% CI  =  -0.13 to -0.01, P  =  0.021). Finally, in the subgroups of gender, age, BMI, and iodine content, no interaction was found.

CONCLUSIONS

To summarise, we believe that vitamin B6 may reduce serum TT4 levels by inhibiting inflammation.

Obesity and thyroid cancer: unraveling the connection through a systematic review and meta-analysis of cohort studies

Background

The relationship between adiposity indicators and thyroid cancer (TC) risk has garnered increasing attention due to the rising prevalence of obesity and its potential impact on cancer incidence. We conducted a comprehensive meta-analysis to investigate this association across various effect measures.

Method

Until July 2022, a comprehensive search of databases was conducted to identify cohort studies that assessed the association between adiposity and the development of TC. Meta-analysis was performed using random effects models. Subgroup analyses were conducted to explore heterogeneity. Publication bias was assessed using Begg's tests.

Results

A systematic literature search identified 27 eligible studies reporting odds ratios (OR), relative risks (RR), or hazard ratios (HR) as effect measures. Pooling the studies irrespective of the effect measure, a significant positive association between adiposity indicators and TC risk was observed, yielding an effect estimate of 1.16 (95% CI 1.12-1.21). The combined effect estimate for OR/RR studies was 1.10 (95%CI 1.04-1.17), while HR studies yielded an effect estimate of 1.20 (95%CI 1.13-1.26). Subgroup analyses revealed associations across different age groups, obesity indices, and regions, with some variations based on effect measure. Meta-regression identified follow-up duration as a confounding factor only in HR studies.

Conclusion

The synthesis of 27 studies with diverse designs and populations underscores a robust positive association between adiposity and TC risk, providing compelling evidence for the potential role of increased adiposity in TC development.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40200-024-01425-3.

Associations of TSH, free T3, free T4, and conversion ratio with incident hypertension: results from the prospective Brazilian Longitudinal Study of Adult Health (ELSA-Brasil)

Objective

To evaluate the association of TSH, free T3 (FT3), free T4 (FT4), and conversion (FT3:FT4) ratio values with incident hypertension.

Materials and methods

The study included data from participants of the ELSA-Brasil study without baseline hypertension. Serum TSH, FT4 and FT3 levels, and FT3:FT4 ratio values were assessed at baseline, and incident hypertension (defined by blood pressure levels ≥ 140/90 mmHg) was estimated over a median of 8.2 years of follow-up. The risk of incident hypertension was evaluated considering a 1-unit increase in TSH, FT4, FT3, and conversion ratio values and after dividing these variables into quintiles for further analysis using Poisson regression with robust variance. The results are presented as relative risks (RR) and 95% confidence intervals (CIs) before and after adjustment for multiple variables.

Results

The primary analysis incorporated data from 5,915 euthyroid individuals, and the secondary analysis combined data from all euthyroid individuals, 587 individuals with subclinical hypothyroidism, and 31 individuals with subclinical hyperthyroidism. The rate of incident hypertension was 28% (95% CI: 27%-29.3%). The FT4 levels in the first quintile (0.18-1.06 ng/dL) were significantly associated with incident hypertension (RR: 1.03, 95% CI: 1.01-1.06) at follow-up. The association between FT4 levels in the first quintile and incident hypertension was also observed in the analysis of combined data from euthyroid individuals and participants with subclinical thyroid dysfunction (RR: 1.04, 95% CI: 1.01-1.07). The associations were predominantly observed with systolic blood pressure levels in euthyroid individuals. However, in the combined analysis incorporating euthyroid participants and individuals with subclinical thyroid dysfunction, the associations were more pronounced with diastolic blood pressure levels.

Conclusion

Low FT4 levels may be a mild risk factor for incident hypertension in euthyroid individuals and persons with subclinical thyroid dysfunction.

Association of Maternal TSH, FT4 With Children's BMI Trajectories, and Obesity: A Birth Cohort Study

OBJECTIVE

To investigate the association between maternal TSH, free thyroxine (FT4), and children's body mass index (BMI) trajectories and obesity.

METHOD

Based on the Ma'anshan Birth Cohort in China, we repeatedly assayed maternal thyroid functions in 3 trimesters of pregnancy. Children's height and weight were measured 15 times before they were age 6 years. Body fat was assessed when children were aged 6 years. Mplus software was used to fit maternal thyroid hormone trajectories and BMI trajectories. Multivariate logistic regression models and generalized linear models were used in data analysis.

RESULTS

Low maternal FT4 trajectory was observed to be related to an increased risk of a high children's BMI trajectory and overweight, with an odds ratio and 95% CI of 1.580 (1.169-2.135) and 1.505 (1.064-2.129), respectively. Increased maternal FT4 concentrations in the first, second, and third trimesters were associated with a decreased risk of high children's BMI trajectories and obesity. There was a positive association between low maternal FT4 trajectory and 6-year-old children's body fat ratio with β and 95% CI of 0.983 (0.138-1.829). Furthermore, negative correlations between maternal FT4 concentration in the first, second, and third trimesters of pregnancy and body fat ratio were observed.

CONCLUSIONS

Low maternal FT4 trajectory during pregnancy may predict a high BMI trajectory in children and relate to overweight and high body fat ratio in 6-year-old children. High maternal FT4 concentrations throughout pregnancy may be associated with the decreasing risk of obesity and low body fat ratio in 6-year-old children.

Effects of the abdominal fat distribution on the relationship between exposure to air pollutants and thyroid hormones among Korean adult males

BACKGROUND

Several significant associations between air pollution and thyroid function have been reported, but few studies have identified whether these associations differ by obesity, particularly its regional distribution. We assessed the relationship between ambient air pollution and thyroid hormone, and whether this relationship is modified by abdominal adiposity, as indicated by the waist circumference, visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and visceral-to-subcutaneous fat ratio (VSR) in Korean men.

METHODS

We included 2440 male adults in the final analysis and used each person's annual average exposure to four air pollutants: particulate matter with an aerodynamic diameter ≤ 10 μm (PM10), nitrogen dioxide, sulfur dioxide (SO2), and carbon monoxide (CO). Abdominal fat deposition was quantified by computed tomography. Serum thyrotropin (TSH) and free thyroxine (FT4) concentrations were measured for thyroid hormone. To evaluate the relationship between air pollution and thyroid hormone according to adiposity, we performed multiple linear regression analysis on the two subgroups stratified by abdominal fat level.

RESULTS

Abdominal adiposity was significantly related to FT4 concentration. The exposures to air pollutants were associated with increased TSH and decreased FT4 concentrations. In stratified analysis using abdominal fat traits, ambient air pollution except for SO2 was significantly related to increased TSH and decreased FT4 concentrations in the high adiposity group (all p < 0.05), but not in the normal adiposity group. Among the air pollutants, PM10 showed an association with an increase of TSH concentration in all group with high adiposity, including high VAT, high SAT, and high VSR groups (all p < 0.05). In case of FT4, CO showed a similar pattern. Among the abdominal fat-related traits, the VSR in the high adiposity group had the largest effect on the relationship between exposure to air pollutants and thyroid hormone.

CONCLUSIONS

This study suggests the first clue that the relationship between air pollution exposure and thyroid hormone differs according to abdominal fat distribution among Korean adult males.

Influence of the TAS2R38 Gene Single Nucleotide Polymorphisms in Metabolism and Anthropometry in Thyroid Dysfunction

The gene TAS2R38 single nucleotide polymorphisms (SNPs-P49A, A262V and V296I) can condition bitter tasting by PAV (proline-alanine-valine) and non-bitter-tasting by AVI (alanine-valine-isoleucine) homozygosity. We evaluated this polymorphisms association with thyroid function, metabolism and anthropometry parameters determined by: Endpoint analysis (SNPs); DXA (fat mass-%, total fat mass-kg, lean mass-kg); Standard methods (lipid metabolism parameters, HbA1c-%, glycemia-mg/dL, insulinemia-µIU/mL, HOMA-IR, uricemia-mg/dL, calcemia-mg/dL and BMI-kg/m²); ELISA (leptinemia-ng/mL); Spectrophotometry (Angiotensin Converting Enzyme activity-UI/L). Statistics: SPSS program; OR [IC95%]; p < 0.05. Sample: 114 hypothyroid, 49 hyperthyroid, and 179 controls. An association between A262V-valine-valine and hypothyroidism/hyperthyroidism was verified (OR = 2.841; IC95% [1.726-4.676]), p < 0.001/OR = 8.915; IC95% [4.286-18.543]), p < 0.001). Protector effect from thyroid dysfunction: A262V-alanine-valine (OR = 0.467; IC95% [0.289-0.757], p = 0.002/OR = 0.132; IC95% [0.056-0.309], p < 0.001) and PAV (OR = 0.456; IC95% [0.282-0.737], p = 0.001/OR = 0.101; IC95% [0.041-0.250], p < 0.001). Higher parameter values associated with genotypes were: fat-mass-% (V296I-valine-isoleucine), lean-mass (P49A-proline-proline; PVI), leptin (AVI), HbA1c (A262V-alanine-valine) and lower values in lean-Mass (AVI; PVV), leptin (A262V-alanine-alanine), HbA1c (PVV), uricemia (V296I-valine-isoleucine), glycemia (A262V-alanine-alanine; AAV) and plasma triglycerides (PVV). In conclusion, TAS2R38 influences thyroid function, body composition and metabolism. Bitter taste perception (PAV) and the genotype A262V-alanine-valine can protect from thyroid dysfunction. AVV, PVV and genotype A262V-valine-valine may confer higher predisposition for thyroid dysfunction, particularly PVV for hyperthyroidism.

Short-term Changes in Thyroid-Stimulating Hormone Level after Body Fat Reduction via Partial Meal Replacement

BACKGROUND

Serum thyroid-stimulating hormone (TSH) levels change during body weight reduction. However, the changes that occur during short-term body weight control interventions remain controversial. Thus, this study aimed to evaluate the changes in TSH levels according to body fat reduction.

METHODS

We performed a 3-month intervention study involving partial meal replacement. Forty-nine participants completed the study. Correlations between changes in TSH levels and other body composition parameters were determined. The subjects were divided into two groups according to their body fat reduction (>1 kg, n=20; <1 kg, n=29). The changes in metabolic parameters, including TSH levels, were compared. For significant values, a multivariate analysis was performed after adjustment to evaluate the relationship between TSH changes and body fat reduction.

RESULTS

The 3-month intervention caused favorable changes in body proportions and metabolic parameters. TSH levels changed significantly only after changes in total body fat, showing a partial correlation. Changes in TSH levels were significantly different between groups (P=0.014). Moreover, the change in TSH levels was significantly different after adjustment (P=0.012).

CONCLUSION

A body fat reduction, especially >1 kg, can reduce serum TSH concentrations in subjects with metabolic syndrome after short-term body weight intervention.

“Ferrocrinology”—Iron Is an Important Factor Involved in Gluco- and Lipocrinology

“Ferrocrinology” is the term used to describe the study of iron effects on the functioning of adipose tissue, which together with muscle tissue makes the largest endocrine organ in the human body. By impairing exercise capacity, reducing AMP-activated kinase activity, and enhancing insulin resistance, iron deficiency can lead to the development of obesity and type 2 diabetes mellitus. Due to impaired browning of white adipose tissue and reduced mitochondrial iron content in adipocytes, iron deficiency (ID) can cause dysfunction of brown adipose tissue. By reducing ketogenesis, aconitase activity, and total mitochondrial capacity, ID impairs muscle performance. Another important aspect is the effect of ID on the impairment of thermogenesis due to reduced binding of thyroid hormones to their nuclear receptors, with subsequently impaired utilization of norepinephrine in tissues, and impaired synthesis and distribution of cortisol, which all make the body’s reactivity to stress in ID more pronounced. Iron deficiency can lead to the development of the most common endocrinopathy, autoimmune thyroid disease. In this paper, we have discussed the role of iron in the cross-talk between glucocrinology, lipocrinology and myocrinology, with thyroid hormones acting as an active bystander.

ENDOCRINE OBESITY: Pituitary dysfunction in obesity

Obesity, the growing pandemic of the 21st century, is associated with multiple organ dysfunction, either by a direct increase in fatty organ content or by indirect modifications related to general metabolic changes driven by a specific increase in biologic products. The pituitary gland is not protected against such a situation. Different hypothalamic-pituitary axes experience functional modifications initially oriented to an adaptive situation that, with years of obesity, turn to maladaptive dynamics that contribute to perpetuating obesity and specific symptoms of their hormonal nature. This paper reviews the recent knowledge on obesity-related pituitary dysfunction and its pathogenic mechanisms and discusses potential therapeutic actions aimed at contributing to ameliorating the complex treatment of severe cases of obesity.

Association of thyroid function, within the euthyroid range, with cardiovascular risk: The EPIPorto study

BACKGROUND

Thyroid hormones are important modulators of cardiovascular function. Both hypothyroidism and hyperthyroidism are known to contribute to an increased cardiovascular risk. It remains uncertain whether thyroid hormones level within the euthyroid range are associated with cardiometabolic risk. We aimed to evaluate the association between thyroid function levels within the euthyroid range and cardiovascular risk in a population-based cohort.

METHODS

Eight hundred thirty-five subjects aged ≥45 years from the EPIPorto population-based cohort were included. We excluded participants with TSH, free T4 (FT4), or free T3 (FT3) outside of the reference range, or with previous cardiovascular or thyroid disease. The associations between thyroid function, cardiovascular risk factors and the 10-year estimated risk of cardiovascular events (using SCORE2 and SCORE2-OP) were evaluated in linear and logistic regression models, crudely and adjusting for age, sex, BMI, diabetes, and smoking.

RESULTS

The mean age of the participants was 61.5 (SD 10.5) years and 38.9% were men. Eleven percent of the participants had diabetes, 47.8% had dyslipidemia, and 54.8% had hypertension. The mean body mass index (BMI) was 27.4 (SD 4.6) kg/m², and the median (percentile25-75) 10-year risk of cardiovascular events was 5.46% (2.92, 10.11). Participants with higher BMI, larger waist circumference and higher hs-CRP had higher levels of FT3 and FT3/FT4 ratio. Lower FT3/FT4 ratio and higher FT4 levels were associated with higher prevalence of diabetes and more adverse lipid profile. Higher TSH, lower FT3 and lower FT3/FT4 ratio were associated with lower eGFR. Lower FT3, lower FT3/FT4 ratio and higher FT4 were associated with an increased 10-year risk of cardiovascular events.

CONCLUSIONS

In a population-based study, variations of thyroid function within the euthyroid range were associated with cardiovascular risk factors. On one hand, individuals with higher BMI, larger waist circumference and higher hs-CRP had higher levels of FT3 and FT3/FT4 ratio. On the other hand, a decreased conversion of T4 to T3 (lower FT3, lower FT3/FT4 ratio and/or higher FT4) was associated with a higher prevalence of diabetes, a more adverse lipid profile, a lower eGFR and an increased 10-year risk of cardiovascular events.

Central Obesity Is Associated with Variations in TSH and ACTH Levels among Euthyroid Obese Individuals

INTRODUCTION

The interactions of central obesity and body composition with thyroid hormones and the hypothalamus-pituitary-adrenal (HPA) axis are unclear; both central obesity and body composition have an impact on energy homeostasis. Our study aimed to investigate the association between body composition and pituitary hormones, including the HPA axis and pituitary-thyroid axis, in a Chinese population of euthyroid overweight and obese individuals.

METHODS

This was a cross-sectional study. Overweight and obese patients who regularly visited the multidisciplinary team (MDT) for obesity at Peking University First Hospital were enrolled in the study. Thyroid function, morning serum ACTH and cortisol levels, thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), body composition, and metabolic indicators, including liver function and the lipid profile, were measured at the first visit. Statistical analysis was performed using SPSS version 21.0 (IBM, USA).

RESULTS

In total, 441 patients with overweight or obesity were enrolled (male/female, 123/318). Patients were assigned to four groups according to the thyroid-stimulating hormone (TSH) level stratified by quartiles, and increased body mass index (BMI) was revealed in the highest TSH quartile group (p=0.002). Hip circumference (HC) of patients in the highest TSH quartile group was significantly increased (p=0.021). Morning ACTH levels and fasting insulin levels were significantly elevated in patients in the highest TSH quartile group (p=0.027 for fasting insulin, p < 0.001 for ACTH). In the female subgroup, patients in the highest TSH quartile group showed increases in BMI (p=0.010), waist circumference (WC) (p=0.007), muscle mass of the lower extremities (p=0.020), fasting C-peptide (p=0.031), and ACTH (p=0.002). In the male subgroup, patients in the highest TSH quartile group exhibited higher BMI (p=0.017), HC (p=0.036), and ACTH (p=0.003). Among patients in the highest ACTH quartile group, there was an elevated proportion of males (p=0.003), and FT3 (p=0.005), fasting insulin (p=0.037), and cortisol (p < 0.001) levels were increased. Weight (p < 0.001), BMI (p < 0.001), WC (p < 0.001), HC (p < 0.001), muscle mass of the upper extremities (p=0.003), muscle mass of the lower extremities (p=0.005), and total muscle mass (p=0.003) were elevated in patients in the highest ACTH quartile group. HC was found to be an independent factor after adjustment for other confounders and was positively associated with the TSH level (p=0.004 for the regression model, B = 0.152, p=0.004).

CONCLUSIONS

BMI is positively correlated with TSH and ACTH levels in both male and female obese individuals. The ACTH level was positively associated with male sex and increased BMI and muscle mass. Hip circumference was an independent factor that was positively related to TSH levels.

The relationship between overweight and thyroid function in first-episode, untreated Chinese patients with major depressive disorder with different ages of onset

BACKGROUND

Major depressive disorder (MDD) and obesity are common. There are many differences in many aspects of MDD patients at different ages of onset (AOO); however, there are currently no studies on differences in obesity or overweight. This study aims to evaluate whether thyroid function changes with body weight, and to explore the related factors of overweight in MDD patients with different AOOs.

METHODS

A total of 1716 first-episode, untreated Chinese Han outpatients with MDD were recruited from a general hospital. Hamilton Anxiety Scale (HAMA), Hamilton Depression Scale (HAMD) and Positive Symptom subscale of the Positive and Negative Syndrome Scale (PANSS) were used to evaluate anxiety, depression and psychotic symptoms, respectively. The participants were divided into two groups: early adulthood onset (EAO, <45 years old) and mid-adulthood onset (MAO, >=45 years old).

RESULTS

Compared with EAO patients, MAO patients scored higher on the HAMD, HAMA, CGI-S and PANSS positive symptoms subscale, and they also had higher systolic and diastolic blood pressure (BP), higher serum levels of thyroid stimulating hormone (TSH), FBG, cholesterol (TC) and low-density lipoprotein, but they had lower serum levels of free triiodothyronine 3 and high-density lipoprotein. TSH, anti-thyroglobulin (TgAb), TC and systolic BP were correlated with overweight in MAO patients, while TSH and FBG were correlated with overweight of EAO patients.

CONCLUSIONS

The results indicate that TSH is related to overweight in both AOO subgroups, and the influencing factors of overweight related to thyroid function may be different in different AOOs.

Thyroid diseases and second to fourth digit ratio in Polish adults

The association between second to fourth finger ratio and thyroid diseases is unexplained. There is a possible interaction between prenatal exposition to sex hormone and thyroid functions in the adulthood. The study included 175 adults investigated in Łódź in the central Poland. It consisted of two main parts: a survey including questions about occurrence of thyroid gland dysfunction and anthropometric measurements (body mass and height and length of the second and fourth finger, waist and hip circumferences). The women who had thyroid disease had higher 2D:4D digit ratio (left hand) (mean = 1.004; SD = 0.036) than healthy ones (mean = 0.989; SD = 0.030) (t = − 2105; p = 0.038; d = 0.707). The association between thyroid diseases occurrence and prenatal steroid hormone exposition is noticed. Only females who had thyroid diseases tend to have higher 2D:4D digit ratio, for left hand.

The Association between the Dietary Inflammatory Index and Thyroid Function in U.S. Adult Males

Thyroid function has a close link with inflammation. However, it is still unknown whether the dietary inflammatory potential is associated with thyroid function. We aimed to assess the relationship among them using the data from the National Health and Nutrition Examination Survey (NHANES). This study was a cross-sectional study, where weighted multivariable linear regression, subgroup analyses, and interaction terms were employed. Thyroid function was assessed by eight indexes, including total and free T4 and T3, Tg, TgAb, TPOAb, and TSH. A total of 2346 male participants aged ≥20 years with an average age of 50.74 ± 17.68 years were enrolled. The mean DII score among participants was −0.46 ± 1.73, ranging from −4.12 to 4.41, and mean total thyroxine (T4) was 7.61 ± 1.51 μg/dL. We found a positive association between DII and total T4 (β = 0.07; p = 0.0044). Using subgroup analysis, this association became stronger in both the iodine-deficient and obese group (iodine-deficient group: β = 0.15, p < 0.0001; obese group: β = 0.14, p < 0.0001). In conclusion, men adhering to a more pro-inflammatory diet appeared to have higher total T4 levels. However, these hormone variations were still within the normal clinical range and more well-designed studies are still needed to validate the causal relationship between DII and thyroid function.

Environmental Factors Affecting Thyroid-Stimulating Hormone and Thyroid Hormone Levels

Thyroid hormones are necessary for the normal functioning of physiological systems. Therefore, knowledge of any factor (whether genetic, environmental or intrinsic) that alters the levels of thyroid-stimulating hormone (TSH) and thyroid hormones is crucial. Genetic factors contribute up to 65% of interindividual variations in TSH and thyroid hormone levels, but many environmental factors can also affect thyroid function. This review discusses studies that have analyzed the impact of environmental factors on TSH and thyroid hormone levels in healthy adults. We included lifestyle factors (smoking, alcohol consumption, diet and exercise) and pollutants (chemicals and heavy metals). Many inconsistencies in the results have been observed between studies, making it difficult to draw a general conclusion about how a particular environmental factor influences TSH and thyroid hormone levels. However, lifestyle factors that showed the clearest association with TSH and thyroid hormones were smoking, body mass index (BMI) and iodine (micronutrient taken from the diet). Smoking mainly led to a decrease in TSH levels and an increase in triiodothyronine (T3) and thyroxine (T4) levels, while BMI levels were positively correlated with TSH and free T3 levels. Excess iodine led to an increase in TSH levels and a decrease in thyroid hormone levels. Among the pollutants analyzed, most studies observed a decrease in thyroid hormone levels after exposure to perchlorate. Future studies should continue to analyze the impact of environmental factors on thyroid function as they could contribute to understanding the complex background of gene-environment interactions underlying the pathology of thyroid diseases.

Cumulative Effects of Thyroid Hormones Over 10 Years and Risk of General and Abdominal Obesity

We aimed to assess if changes in thyrotropin (TSH) and free thyroxine (FT4) over 10 years of follow-up would be associated with changes in body mass index (BMI) and waist circumference (WC) or risk of obesity. We enrolled 2317 out of 4179 participants in Tehran Thyroid Study with serum TSH between 0.1-10 mU/l and without history of thyroid medication or surgery. Serum concentrations of FT4 and TSH were measured at baseline and three follow-ups (1999-2011). To account for within-subject correlation, the generalized estimating equation was used to assess the association between one standard deviation(SD) change in the main exposures [cumulative excess (CE)TSH and CEFT4] and changes in BMI and WC; calculated scores of CETSH and CEFT4 were included in models as time-varying exposures. Cumulative excess of TSH or FT4 was not associated with increased incidence of general or abdominal obesity. However, CEFT4 was negatively associated with BMI only in overweight and obese subjects. In GEE analysis, one unit increase in TSH was associated with 0.02 kg/m² increase in BMI (95% CI: 0.01, 0.03), which remained significant only in women; although the association was not significant after adding FT4 to model. One unit increase in FT4 was associated with 1.5 kg/m² decrease in BMI (95% CI:-1.8,-1.2) and 4.1 cm decrease in WC (95% CI:-5.1,-3.1) in both sexes independent of TSH and other confounders. Cumulative excess of TSH or FT4 indicated no risk for general or abdominal obesity. However, FT4 was negatively associated with BMI and WC independent of TSH.

Association Between Thyroid Function and Body Composition in Type 2 Diabetes Mellitus (T2DM) Patients: Does Sex Have a Role?

BACKGROUND The relationship between thyroid hormones and body anthropometric measures in type 2 diabetes mellitus (T2DM) patients with normal thyroid function is unclear. The purpose of this study was to evaluate the association between thyroid hormones and body composition in euthyroid T2DM patients in men and women. MATERIAL AND METHODS This was a cross-sectional study that included 561 euthyroid T2DM patients. Fasting venous blood was collected to test laboratory indexes. Bioelectric impedance analysis (BIA) was used to measure body composition. Propensity score matching (PSM) was used to enroll patients with similar baseline characteristics. The least absolute shrinkage and selection operator (LASSO) algorithm was used to establish a linear regression model of thyroid hormone and body composition. PSM was performed to match 159 men and 159 women. RESULTS The LASSO regression analysis suggested that thyroid-stimulating hormone (TSH) level was not correlated with body composition parameters in females. In females, free triiodothyronine (FT3) level was positively correlated with body mass index (BMI), fat-free mass index (FFMI), and skeletal muscle index (SMI), and was negatively correlated with extracellular water fraction (EWF). In males, FT3 level was positively correlated with waist circumference (WC) and SMI and negatively correlated with EWF. Free thyroxine (FT4) level in both women and men was positively correlated with body fat mass (BFM) and left lower-limb muscle mass (LLLMM). Moreover, in males, FT4 level was correlated with more body composition parameters. In euthyroid T2DM patients, FT3 level was positively correlated with SMI and negatively correlated with EWF, while FT4 level was positively correlated with BFM and LLLMM. CONCLUSIONS Thyroid function can affect body composition in euthyroid T2DM patients. Thyroid function is more likely to affect the fat and muscle distribution of males than females.

The role of thyroid hormone in metabolism and metabolic syndrome

Metabolic syndrome (MetS) and thyroid dysfunction are common in clinical practice. The objectives of this review are to discuss some proposed mechanisms by which thyroid dysfunctions may lead to MetS, to describe the bidirectional relationship between thyroid hormones (THs) and adiposity and finally, to resume a list of recent studies in humans that evaluated possible associations between thyroid hormone status and MetS or its clinical components. Not solely THs, but also its metabolites regulate metabolic rate, influencing adiposity. The mechanisms enrolled are related to its direct effect on adenosine triphosphate (ATP) utilization, uncoupling synthesis of ATP, mitochondrial biogenesis, and its inotropic and chronotropic effects. THs also act controlling core body temperature, appetite, and sympathetic activity. In a bidirectional way, thyroid function is affected by adiposity. Leptin is one of the hallmarks, but the pro-inflammatory cytokines and also insulin resistance impact thyroid function and perhaps its structure. MetS development and weight gain have been positively associated with thyroid-stimulating hormone (TSH) in several studies. Adverse glucose metabolism may be related to hyperthyroidism, but also to reduction of thyroid function or higher serum TSH, as do abnormal serum triglyceride levels. Hypo- and hyperthyroidism have been related to higher blood pressure (BP), that may be consequence of genomic or nongenomic action of THs on the vasculature and in the heart. In summary, the interaction between THs and components of MetS is complex and not fully understood. More longitudinal studies controlling each of all confounding variables that interact with endpoints or exposure factors are still necessary.

Association between TSH levels within the reference range and adiposity markers at the baseline of the ELSA-Brasil study

Background

The association of thyrotropin (TSH) with overall (body mass index, BMI), visceral (waist circumference and steatosis), and upper subcutaneous (neck circumference, NC) adiposity markers is still controversial, and the aim of this study is to assess these associations in the baseline data of a large cohort from ELSA-Brasil.

Methods and findings

This cross-sectional study included 11,224 participants with normal thyroid function (normal TSH levels). BMI, waist circumference, NC and steatosis, defined by hepatic attenuation (mild or moderate/severe) were the explicative variables. TSH levels were log transformed (logTSH), and multivariate linear regression models were generated to estimate the associations between logTSH and BMI (continuous and categorized), waist circumference, NC, and steatosis after adjusting for sociodemographic characteristics, health behaviors, and comorbidities. The mean age was 51.5±8.9 years, 5,793 (51.6%) participants were women, 21.8% (n = 2,444) were obese, and 15.1% of the sample was TPOAb positive. The TSH levels were significantly higher in the obese group than in the reference group (<25.0 kg/m²). In the multivariable linear regression models, significant associations of logTSH with BMI and obesity were found. LogTSH was associated with waist circumference only among women. NC and steatosis were not related to TSH levels.

Conclusions

TSH levels were associated with overall adiposity and obesity. Further studies may elucidate reference levels of TSH according to BMI status.

Association between Abdominal Fat Distribution and Free Triiodothyronine in a Euthyroid Population

BACKGROUND

Obesity is closely related to thyroid hormones; however, the relationship between abdominal fat distribution and thyroid hormones has rarely been explored.

OBJECTIVES

This study aimed to explore the relationship between abdominal fat distribution and free triiodothyronine (FT3) and FT3 to free thyroxine (FT4) ratio (FT3/FT4) in a euthyroid population.

METHODS

The present study enrolled 1,036 participants (age range 27-81 years; 445 men and 591 women). The visceral fat area (VFA) and the subcutaneous fat area (SFA) were determined by magnetic resonance imaging. FT3, FT4, and thyroid-stimulating hormone were measured by an electrochemical luminescence immunoassay.

RESULTS

In both men and women, SFA increased according to the increase of FT3 and FT3/FT4 tertiles (p for trend <0.05), while VFA did not significantly change. In the multivariate stepwise regression analysis, SFA was independently and positively related to FT3 in both men and women, the standardized β (95% CI) were 0.183 (0.094, 0.272) (p < 0.001) and 0.089 (0.007, 0.171) (p = 0.033), respectively. Moreover, SFA was independently and positively related to FT3/FT4 in men, the standardized β (95% CI) was 0.196 (0.101, 0.290) (p < 0.001). However, VFA was not related to either FT3 or FT3/FT4 in both genders.

CONCLUSIONS

Abdominal subcutaneous fat was independently related to increased FT3 in a euthyroid population.

The Association of Obesity with Autoimmune Thyroiditis and Thyroid Function-Possible Mechanisms of Bilateral Interaction

A growing number of patients suffer from autoimmune diseases, including autoimmune thyroid disease. There has simultaneously been a significant increase in the prevalence of obesity worldwide. It is still an open question whether adiposity can directly influence activation of inflammatory processes affecting the thyroid in genetically predisposed individuals. Adipokines, biologically active substances derived from the adipocytes, belong to a heterogenic group of compounds involved in numerous physiological functions, including the maintenance of metabolism, hormonal balance, and immune response. Notably, the presence of obesity worsens the course of selected autoimmune diseases and impairs response to treatment. Moreover, the excess of body fat may result in the progression of autoimmune diseases. Nutritional status, body weight, and energy expenditure may influence thyroid hormone secretion. Interestingly, thyroid hormones might influence the activity of adipose tissue as metabolic alterations related to fat tissue are observed under pathological conditions in which there are deficits or overproduction of thyroid hormones. Functioning TSH receptors are expressed on adipocytes. Thermogenesis may presumably be stimulated by TSH binding to its receptor on brown adipocytes. There could be a bilateral interaction between the thyroid and adipose. Obesity may influence the onset and course of autoimmune disease.

Obese Subjects with Non-Alcoholic Fatty Liver Disease Have a Higher Risk of Thyroid Dysfunction

Objectives The effects of obesity on thyroid function have not been well established. The aim of this study was to investigate the effects of body mass index (BMI) and/or non-alcoholic fatty liver disease (NAFLD) on thyroid function. Methods A retrospective longitudinal analysis was conducted among subjects who underwent comprehensive health check-ups at least four times between 2008 and 2017. Thyroid function was investigated according to BMI or presence of NAFLD at the end of follow-up. The subjects were divided into four groups: control (n = 216), subjects with obese (n = 94), subjects with NAFLD (n = 48), and subjects with obese + NAFLD (n = 93). Obesity was defined as BMI ≥ 25 kg/m2. Results During the mean follow-up of 6.8 years (6.8 ± 1.2 years), 42 of the 451 subjects (9.3%) had subclinical hypothyroidism (SCH) but no subjects developed overt hypothyroidism. In multivariate Cox proportional hazard analysis, after adjustment for age, sex, smoking, and baseline thyroid stimulating hormone level, obese subjects with NAFLD had a higher risk of SCH than the control group. Conclusions The obese subjects with NAFLD had a higher risk for SCH in the future.

Association of thyroid hormones with resting energy expenditure and complement C3 in normal weight high body fat women

BACKGROUND

A high body fat percentage has a specific effect on activation of the hypothalamic-pituitary-thyroid axis. On the other hand, a slight change in thyroid hormones can affect metabolism and body composition as well as immune function. This study aims to examine the relationship between adiposity, thyroid hormone levels and immunity by comparing resting energy expenditure (REE), serum thyroid hormone levels and complement C3 in normal-weight high body fat (normal weight obesity) women and normal-weight normal body fat women.

METHODS

In this case-control study, 40 women with normal body weight (BMI < 24.9 kg/m2) and body fat mass (FM) ≥ 30% (normal-weight obesity (NWO) group), and 30 non-obese women (BMI < 24.9 kg/m2) and FM < 30% (non-normal weight obesity (non-NWO) group) were recruited from a sport club in Tehran. Body composition was analyzed using bioimpedance analyzer. Blood samples were collected and analyzed for fasting serum concentration of thyroid hormones (including total T3 and total T4), thyroid-stimulating hormone (TSH), and serum complement C3. REE was measured by an indirect calorimetry.

RESULTS

Serum T3 and T4 and also complement C3 were higher in the NWO group than in the non-NWO group. Body fat percentages had significant positive correlation with T3 (r; 0.344, P <  0.05), T4 (r; 0.294, P <  0.05), and complement C3 (r; 0.417, P <  0.05). Serum T3 and T4 were also positively correlated with C3 concentration (r; 0.417, p <  0.001) and (r; 0.349, p <  0.05); respectively, but there was no significant correlation between TSH and C3. REE was not significantly different between the two groups. REE only had a significant positive correlation with fat-free mass (r; 0.421, P <  0.001).

CONCLUSION

An increase in body fat even in the presence of a normal body weight can be accompanied by the changes in thyroid function and inflammatory markers such as complement C3.

Thyroid disease and the metabolic syndrome

PURPOSE OF REVIEW

To summarize recent developments in the association of thyroid function with metabolic syndrome (MetS).

RECENT FINDINGS

Although thyroid hormones even within low normal range are associated with various metabolic abnormalities, the risk of MetS remains a controversial issue. Hyperthyroid state might be associated only with insulin resistance and dysglycemia. Autoimmune thyroid diseases may be a potential risk factor for metabolic abnormalities even in those with low normal thyroid function.

SUMMARY

The interrelation between thyroid stimulating hormone, free T3, freeT4 and metabolic parameters is complex and might be affected by age, sex, BMI, insulin resistance, smoking, iodine intake and inflammatory markers.

Thyroid hormone concentrations in second trimester of gestation and birth outcomes in Shanghai, China

BACKGROUND

Previous studies suggested that thyroid function in the first trimester of gestation played an important role in fetal growth. However, limited clinical data are available to support this relationship in the second trimester of gestation.

OBJECTIVE

We examined the relationship of maternal thyroid function parameters in early second trimester of gestation with birth outcomes in China.

METHODS

Participating 5016 mother-infant pairs were recruited from the Songjiang District Maternity and Infant Hospital, Shanghai, China, between July and December 2016. Linear regression assessed the associations of thyroid hormones with birthweight. Logistic regression tested the correlations between thyroid hormones and low birthweight, macrosomia, small for gestational age , and large for gestational age infants. Maternal serum TSH, TT3, TT4, FT3, and FT4 concentrations were measured in 16-20 weeks of gestation.

RESULTS

TT3 and FT3 were positively associated with birthweight, while TT4 and FT4 were negatively associated with birthweight, respectively. Furthermore, higher TT3 and FT3 were associated with increased risks of LGA infants (OR = 1.48, 95% CI: 1.15-1.9; OR = 1.22, 95% CI: 1.01-1.46), respectively. Higher TT3 was associated with an increased risk of macrosomic infants (OR = 1.35, 95% CI: 1.04-1.74). In contrast, higher FT4 was associated with decreased risks of LGA (OR = 0.87, 95% CI: 0.81-0.93) and macrosomic infants (OR = 0.90, 95% CI: 0.84-0.96), respectively. No associations were found between TSH and any of the outcomes.

CONCLUSIONS

TT3 and FT3 were positively associated with fetal growth, while TT4 and FT4 were negatively associated with fetal growth. Our findings suggested that thyroid function in early second trimester of gestation is a potential risk factor for abnormal fetal growth.

High TSH Level within Normal Range Is Associated with Obesity, Dyslipidemia, Hypertension, Inflammation, Hypercoagulability, and the Metabolic Syndrome: A Novel Cardiometabolic Marker

(1) Background: Overt and subclinical hypothyroidism has been associated with increased cardiometabolic risks. Here we further explore whether thyroid function within normal range is associated with cardiometabolic risk factors in a large population-based study. (2) Methods: We screened 24,765 adults participating in health examinations in Taiwan. Participants were grouped according to high-sensitive thyroid-stimulating hormone (hsTSH) level as: <50th percentile (0.47–1.48 mIU/L, the reference group), 50–60th percentile (1.49–1.68 mIU/L), 60–70th percentile (1.69–1.94 mIU/L), 70–80th percentile (1.95–2.3 mIU/L), 80–90th percentile (2.31–2.93 mIU/L), and >90th percentile (>2.93 mIU/L). Cardiometabolic traits of each percentile were compared with the reference group. (3) Results: Elevated hsTSH levels within normal range were dose-dependently associated with increased body mass index, body fat percentage, waist circumferences, blood pressure, hemoglobin A1c (HbA1c), fasting insulin, homeostasis model assessment of insulin resistance (HOMA-IR), high homeostasis model of assessment of beta-cell (HOMA-β), triglycerides, total cholesterols, fibrinogen, and uric acids (p-for-trend <0.001), but not with fasting glucose levels. The association remained significant after adjustment of age, sex, and lifestyle. As compared to the reference group, subjects with the highest hsTSH percentile had significantly increased risk of being overweight (adjusted odds ratio (adjOR): 1.35), increased body fat (adjOR: 1.29), central obesity (adjOR: 1.36), elevated blood pressure (adjOR: 1.26), high HbA1c (adjOR: 1.20), hyperinsulinemia (adjOR: 1.75), increased HOMA-IR (adjOR: 1.45), increased HOMA-β (adjOR: 1.40), hypertriglyceridemia (adjOR: 1.60), hypercholesterolemia (adjOR: 1.25), elevated hsCRP (adjOR: 1.34), increased fibrinogen (adjOR: 1.45), hyperuricemia (adjOR: 1.47), and metabolic syndrome (adjOR: 1.42), but significant risk of low fasting glucose (adjOR: 0.89). Mediation analysis indicates that insulin resistance mediates the majority of the association between thyroid hormone status and the metabolic syndrome. (4) Conclusion: Elevated hsTSH within the normal range is a cardiometabolic risk marker associated with central obesity, insulin resistance, elevated blood pressure, dyslipidemia, hyperuricemia, inflammation, and hypercoagulability.

The Role of Oxidative Stress and Hormones in Controlling Obesity

The accumulation of adipose tissue in the body occurs because the energy introduced with food and drink exceeds that expense, but to understand why this imbalance is established and why it is maintained over time, it is important to consider the main causes and risk factors of excess weight. In this review, we will refer to the main factors linked to obesity, starting from oxidative stress to hormonal factors including the role of obesity in breast cancer. Among the many hypotheses formulated on the etiopathology of obesity, a key role can be attributed to the relationship between stress oxidative and intestinal microbiota. Multiple evidences tend to show that genetic, epigenetic, and lifestyle factors contribute to determine in the obese an imbalance of the redox balance correlated with the alteration of the intestinal microbial flora. Obesity acts negatively on the wound healing, in fact several studies indicate morbid obesity significantly increased the risk of a post-operative wound complication and infection. Currently, in the treatment of obesity, medical interventions are aimed not only at modifying caloric intake, but also to modulate and improve the composition of diet with the aim of rebalancing the microbiota-redox state axis.

Relationship of metabolic syndrome and its components with thyroid dysfunction in Algerian patients

AIMS

The aim of this study is to evaluate the prevalence of the metabolic syndrome and its compounds in subjects with different thyroid status.

MATERIALS AND METHODS

A prospective cross-sectional study was conducted in the internal medicine department at El Okbi Hospital of Guelma (East of Algeria) from January 2014 to September 2015. Eighty six patients attending the specialist consultation for suspected thyroid disorders were included in the study. Gender; blood pressure; body mass index; and serum levels of fasting glucose, total cholesterol (TC), high-density-lipoprotein cholesterol (HDL-C), low-density-lipoprotein cholesterol, and triglyceride were compared between subjects with hypothyroidism, hyperthyroidism and euthyroidism.

RESULTS AND CONCLUSION

Thyroid dysfunction was found in 59.3% (n=42) patients, hypothyroidism (45.3%) was the major thyroid dysfunction followed by hyperthyroidism (14.0%). Overall, the prevalence of metabolic syndrome was 48.8% (n=42). Subjects with hypothyroidism had significantly higher level of BMI, WC, TC, LDL-C, and higher prevalence of abdominal obesity (84.6%, p<0.01) and hypertension (51.2%, p<0.05). The hyperthyroid group had significantly lower level of TC, LDL-C and HDL-C but a higher level of SBP and UA. Furthermore, abdominal obesity, hypertension and low HDL-C level were the most common metabolic syndrome compounds found in the hyperthyroid group compared to the euthyroid group. We found a positive association between TSH level and the prevalence of the metabolic syndrome.

Maternal thyroid function, prepregnancy obesity and gestational weight gain-The Generation R Study: A prospective cohort study

OBJECTIVE

Maternal prepregnancy obesity and excessive gestational weight gain are associated with pregnancy complications. Thyroid function is related to differences in body mass index (BMI) in adult populations. We examined the associations of maternal thyroid function in early pregnancy with maternal BMI and weight gain during pregnancy.

DESIGN AND METHODS

In a population-based prospective cohort study among 5726 mothers, we measured maternal TSH and FT4 levels at 13.5 weeks of gestation (95% range: 9.7-17.6 weeks). Maternal weight was assessed before pregnancy and in each trimester.

RESULTS

Higher maternal TSH levels were associated with higher prepregnancy BMI (difference: 0.18 kg/m² [95% CI: 0.01, 0.36] per SD increase in maternal TSH level) and higher total gestational weight gain (difference: 0.02 kg/wk [95% CI: 0.01, 0.03] per SD increase in maternal TSH level). Higher maternal FT4 levels were associated with lower prepregnancy BMI (difference: -0.44 kg/m² [95% CI: -0.63, -0.26] per SD increase in maternal FT4 level) and lower total gestational weight gain (difference: -0.01 kg/wk [95% CI: -0.02, -0.01] per SD increase in maternal FT4 level). The associations of maternal thyroid function with weight gain in early pregnancy were stronger than those with weight gain in mid and late-pregnancy. Maternal hypothyroidism was associated with higher prepregnancy BMI and early pregnancy weight gain, whereas opposite effects were observed for maternal hyperthyroidism (P<.05).

CONCLUSIONS

Higher maternal TSH level and lower FT4 level in early pregnancy are associated with higher prepregnancy BMI and higher gestational weight gain. Further studies are needed to explore maternal and foetal consequences.

Reference values for TSH may be inadequate to define hypothyroidism in persons with morbid obesity: Di@bet.es study

OBJECTIVE

To analyze the reference range of thyroid-stimulating hormone (TSH) in different BMI categories and its impact on the classification of hypothyroidism.

METHODS

The study included 3,928 individuals free of thyroid disease (without previous thyroid disease, no interfering medications, TSH <10 µUI/mL and thyroid peroxidase antibodies [TPO Abs] <50 IU/mL) who participated in a national, cross-sectional, population-based study and were representative of the adult population of Spain. Data gathered included clinical and demographic characteristics, physical examination, and blood and urine sampling. TSH, free thyroxine, free triiodothyronine, and TPO Ab were analyzed by electrochemiluminescence (E170, Roche Diagnostics, Basel, Switzerland).

RESULTS

The reference range (p2.5-97.5) for TSH was estimated as 0.6 to 4.8 µUI/mL in the underweight category (BMI<20 kg/m² ), 0.6 to 5.5 µUI/mL in the normal-weight category (BMI 20-24.9 kg/m² ), 0.6 to 5.5 µUI/mL in the overweight category (BMI 25-29.9 kg/m² ), 0.5 to 5.9 µUI/mL in the obesity category (BMI 30-39.9 kg/m² ), and 0.7 to 7.5 µUI/mL in the morbid obesity category (BMI ≥40). By using the reference criteria for the normal-weight population, the prevalence of high TSH levels increased threefold in the morbid obesity category (P < 0.01).

CONCLUSIONS

Persons with morbid obesity might be inappropriately classified if the standard ranges of normality of TSH for the normal-weight population are applied to them.

Association of triiodothyronine levels with future development of metabolic syndrome in euthyroid middle-aged subjects: a 6-year retrospective longitudinal study

BACKGROUND

Several cross-sectional studies have reported that thyroid hormone levels are associated with cardiovascular risk markers and metabolic syndrome (MetS) even in euthyroid subjects. However, the prognostic role of serum thyroid hormone levels in the risk of incident MetS has not been elucidated.

AIM

We aimed to investigate the associations of baseline serum thyroid hormone levels with the development of MetS in healthy subjects.

METHODS

This 6-year, cross-sectional, longitudinal and follow-up study was conducted in 12 037 euthyroid middle-aged subjects without MetS subjected to comprehensive health examinations. Subjects were grouped according to total triiodothyronine (T3) quartiles. The hazard ratio (HR) for the development of MetS according to T3 quartiles was estimated using Cox proportional hazards model.

RESULTS

During the 6-year period, 3544 incident cases of MetS (29%) were identified. The proportion of subjects with incident MetS increased across the T3 quartiles (P for trend <0.001). The HR and 95% confidence interval (CI) for the development of MetS were significantly higher in the highest T3 quartile compared with the lowest T3 quartile even after adjusting for confounding variables including gender, age and smoking (HR: 1.238, 95% CI: 1.128-1.358, P < 0.001).

CONCLUSION

In euthyroid middle-aged subjects, serum T3 levels are associated with increased risk for future development of MetS.

Association between Serum Thyroid-Stimulating Hormone Levels and Visceral Adipose Tissue: A Population-Based Study in Northeast Germany

BACKGROUND

Abdominal obesity is a major driver for adverse medical conditions. While an interaction between adipose tissue and thyroid function is thought to exist, to our knowledge, no study has examined the effect of thyroid-stimulating hormone (TSH) on visceral adipose tissue (VAT) in a population-based context.

OBJECTIVE

We determined an association between serum TSH levels and VAT.

METHODS

A sample of 1,021 female and 956 male adults aged 20-79 years was drawn from registry offices in the cross-sectional, population-based Study of Health in Pomerania Trend (SHIP Trend) in Northeast Germany from 2008 to 2012. Our main exposure was serum TSH levels. Our main outcome was VAT measured using magnetic resonance imaging. The possibly mediating role of leptin on the TSH-VAT association was also assessed.

RESULTS

A total of 1,719 participants (87.9%) had serum TSH levels within the reference range. The mean volume of VAT was 5.33 liters for men and 2.83 liters for women. No association between TSH and VAT (β = 0.06, 95% CI: -0.02, 0.14) was observed, and there were no differences detected between sexes. VAT was strongly associated with leptin with a greater effect in women than in men. Leptin was strongly associated with TSH.

CONCLUSIONS

No association between TSH and VAT was observed. Other biomarkers such as leptin may play a role in the relationship between thyroid function and metabolic risk.

Thyroid function and cardiovascular disease risk factors in euthyroid adults: a cross-sectional and longitudinal study

OBJECTIVE AND DESIGN

We explored the cross-sectional and longitudinal associations of thyroid function within the normal range with cardiovascular disease (CVD) risk factors and adiposity measures.

PATIENTS AND MEASUREMENTS

A total of 3483 (50·4% women) participants for the cross-sectional CVD study and 1630 (41·2% women) participants for the cross-sectional body composition substudy were drawn from the Framingham Third Generation Exam 1; 2912 participants (50·1% women) for the longitudinal CVD study and 713 participants (35·9% women) for the longitudinal body composition substudy were drawn from the Framingham Third Generation Exams 1-2. Thyroid function was assessed by thyrotropin [thyroid-stimulating hormone (TSH)] and free thyroxine (fT4) concentrations within the reference range at Exam 1. The associations between thyroid function and CVD risk factors were modelled via multivariable-adjusted regression models. Multivariable adjustment included age, sex, current smoking, postmenopausal status and BMI.

RESULTS

Cross-sectionally, higher TSH concentration was associated with increased odds of hypertriglyceridaemia [odds ratio (OR)=1·10], and higher BMI (β = 0·19 kg/m² ), total cholesterol (β = 0·05 mmol/l), triglycerides (β = 0·0006 mmol/l) and subcutaneous adipose tissue (SAT) volume (β = 38·8 cm³ ) (all P < 0·05). Cross-sectionally, fT4 was inversely associated with metabolic and adiposity-related CVD risk factors, including obesity (OR = 1·17), hypertriglyceridaemia (OR = 1·09), BMI (β = 0·42 kg/m² ), total cholesterol (β = 0·05 mmol/l), triglycerides (β = 0·0002 mmol/l), visceral adipose tissue (VAT) volume (β = -20·7 cm³ ) and attenuation (0·17 HU) and VAT/SAT ratio (β = -0·01) (all P < 0·05). However, during 6·1 years of follow-up, baseline TSH and fT4 levels were not longitudinally associated with CVD risk factors and adiposity measures.

CONCLUSIONS

Thyroid function within the normal range is cross-sectionally, but not longitudinally, associated with CVD risk factors and adiposity measures.

The relation between changes in thyroid function and anthropometric indices during long-term follow-up of euthyroid subjects: the Tehran Thyroid Study (TTS)

OBJECTIVE

Uncertainties exist regarding the causal relationship between thyroid function tests (TFT) within the euthyroid range and anthropometric measures. This longitudinal cohort is aimed to examine the relationship between the two conditions.

SUBJECTS AND METHODS

Euthyroid participants of Tehran Thyroid Study (TTS) attending phase I (1999-2001) were included in this study and were followed up to phase IV (2008-2011). TSH and free T4 (fT4) levels as well as weight (Wt), waist circumference (WC), hip circumference (HC) and waist-to-hip ratio (WHR) were measured at both phases.

RESULTS

971 women and 784 men were included in the analysis. During 9.7years of follow-up, increases in TSH levels, Wt and WHR as well as a decrease in fT4 level were observed. Multivariable regression analysis showed a significant relationship between TSH changes and alterations in WC in women (β=0.69, P=0.021) and men (β=0.61, P=0.038). Moreover, a significant negative association of ΔfT4 with changes in weight was documented (β=-0.49, P=0.001 in women and β=-0.56, P<0.001 in men). Additionally, we found a negative relationship between ΔfT4 and ΔHC in men (β=-0.36, P=0.001).

CONCLUSION

In both genders, there was a positive relationship between changes in TSH and waist circumference and conversely a negative association of changes of fT4 levels with weight over time.

Plasma Levels of Free Thyroxine and Risk of Major Bleeding in Bariatric Surgery

BACKGROUND

In a recent study of patients using vitamin K antagonists, those with low free thyroxin (FT4) levels within the normal range had a 3- to 5-fold increased risk of major bleeding. We tested the hypothesis that low levels of preoperative FT4 within the reference range are associated with an increased risk of major bleeding during and after bariatric surgery.

METHODS

The charts of 2,872 consecutive patients undergoing bariatric surgery were retrospectively screened for bleeding episodes. Patients with major bleeding until 1 month after surgery were compared to randomly selected control patients without bleeding, in a ratio of 1:4. We evaluated the association between preoperative FT4 levels and the risk of major bleeding by logistic regression.

RESULTS

Seventy-two cases (2.5%) with major bleeding were identified and 288 controls were selected. The median plasma level of FT4 was 13 pmol/l (interquartile range: 12-14) in the cases as well as in the controls. No clear effect was observed of low levels of FT4 on the risk of major bleeding: odds ratio 1.48 (95% CI: 0.46-4.80) for patients with an FT4 level <11 pmol/l, 1.03 (0.49-2.18) for patients with an FT4 level <12 pmol/l, and 1.12 (0.65-1.94) for patients with an FT4 level <13 pmol/l as compared to patients with FT4 values greater than or equal to these cutoff levels.

INTERPRETATION

We did not observe an increased risk of major bleeding with low levels of FT4 in patients undergoing bariatric surgery.

Thyroid Function and Thyroid Autoimmunity in Relation to Weight Status and Cardiovascular Risk Factors in Children and Adolescents: A Population-Based Study

OBJECTIVE

In obese subjects, slight increases have been observed in thyrotropin [thyroid-stimulating hormone (TSH)] levels, but data in children are scarce. The aim of this study was to evaluate whether thyroid function and autoimmunity vary with weight status in a healthy population of children and adolescents and to determine whether hyperthyrotropinemia is associated with any cardiovascular risk factor.

METHODS

This cross-sectional epidemiological study was conducted in Almería (Spain) on a representative sample of 1317 healthy subjects aged 2-16 years. Thyroid function, thyroid autoimmunity and cardiovascular risk factors were measured. Chi-square test, analysis of variance and multiple linear regression were used in the statistical analyses.

RESULTS

The obese children and adolescents had thyrotropin levels (mean ± standard deviation) of 3.12±2.44 mU/L. These levels were higher than those of overweight subjects (2.79±1.51 mU/L) and of normal weight subjects (2.73±1.30 mU/L) (p=0.02). Levels of free thyroxine and urinary iodine did not differ significantly between the groups. The prevalence (95% confidence interval) of thyroid autoimmunity was lower in the individuals with normal weight (2.9%; 2.0-4.2) than in the overweight (6.3%; 3.9-9.9) and obese subjects (5.6%, 2.5-11.3) (p=0.02). TSH levels were associated with obesity (β=0.36; p<0.001) and thyroid autoimmunity (β=1.10; p<0.001). They were not associated with any cardiovascular risk factor.

CONCLUSION

Obese children and adolescents had higher levels of thyrotropin than those who were overweight and of normal weight. The differences among the groups were of very little clinical significance and could possibly be linked to the higher prevalence of thyroid autoimmunity in obese subjects. The hyperthyrotropinemia in these subjects was not associated with any cardiovascular risk factor.

Triiodothyronine Levels Are Independently Associated with Metabolic Syndrome in Euthyroid Middle-Aged Subjects

BACKGROUND

Recent studies have shown an association between thyroid hormone levels and metabolic syndrome (MetS) among euthyroid individuals; however, there have been some inconsistencies between studies. Here, we evaluated the relationship between thyroid hormone levels and MetS in euthyroid middle-aged subjects in a large cohort.

METHODS

A retrospective analysis of 13,496 euthyroid middle-aged subjects who participated in comprehensive health examinations was performed. Subjects were grouped according to thyroid stimulating hormone, total triiodothyronine (T3), total thyroxine (T4), and T3-to-T4 ratio quartile categories. We estimated the odds ratios (ORs) for MetS according to thyroid hormone quartiles using logistic regression models, adjusted for potential confounders.

RESULTS

Of the study patients, 12% (n=1,664) had MetS. A higher T3 level and T3-to-T4 ratio were associated with unfavourable metabolic profiles, such as higher body mass index, systolic and diastolic blood pressure, triglycerides, fasting glucose and glycated hemoglobin, and lower high density lipoprotein cholesterol levels. The proportion of participants with MetS increased across the T3 quartile categories (P for trend <0.001) and the T3-to-T4 ratio quartile categories (P for trend <0.001). The multi-variate-adjusted OR (95% confidence interval) for MetS in the highest T3 quartile group was 1.249 (1.020 to 1.529) compared to the lowest T3 quartile group, and that in the highest T3-to-T4 ratio quartile group was 1.458 (1.141 to 1.863) compared to the lowest T3-to-T4 ratio quartile group, even after adjustment for potential confounders.

CONCLUSION

Serum T3 levels and T3-to-T4 ratio are independently associated with MetS in euthyroid middle-aged subjects. Longitudinal studies are needed to define this association and its potential health implications.

Paradoxical Relationship Between Body Mass Index and Thyroid Hormone Levels: A Study Using Mendelian Randomization

CONTEXT

Free T3 (FT3) has been positively associated with body mass index (BMI) in cross-sectional studies in healthy individuals. This is difficult to reconcile with clinical findings in pathological thyroid dysfunction.

OBJECTIVE

We aimed to investigate whether childhood adiposity influences FT3 levels.

DESIGN

Mendelian randomization using genetic variants robustly associated with BMI.

SETTING

Avon Longitudinal Study of Parents and Children, a population-based birth cohort.

PARTICIPANTS

A total of 3014 children who had thyroid function measured at age 7, who also underwent dual x-ray absorptiometry scans at ages 9.9 and 15.5 years and have genetic data available.

MAIN OUTCOME MEASURES

FT3.

RESULTS

Observationally at age 7 years, BMI was positively associated with FT3: β-standardized (β-[std]) = 0.12 (95% confidence interval [CI]: 0.08, 0.16), P = 4.02 × 10(-10); whereas FT4 was negatively associated with BMI: β-(std) = -0.08 (95% CI: -0.12, -0.04), P = 3.00 × 10(-5). These differences persisted after adjustment for age, sex, and early life environment. Genetic analysis indicated 1 allele change in BMI allelic score was associated with a 0.04 (95% CI: 0.03, 0.04) SD increase in BMI (P = 6.41 × 10(-17)). At age 7, a genetically determined increase in BMI of 1.89 kg/m(2) was associated with a 0.22 pmol/L (95% CI: 0.07, 0.36) increase in FT3 (P = .004) but no substantial change in FT4 0.01 mmol/L, (95% CI: -0.37, 0.40), P = .96.

CONCLUSION

Our analysis shows that children with a genetically higher BMI had higher FT3 but not FT4 levels, indicating that higher BMI/fat mass has a causal role in increasing FT3 levels. This may explain the paradoxical associations observed in observational analyses. Given rising childhood obesity levels, this relationship merits closer scrutiny.

Maternal hypothyroxinaemia in pregnancy is associated with obesity and adverse maternal metabolic parameters

OBJECTIVE

Subclinical hypothyroidism and isolated hypothyroxinaemia in pregnancy have been associated with an increased risk of gestational diabetes. We aimed to ascertain if these women have a worse metabolic phenotype than euthyroid pregnant women.

DESIGN, SUBJECTS AND METHODS

We recruited 956 healthy Caucasian women with singleton, non-diabetic pregnancies from routine antenatal clinics. Detailed anthropometric measurements (including BMI and skinfold thickness) and fasting blood samples (for TSH, free thyroxine (FT4), free triiodothyronine (FT3), HbA1c, lipid profile, plasma glucose and insulin resistance (HOMA-IR) analysis) were obtained at 28 weeks gestation.

RESULTS

In comparison to euthyroid women (n=741), women with isolated hypothyroxinaemia (n=82) had significantly increased BMI (29.5 vs 27.5 kg/m(2), P<0.001), sum of skinfolds (57.5 vs 51.3 mm, P=0.002), fasting plasma glucose (4.5 vs 4.3 mmol/l, P=0.01), triglycerides (2.3 vs 2.0 mmol/l, P<0.001) and HOMA-IR (2.0 vs 1.3, P=0.001). Metabolic parameters in women with subclinical hypothyroidism (n=133) were similar to those in euthyroid women. Maternal FT4 was negatively associated with BMI (r=-0.22), HbA1c (r=-0.14), triglycerides (r=-0.17), HOMA-IR (r=-0.15) but not total/HDL cholesterol ratio (r=-0.03). Maternal FT3:FT4 ratio was positively associated with BMI (r=0.4), HbA1c (r=0.21), triglycerides (r=0.2), HOMA-IR (r=0.33) and total/HDL cholesterol ratio (r=0.07). TSH was not associated with the metabolic parameters assessed.

CONCLUSIONS

Isolated hypothyroxinaemia, but not subclinical hypothyroidism, is associated with adverse metabolic phenotype in pregnancy, as is decreasing maternal FT4 and increasing FT3:FT4 ratio. These associations may be a reflection of changes in the thyroid hormone levels secondary to increase in BMI rather than changes in thyroid hormone levels affecting body weight and related metabolic parameters.

Adiposity and risk of thyroid cancer: a systematic review and meta-analysis

Thyroid cancer incidence has increased rapidly over time, as has obesity prevalence. A link between the two appears plausible, but the relation of adiposity to thyroid cancer remains incompletely understood. We performed a meta-analysis of adiposity measures and thyroid cancer using studies identified through October 2014. Twenty-one articles yielded data on 12,199 thyroid cancer cases. We found a statistically significant 25% greater risk of thyroid cancer in overweight individuals and a 55% greater thyroid cancer risk in obese individuals as compared with their normal-weight peers. Each 5-unit increase in body mass index (BMI), 5 kg increase in weight, 5 cm increase in waist or hip circumference and 0.1-unit increase in waist-to-hip ratio were associated with 30%, 5%, 5% and 14% greater risks of thyroid cancer, respectively. When evaluated by histologic type, obesity was significantly positively related to papillary, follicular and anaplastic thyroid cancers, whereas it revealed an inverse association with medullary thyroid cancer. Both general and abdominal adiposity are positively associated with thyroid cancer. However, relations with BMI vary importantly by tumour histologic type.

Indices of adiposity and thyroid hormones in euthyroid postmenopausal women

OBJECTIVE

We aimed to evaluate the association between thyroid hormones and indices of obesity in a sample of euthyroid postmenopausal women.

DESIGN

Cross-sectional study.

METHODS

Serum levels of TSH, free triiodothyronine (FT3), and free thyroxine (FT4) as well as BMI and waist:hip ratio (WHR) were evaluated in 194 healthy euthyroid postmenopausal women. Ultrasonography was used to assess abdominal fat layers (subcutaneous fat (SF), preperitoneal fat (PF), and SF:PF ratio). Indices of adiposity were defined as high vs low depending on the median value of the assessed parameters.

RESULTS

After multivariate adjustment for traditional risk factors, lower FT4 levels and a higher FT3:FT4 ratio predicted higher SF mass (FT4, Exp(β)=0.035, P=0.020 and FT3:FT4, Exp(β)=2.374, P=0.018), whereas higher FT3 predicted higher PF mass (Exp(β)=2.815, P=0.032). Women with FT3:FT4 above the highest quartile had a significantly higher SF mass as compared to women in the lowest quartile (1.81 ± 0.62 cm vs 1.54 ± 0.46 cm, P=0.027). BMI had a positive independent association with TSH (Exp(β)=1.829, P=0.018). Finally, FT3 was significantly associated with SF mass among women with higher BMI (FT3, β=0.259, P=0.040) and women with higher WHR (β=0.309, P=0.020) but not among women with lower BMI or WHR values.

CONCLUSION

Thyroid hormone levels, and in particular FT3, were independently associated with SF and PF in euthyroid postmenopausal women, and this association was mainly evident in women with higher BMIs. On the other hand, among traditional indices of adiposity, only TSH was positively associated with BMI. Larger prospective studies are needed to evaluate the significance of the present findings.

Effects of restoration of the euthyroid state on epicardial adipose tissue and carotid intima media thickness in subclinical hypothyroid patients

Epicardial adipose tissue (EAT) thickness is a novel cardiovascular risk indicator, whereas carotid intima media thickness (CIMT) is a known marker of subclinical atherosclerosis. The aim of this study was to measure EAT thickness and CIMT in subclinical hypothyroid (SCH) patients and to determine the effect of restoration of the euthyroid state on these variables. The study included 43 patients with SCH (mean age: 35.2±10.7 years; F-M ratio: 42:1) and 30 healthy controls (mean age: 34.5±8.2 years; F-M ratio: 25:5). EAT thickness and CIMT were measured via conventional echocardiography and ultrasonography. Among the patients, 23 were followed up with L-thyroxine replacement until restoration of the euthyroid state and re-evaluated 6 months after baseline. Basal EAT thickness was higher in the SCH patients than in the controls (3.2±0.7 vs. 2.3±0.3 mm-p<0.0001), whereas CIMT was similar in both groups (0.50±0.09 vs. 0.48±0.04 mm). EAT thickness was correlated with CIMT in the patient group (r=0.39, p=0.01), but this correlation was not evident based on multivariate analysis when corrected for age and body mass index (r2=0.47 and p=0.0001 for the model). In the follow-up patient subgroup, both EAT thickness and CIMT decreased significantly following L-thyroxine treatment, when normal TSH levels were attained (3.4±0.7 vs. 2.3±0.5 mm-p=0.007 and 0.51±0.09 vs. 0.46±0.07 mm-p=0.01, respectively). EAT thickness was greater in the SCH patients than in controls, whereas CIMT was similar in both groups. Restoration of the euthyroid state with L-thyroxine treatment was associated with significant decreases in EAT thickness and CIMT in the group of patients that received L-thyroxine treatment and, as such, might reduce the cardiovascular risk associated with SCH.

Changes in body weight after treatment of primary hypothyroidism with levothyroxine

OBJECTIVE

Surprisingly few studies have examined weight change in hypothyroid patients after initiation of levothyroxine (LT4) therapy. Our study aimed to investigate weight change after initiation of LT4 treatment for primary hypothyroidism.

METHODS

Using electronic medical records from Boston Medical Center, Boston, Massachusetts, we performed a retrospective cohort study between January 1, 2003, and February 1, 2011. Adults ≥18 years of age with newly diagnosed primary hypothyroidism with an initial thyroid-stimulating hormone (TSH) level ≥10 mIU/L were identified. Patients with postsurgical hypothyroidism, thyroid cancer, and a history of radioactive iodine or head/neck irradiation, congestive heart failure, anorexia nervosa, end-stage renal disease, cirrhosis, pregnancy, or use of prescription weight-loss medications were excluded. TSH and weight at diagnosis and up to 24 months after LT4 initiation were collected. Weight change was assessed at the first posttreatment serum TSH level <5 mIU/L.

RESULTS

A total of 101 patients (mean age, 48 ± 15 years; 71% women) were included. Initial median TSH was 18.3 mIU/L (range, 10.1 to 710.5 mIU/L) and initial median weight was 79.6 kg (range 41.5 to 167.5 kg). Posttreatment median TSH level was 2.3 mIU/L (range, 0.04 to 5 mIU/L), and weight change at a median of 5 months (range, 1.1 to 25.6 months) was -0.1 kg (range, -20.6 to 7.7 kg). Initial median body mass index (BMI) of 95 of the patients was 29.3 kg/m2 (range, 19.5 to 56.1 kg/m2), and the median change in BMI was -0.1 kg/m2 (range, -7.1 to 3.3 kg/m2). Only 52% of patients lost weight, with a mean weight loss of 3.8 ± 4.4 kg. Gender, race, education, insurance type, age, initial TSH level, time to normalization of TSH, and initial weight were not associated with changes in weight or BMI.

CONCLUSION

Contrary to popular belief, our study of 101 patients with primary hypothyroidism showed that no significant weight change occurs after initiation of LT4 treatment.

Mechanisms in endocrinology: the crosstalk between thyroid gland and adipose tissue: signal integration in health and disease

Obesity and thyroid diseases are common disorders in the general population and they frequently occur in single individuals. Alongside a chance association, a direct relationship between 'thyroid and obesity' has been hypothesized. Thyroid hormone is an important determinant of energy expenditure and contributes to appetite regulation, while hormones and cytokines from the adipose tissue act on the CNS to inform on the quantity of energy stores. A continuous interaction between the thyroid hormone and regulatory mechanisms localized in adipose tissue and brain is important for human body weight control and maintenance of optimal energy balance. Whether obesity has a pathogenic role in thyroid disease remains largely a matter of investigation. This review highlights the complexity in the identification of thyroid hormone deficiency in obese patients. Regardless of the importance of treating subclinical and overt hypothyroidism, at present there is no evidence to recommend pharmacological correction of the isolated hyperthyrotropinemia often encountered in obese patients. While thyroid hormones are not indicated as anti-obesity drugs, preclinical studies suggest that thyromimetic drugs, by targeting selected receptors, might be useful in the treatment of obesity and dyslipidemia.

Obesity and thyroid cancer

Many studies have provided observational data on the association of obesity and thyroid cancers, but only few of them propose mechanisms that would permit a better understanding of the causal molecular mechanisms of this association. Considering that there is an increasing incidence of both obesity and thyroid cancers, we need to summarize and link recent studies in order to characterize and understand the contribution of obesity-related factors that might affect thyroid cancer development and progression. Adipose tissue is involved in many vital processes, including insulin sensitivity, angiogenesis, regulation of energy balance, activation of the complement system, and responses such as inflammation. Although these processes have their own molecular pathways, they involve the same molecules through which obesity and adipose tissue might exert their roles in carcinogenesis, not only affecting MAPK and PI3K or even insulin pathways, but also recruiting local inflammatory responses that could result in disease formation and progression. This review describes five important issues that might explain the link between excessive weight and thyroid cancer: thyroid hormones, insulin resistance, adipokines, inflammation, and sexual hormones.

Maternal thyroid hormones during pregnancy, childhood adiposity and cardiovascular risk factors: the Generation R Study

OBJECTIVE

Variation in maternal thyroid function during early pregnancy may permanently affect childhood growth and cardiovascular development. We examined the associations of early pregnancy maternal TSH and free T4 (FT4) levels with childhood growth, body composition and cardiovascular characteristics.

METHODS

We performed a population-based prospective cohort study among 5646 mothers and their children. Maternal thyroid parameters were assessed in early pregnancy (median: 13·2 weeks; 95% range: 9·7-17·6 weeks). Childhood growth was repeatedly measured from birth to 6 years. At the age of 6 years, childhood body mass index (BMI), total body and abdominal fat distribution, blood pressure and left ventricular mass were measured.

RESULTS

Maternal thyroid parameters were not consistently associated with childhood length and weight growth characteristics. Lower maternal TSH levels were associated with lower childhood BMI, total fat mass, abdominal subcutaneous fat mass area and diastolic blood pressure (P-values <0·05), but not with preperitoneal abdominal fat mass area, systolic blood pressure or left ventricular mass. Higher maternal FT4 levels were associated with lower childhood BMI, abdominal subcutaneous and preperitoneal fat mass area (P for trends <0·05), but not with other cardiovascular characteristics. Clinically maternal hypothyroid or hyperthyroid statuses were not associated with childhood growth, body fat or cardiovascular outcomes.

CONCLUSIONS

Maternal thyroid function during early pregnancy may influence childhood body composition and cardiovascular development. Further studies are needed to replicate these findings and to examine the influence of maternal thyroid hormone levels during pregnancy on long-term growth and cardiovascular development in the offspring.

Subclinical hypothyroidism, weight change, and body composition in the elderly: the Cardiovascular Health Study

BACKGROUND

Subclinical hypothyroidism is common in the elderly, yet its relationship with weight and body composition is unclear.

OBJECTIVE

We examined the relationship between subclinical hypothyroidism and weight change and body composition in older adults.

METHODS

A total of 427 subclinically hypothyroid and 2864 euthyroid U.S. individuals ≥65 years old enrolled in the Cardiovascular Health Study and not taking thyroid preparations were included. Analyses of 6-year weight change were performed, compared by thyroid status. A cross-sectional analysis of thyroid status and body composition was performed in a subset of 1276 participants who had dual-energy x-ray absorptiometry scans. Models were risk factor-adjusted and stratified by sex.

RESULTS

Overall, participants lost weight during follow-up (-0.38 kg/y in men, -0.37 kg/y in women). Subclinical hypothyroidism, when assessed at a single time point or persisting over 2 years, was not associated with a difference in weight change compared with euthyroidism. Subclinical hypothyroidism was also not associated with differences in lean mass, fat mass, or percent fat compared with euthyroidism. A TSH level 1 mU/L higher within the euthyroid or subclinical hypothyroid range was associated with a 0.51-kg higher baseline weight in women only (P < .001) but not with weight change in either sex. A 1 ng/dL higher free T4 level was associated with lower baseline weight and 0.32 kg/y greater weight loss in women only (P = .003). Baseline weight and weight change did not differ by T3 levels.

CONCLUSIONS

Our data do not support a clinically significant impact of subclinical hypothyroidism on weight status in the elderly.

Triiodothyronine and free thyroxine levels are differentially associated with metabolic profile and adiposity-related cardiovascular risk markers in euthyroid middle-aged subjects

BACKGROUND

We have previously shown that in healthy young men, a less favorable body composition is associated with higher free triiodothyronine (fT3) levels within the euthyroid range. Besides, a higher free-triiodothyronine-to-free-thyroxin (fT3-to-fT4) ratio has been related to a less favorable metabolic phenotype and more placental growth in pregnant women. In the present study, we therefore investigated whether serum thyrotropin (TSH), thyroid hormone levels, and the fT3-to-fT4 ratio are associated with metabolic and adiposity-related cardiovascular risk markers in a healthy population of middle-aged euthyroid men and women.

METHODS

Thyroid parameters were measured in 2524 generally healthy subjects from the Asklepios Study (35-55 years, mean age 46 years). Analyses were restricted to 2315 subjects (1138 women and 1177 men), not using thyroid medication, not having anti-TPO levels above clinical cutoff values or TSH levels outside the reference range (0.27-4.2 mU/L). Twenty-seven percent of the women and 47.5% of the men were overweight, while 13% of women and 17% of men were obese. Twenty percent of the subjects were active smokers. Serum thyroid function parameters were determined by electrochemiluminescence.

RESULTS

fT3 and the fT3-to-fT4 ratio were positively related to body mass index (BMI), waist circumference, and components of metabolic syndrome, that is, triglycerides, systolic and diastolic blood pressure, and fasting plasma glucose, and negatively with HDL-cholesterol levels, whereas fT4 was negatively associated with BMI, waist circumference, and triglycerides (p<0.001). TSH related positively with total cholesterol levels (p<0.01), triglycerides, and systolic and diastolic blood pressure (p<0.001). The fT3-to-fT4 ratio was further positively associated with the adiposity-related inflammation markers interleukin-6 and high-sensitivity C-reactive protein and to pulse wave velocity. All associations were adjusted for sex, age, height, and smoking, and most associations persisted after additional adjustment for weight or waist circumference.

CONCLUSION

In healthy euthyroid middle-aged men and women, higher fT3 levels, lower fT4 levels, and thus a higher fT3-to-fT4 ratio are consistently associated with various markers of unfavorable metabolic profile and cardiovascular risk.