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

Triiodothyronine Is Associated with Incidence/Resolution of Steatotic Liver Disease: Longitudinal Study in Euthyroid Korean

BACKGRUOUND

The positive relationship between triiodothyronine (T3) and steatotic liver disease (SLD) demonstrated only in crosssectional study. We aimed to evaluated whether total T3 (TT3) is associated with the development/resolution of SLD in longitudinal design.

METHODS

This retrospective, longitudinal, population-based cohort study included 1,665 South Korean euthyroid adults with ≥4 thyroid function test. We explored the impact of mean TT3 during follow-up on development/resolution of either SLD (diagnosed by ultrasound) or modified metabolic dysfunction-associated steatotic liver disease (MASLD) using Cox proportional hazards regression models.

RESULTS

During about median 5 years follow-up, 807/1,216 (66.3%) participants among participants without SLD at baseline developed SLD, and 253/318 (79.5%) participants among participants with SLD at baseline SLD resolved fatty liver. Mean TT3 rather than thyroid stimulating hormone or mean free thyroxine was significantly related with development (adjusted hazard ratio [HR], 1.01; 95% confidence interval [CI], 1.00 to 1.02; P=0.002) and resolution (adjusted HR, 0.97; 95% CI, 0.96 to 0.99; P=0.005) of SLD. Compared with low mean TT3 group, high mean TT3 group was positively associated with development of SLD (adjusted HR, 1.20; 95% CI, 1.05 to 1.38; P=0.008) and inversely associated with resolution of SLD (adjusted HR, 0.66; 95% CI, 0.51 to 0.85; P=0.001). The statistical significance remained for development (adjusted HR, 1.29; 95% CI, 1.10 to 1.51; P=0.001) and resolution (adjusted HR, 0.71; 95% CI, 0.54 to 0.94; P=0.018) of modified MASLD.

CONCLUSION

In Korean euthyroid adults, TT3 level was associated with development and resolution of either SLD or modified MASLD.

Associations of thyroid feedback quantile-based index with diabetes in euthyroid adults in the United States and China

BACKGROUND

We aimed to investigate the associations between thyroid hormone sensitivity indices and diabetes in euthyroid adults in the United States and China.

METHODS

2296 euthyroid adults from the NHANES in the United States and 8319 euthyroid adults from the SPEED-Shunde in China were involved. The thyroid sensitivity indices, namely TFQIFT4 and TFQIFT3, were calculated. Multivariable logistic regression, restricted cubic spline analysis, and general ordinal logit regression were utilized.

RESULTS

In the NHANES, compared with participants in quartile 1st (Q1), those in Q4 of TFQIFT3 (OR 2.12, 95% CI (1.18, 3.81)) and those in Q3 of TFQIFT4 (OR 2.31, 95% CI (1.18, 4.53)) (both P for trend < 0.05) were associated with a greater prevalence of diabetes. In the SPEED-Shunde, compared with participants in Q1, those in Q4 of TFQIFT3 had a greater prevalence of diabetes (OR 1.36, 95% CI (1.11, 1.66) (P for trend < 0.05), while no significant associations between TFQIFT4 and diabetes were found.

CONCLUSIONS

TFQIFT3 was associated with a higher prevalence of diabetes both in the United States and China. However, TFQIFT4 was only associated with a higher prevalence of diabetes in the United States, not in China. Further prospective cohort studies are necessary to validate these findings.

Hypothyroidism

Hypothyroidism, the deficiency of thyroid hormone, is a common condition worldwide. It affects almost all body systems and has a wide variety of clinical presentations from being asymptomatic to, in rare cases, life threatening. The classic symptoms of hypothyroidism include fatigue, lethargy, weight gain, and cold intolerance; however, these symptoms are non-specific and the diagnosis is typically made on biochemical grounds through serum thyroid function tests. The most common cause of hypothyroidism is chronic autoimmune thyroiditis (Hashimoto's thyroiditis), although other causes, including drugs (such as amiodarone, lithium, and immune checkpoint inhibitors), radioactive-iodine treatment, and thyroid surgery, are frequent. Historically, severe iodine deficiency was the most common cause. Reference ranges for thyroid function tests are based on fixed percentiles of the population distribution, but there is increasing awareness of the need for more individualised reference intervals based on key factors such as age, sex, and special circumstances such as pregnancy. Levothyroxine monotherapy is the standard treatment for hypothyroidism; it is safe and inexpensive, restores thyroid function tests to within the reference range, and improves symptoms in the majority of patients. However, 10% of patients have persistent symptoms of ill health despite normalisation of thyroid function tests biochemically and a substantial proportion of patients on levothyroxine have thyroid-stimulating hormone concentrations outside the reference range. Ongoing symptoms despite levothyroxine treatment has led to some patients using liothyronine or desiccated thyroid extract. Taken together, these factors have led to intense debate around the treatment thresholds and treatment strategies for hypothyroidism. In this Seminar, we review the epidemiology, genetic determinants, causes, and presentation of hypothyroidism; highlight key considerations and controversies in its diagnosis and management; and provide future directions for research.

Age-related variation in thyroid function - a narrative review highlighting important implications for research and clinical practice

BACKGROUND

Thyroid hormones are key determinants of health and well-being. Normal thyroid function is defined according to the standard 95% confidence interval of the disease-free population. Such standard laboratory reference intervals are widely applied in research and clinical practice, irrespective of age. However, thyroid hormones vary with age and current reference intervals may not be appropriate across all age groups. In this review, we summarize the recent literature on age-related variation in thyroid function and discuss important implications of such variation for research and clinical practice.

MAIN TEXT

There is now substantial evidence that normal thyroid status changes with age throughout the course of life. Thyroid stimulating hormone (TSH) concentrations are higher at the extremes of life and show a U-shaped longitudinal trend in iodine sufficient Caucasian populations. Free triiodothyronine (FT3) levels fall with age and appear to play a role in pubertal development, during which it shows a strong relationship with fat mass. Furthermore, the aging process exerts differential effects on the health consequences of thyroid hormone variations. Older individuals with declining thyroid function appear to have survival advantages compared to individuals with normal or high-normal thyroid function. In contrast younger or middle-aged individuals with low-normal thyroid function suffer an increased risk of adverse cardiovascular and metabolic outcomes while those with high-normal function have adverse bone outcomes including osteoporosis and fractures.

CONCLUSION

Thyroid hormone reference intervals have differential effects across age groups. Current reference ranges could potentially lead to inappropriate treatment in older individuals but on the other hand could result in missed opportunities for risk factor modification in the younger and middle-aged groups. Further studies are now needed to determine the validity of age-appropriate reference intervals and to understand the impact of thyroid hormone variations in younger individuals.

The nonlinear relationship between thyroid function parameters and metabolic dysfunction-associated fatty liver disease

BACKGROUND

The relationship between thyroid function parameters and metabolic dysfunction-associated fatty liver disease (MAFLD) remains controversial. Additionally, little is known about the relationship between thyroid function parameters and MAFLD in the Chinese population.

METHODS

We conducted a retrospective cross-sectional study involving 177,540 individuals with thyroid function tests and MAFLD diagnosis from 2010-2018. The association between thyroid function parameters and MAFLD was evaluated on a continuous scale with restricted cubic spline (RCS) models and by the prior-defined centile categories with multivariable-adjusted logistic regression models. Thyroid function parameters included free triiodothyronine (FT3), free tetra-iodothyronine (FT4), and thyroid stimulating hormone (TSH). Additionally, fully adjusted RCS models stratified by sex, age, and location were studied.

RESULTS

In the RCS models, the risk of MAFLD increased with higher levels of FT3 when FT3 <5.58pmol/L, while the risk of MAFLD decreased with higher levels of FT3 when FT3 ≥5.58pmol/L (P nonlinearity <0.05). While RCS analysis suggested that the FT4 levels had a negative association with MAFLD (P nonlinearity <0.05), indicating an increase in FT4 levels was associated with a decreased risk of MAFLD. RCS analysis suggested an overall positive association between the concentration of TSH and MAFLD risk (P nonlinearity <0.05). The rising slope was sharper when the TSH concentration was less than 1.79uIU/mL, which indicated the association between TSH and MAFLD risk was tightly interrelated within this range. The multivariable logistic regression showed that populations in the 81st-95th centile had the highest risk of MAFLD among all centiles of FT3/TSH, with the 1st-5th centile as the reference category.

CONCLUSIONS

Our study suggested nonlinear relationships between thyroid function parameters and MAFLD. Thyroid function parameters could be additional modifiable risk factors apart from the proven risk factors to steer new avenues regarding MAFLD prevention and treatment.

Subclinical Hypothyroidism in Families Due to Chronic Consumption of Nitrate-Contaminated Water in Rural Areas with Intensive Livestock and Agricultural Practices in Durango, Mexico

Nitrate is a widely disseminated water pollutant and has been linked to health disorders, including hypothyroidism. Here, we evaluated the relationship between thyroid function and chronic exposure to nitrates in rural zone families, in addition to the genetic and autoimmune factors. Exposure and effect biomarkers, thyroid hormones, and autoantibodies of tiroperoxidase were measured, as well the presence of two FOXE1 polymorphisms (rs965513, rs1867277). Pearson’s correlation, principal component analysis, Kruskal–Wallis, and chi-squared tests were used for statistical analysis. A total of 102 individuals were analyzed; 45% presented subclinical hypothyroidism, a negative correlation was observed between methemoglobin and the total T3 (r = −0.43, p = 0.001) and free T3 levels (r = −0.34, p = 0.001), as well as between TSH and the free T4 (r = −0.41, p = 0.0001) and total T4 (r = −0.36, p = 0.0001). A total of 15.7% had positive antithyroid ab-TPO, while the polymorphic genotype (AA) represented only 3% (rs965513) and 4% (rs1867277) among subjects with subclinical hypothyroidism. The high frequency of subclinical hypothyroidism in the population under study could be related, mainly, to chronic exposure through the consumption of nitrate-contaminated water.

DNA methylation at a nutritionally sensitive region of the PAX8 gene is associated with thyroid volume and function in Gambian children

PAX8 is a key thyroid transcription factor implicated in thyroid gland differentiation and function, and PAX8 gene methylation is reported to be sensitive to the periconceptional environment. Using a novel recall-by-epigenotype study in Gambian children, we found that PAX8 hypomethylation at age 2 years is associated with a 21% increase in thyroid volume and an increase in free thyroxine (T4) at 5 to 8 years, the latter equivalent to 8.4% of the normal range. Free T4 was associated with a decrease in DXA-derived body fat and bone mineral density. Furthermore, offspring PAX8 methylation was associated with periconceptional maternal nutrition, and methylation variability was influenced by genotype, suggesting that sensitivity to environmental exposures may be under partial genetic control. Together, our results demonstrate a possible link between early environment, PAX8 gene methylation and thyroid gland development and function, with potential implications for early embryonic programming of thyroid-related health and disease.

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.

Genetically Determined Higher TSH Is Associated With a Lower Risk of Diabetes Mellitus in Individuals With Low BMI

CONTEXT

Thyroid status is hypothesized to be causally related with the risk of diabetes mellitus (DM), but previous results were conflicting possibly because of a complex interaction between thyrotropin (TSH), body mass index (BMI) and DM.

OBJECTIVE

This work aims to investigate the causal association between thyroid status with DM and glucose homeostasis and to what extent this association is dependent on BMI.

METHODS

A mendelian randomization study was conducted of European-ancestry participants from the UK Biobank population. The present study involved 408 895 individuals (mean age 57.4 years [SD 8.0], 45.9% men), of whom 19 773 had DM. Genetic variants for circulatory TSH, free thyroxine (fT4) concentrations and BMI to calculate weighted genetic risk scores. The main outcome measures included self-reported DM-stratified analyses by BMI. Analyses were repeated for nonfasting glucose and glycated hemoglobin A1c (HbA1c) among individuals without DM.

RESULTS

Genetically determined TSH and fT4 levels were not associated with risk of DM in the total UK Biobank population. However, in analyses stratified on genetically determined BMI, genetically determined higher TSH, and not fT4, was associated with a lower risk for DM only in the low BMI group (odds ratio 0.91; 95% CI, 0.85-0.98 in low BMI; P value for interaction = .06). Similar results were observed for glucose and HbA1c among individuals without DM.

CONCLUSION

TSH, but not fT4, is a potential causal risk factor for DM in individuals with genetically determined low BMI highlighting potential protective effects of TSH only in low-risk populations.

Variation in Normal Range Thyroid Function Affects Serum Cholesterol Levels, Blood Pressure, and Type 2 Diabetes Risk: A Mendelian Randomization Study

Background: Observational studies have demonstrated that variation in normal range thyroid function is associated with major cardiovascular risk factors, including dyslipidemia, hypertension, type 2 diabetes (T2D), and obesity. As observational studies are prone to residual confounding, reverse causality, and selection bias, we used a Mendelian randomization (MR) approach to investigate whether these associations are causal or not. Methods: Two-sample MR analysis using data from the largest available genome-wide association studies on normal range thyrotropin (TSH) and free thyroxine (fT4) levels, serum lipid levels, blood pressure measurements, T2D, and obesity traits (body mass index [BMI] and waist/hip ratio). Results: A one standard deviation (SD) increase in genetically predicted TSH levels was associated with a 0.037 SD increase in total cholesterol levels (p = 3.0 × 10-4). After excluding pleiotropic instruments, we also observed significant associations between TSH levels and low-density lipoprotein levels (β = 0.026 SD, p = 1.9 × 10-3), pulse pressure (β = -0.477 mmHg, p = 7.5 × 10-10), and T2D risk (odds ratio = 0.95, p = 2.5 × 10-3). While we found no evidence of causal associations between TSH or fT4 levels and obesity traits, we found that a one SD increase in genetically predicted BMI was associated with a 0.075 SD decrease in fT4 levels (p = 3.6 × 10-4). Conclusions: Variation in normal range thyroid function affects serum cholesterol levels, blood pressure, and T2D risk.

Effect of the Cut-Off Level for Thyroid-Stimulating Hormone on the Prevalence of Subclinical Hypothyroidism among Infertile Mexican Women

The primary aim of this study was to compare the prevalence of subclinical hypothyroidism (SCH) using two different cut-off levels for TSH values (≥2.5 mIU/L versus ≥4.1 mIU/L). The secondary objective was to analyze the clinical-biochemical characteristics in women with and without SCH. This was a retrospective cross-sectional study. In total, 1496 Mexican women with infertility were included: Group 1, women with TSH levels ranging between 0.3 and 2.49 mIU/L, n = 886; Group 2, women with TSH between 2.5 and 4.09 mIU/L, n = 390; and Group 3, women with TSH ≥4.1 mIU/L n = 220. SCH prevalence was 40.7% (CI 95%: 38.3-43.3%) with TSH cut-off ≥ 2.5 mIU/L, and 14.7% (CI 95%: 12.7-16.5%) with TSH cut-off ≥ 4.1 mIU/L, (p = 0.0001). The prevalence of overweight was higher in Group 2 than in Groups 1 and 3. Thyroid autoimmunity, obesity and insulin resistance were higher in Group 3 than in Group 1 (p < 0.05). No other differences were observed between groups. Conclusions: The prevalence of SCH in our selected patients increased almost three times using a TSH cut-off ≥ 2.5 mIU/L compared with a TSH cut-off ≥ 4.1 mIU/L. Women with TSH ≥4.1 mIU/L compared with TSH cut-off ≤ 2.5 mIU/L more often presented with obesity, thyroid autoimmunity and insulin resistance.

Free Triiodothyronine Is Associated With Hepatic Steatosis and Liver Stiffness in Euthyroid Chinese Adults With Non-Alcoholic Fatty Liver Disease

OBJECTIVE

The association between non-alcoholic fatty liver disease (NAFLD) and thyroid hormones in euthyroid subjects is unclear. We investigated the relationship between thyroid function and the severity of hepatic steatosis and liver fibrosis in a large cohort of euthyroid Chinese adults.

METHODS

A total of 3496 participants were enrolled. Liver ultrasonography was used to define the presence of NAFLD (n=2172) or the absence of NAFLD (n=1324). Anthropometric and biochemical measurements were made and thyroid function parameters including free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH) were measured. The severity of hepatic steatosis and liver stiffness was assessed by transient elastography.

RESULTS

Levels of FT3 were significantly higher in the severe NAFLD group and moderate NAFLD group than in the mild NAFLD group (5.18 ± 0.58 vs 5.11 ± 0.57 vs 4.98 ± 0.60 pmol/L, P<0.001). Participants with F4 and F3 liver fibrosis had higher FT3 levels than those with F2 fibrosis (6.33 ± 0.39 vs 5.29 ± 0.48 vs 5.20 ± 0.50 pmol/L, P<0.001). However, FT4 and TSH levels did not correlate with hepatic steatosis or liver fibrosis severity. In addition, the proportions of participants with NAFLD (46.0% vs 63.1% vs 73.3%, P<0.001) and liver fibrosis (11.5% vs 18.6% vs 20.8%, P<0.001) increased as FT3 levels increased. Logistic regression analysis showed that FT3 levels were positively associated with the severity of hepatic steatosis and liver fibrosis presence, even after adjustment for metabolic risk factors including BMI. In non-obese participants, the FT3 level was an independently risk factor for the severity of hepatic steatosis.

CONCLUSIONS

There are positive associations of FT3 levels with the severity of hepatic steatosis and the presence of liver fibrosis in NAFLD with euthyroidism.

Exposure to organophosphate ester flame retardants and plasticizers during pregnancy: Thyroid endocrine disruption and mediation role of oxidative stress

Organophosphate esters (OPEs) are widely used as flame retardants and plasticizers in consumer and industrial products. Human exposure to OPEs raises concerns due to their endocrine disruptive potentials. Till now, the effects of OPEs on thyroid hormones (THs) and the mediating role of oxidative stress in pregnant women have not been studied. In this study, prenatal urinary concentrations of OPE metabolites (mOPEs), levels of free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH), and oxidative stress levels of 8-hydroxy-2-deoxy guanosine (8-OHdG) and malondialdehyde (MDA) were measured in pregnant women (n = 360) from a coastal urbanized region and moderate socioeconomic status. Neonatal TSH in heel blood was also measured in newborns (n = 309). Dibutyl phosphate (DBP) and diphenyl phosphate (DPHP) were extensively detected with a median creatinine-adjusted level of 0.19 μg/g and 0.66 μg/g, respectively, and the median of ∑mOPEs was 1.82 μg/g. DBP and DPHP were included in the analysis. The concentrations of DBP and DPHP were positively associated with either maternal or neonatal TSH levels, while not for maternal FT3 and FT4 levels. Positive associations for maternal and neonatal TSH were particularly observed in girls as stratified by newborn sex suggesting a sex-selective difference. Furthermore, 8-OHdG, the biomarker of DNA damage, was found to be a major mediator (>60%) for the association between neonatal TSH and DPHP, suggesting that DNA damage is involved in fetal thyroid function disruption. On the other hand, MDA showed a partially suppressing effect (<40%) for the associations between mOPEs and neonatal TSH, which needs further clarification. For maternal TSH, both 8-OHdG and MDA showed moderate mediating effects while the direct effects of mOPEs on maternal TSH also contributed. These results suggest thyroid disrupting effects of OPE exposure on mothers and fetuses during pregnancy and the potential influence mediated by the oxidative stresses of DNA damage and lipid peroxidation.

Suppressing Effect of Free Triiodothyronine on the Negative Association between Body Mass Index and Serum Osteocalcin Levels in Euthyroid Population

Previous studies found that thyroid hormones stimulate osteoblast-like cells to secrete osteocalcin. We aimed to investigate the association between serum thyroid hormone and serum osteocalcin in euthyroid population. The study recruited 1152 community-based euthyroid subjects (average age 59 ± 8 years), among whom 677 were women. Serum free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH), and osteocalcin were measured by electrochemiluminescence immunoassays. After adjusting for age and gender, partial correlation analysis showed that FT3 and FT3/FT4 were both positively correlated with body mass index (BMI) and serum osteocalcin levels (all P < 0.05) and BMI was negatively correlated with serum osteocalcin levels (P < 0.01), while FT4 and TSH were not correlated with serum osteocalcin levels (all P > 0.05). Age, gender, blood pressure, thyroid hormones, and multiple metabolic risk factors were included in the ridge regression model. FT3 and FT3/FT4 were independently and positively associated with serum osteocalcin levels (all P < 0.05), while BMI was independently and negatively associated with serum osteocalcin levels (P < 0.01). The mediating effect model showed that FT3 and FT3/FT4 suppressed the negative association between BMI and serum osteocalcin levels, with suppressing effects of 6.41% and 10.39%, respectively. In euthyroid subjects, both FT3 and FT3/FT4 were positively associated with serum osteocalcin levels, and they further suppressed the negative association between BMI and serum osteocalcin levels.

Clinical Parameters Are More Likely to Be Associated with Thyroid Hormone Levels than with Thyrotropin Levels: A Systematic Review and Meta-Analysis

Background: Though the functional states of other endocrine systems are not defined on the basis of levels of controlling hormones, the assessment of thyroid function is based on levels of the controlling hormone thyrotropin (TSH). We, therefore, addressed the question as to whether levels of thyroid hormones [free thyroxine (fT4), total triiodothyronine (TT3)/free triiodothyronine (fT3)], or TSH levels, within and beyond the reference ranges, provide the better guide to the range of clinical parameters associated with thyroid status. Methods: A PubMed/MEDLINE search of studies up to October 2019, examining associations of levels of thyroid hormones and TSH, taken simultaneously in the same individuals, with clinical parameters was performed. We analyzed atrial fibrillation, other cardiac parameters, osteoporosis and fracture, cancer, dementia, frailty, mortality, features of the metabolic syndrome, and pregnancy outcomes. Studies were assessed for quality by using a modified Newcastle-Ottawa score. Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines were followed. A meta-analysis of the associations was performed to determine the relative likelihood of fT4, TT3/fT3, and TSH levels that are associated with the clinical parameters. Results: We identified 58 suitable articles and a total of 1880 associations. In general, clinical parameters were associated with thyroid hormone levels significantly more often than with TSH levels-the converse was not true for any of the clinical parameters. In the 1880 considered associations, fT4 levels were significantly associated with clinical parameters in 50% of analyses. The respective frequencies for TT3/fT3 and TSH levels were 53% and 23% (p < 0.0001 for both fT4 and TT3/fT3 vs. TSH). The fT4 and TT3/fT3 levels were comparably associated with clinical parameters (p = 0.71). More sophisticated statistical analyses, however, indicated that the associations with TT3/fT3 were not as robust as the associations with fT4. Conclusions: Thyroid hormones levels, and in particular fT4 levels, seem to have stronger associations with clinical parameters than do TSH levels. Associations of clinical parameters with TSH levels can be explained by the strong negative population correlation between thyroid hormones and TSH. Clinical and research components of thyroidology currently based on the measurement of the thyroid state by reference to TSH levels warrant reconsideration.

Abdominal Obesity Phenotypes and Incidence of Thyroid Autoimmunity: A 9-Year Follow-up

PURPOSE

The association between obesity and autoimmune diseases has been suggested by several previous studies. The objective of our study was to assess the association of abdominal obesity phenotypes with thyroid autoimmunity.

MATERIALS AND METHODS

This study was conducted within the framework of a population-based cohort study, Tehran Thyroid Study (TTS) on 4708 subjects without thyroid autoimmunity at baseline. Participants were categorized into four abdominal obesity phenotypes according to waist circumference (WC) and other metabolic syndrome components. Serum concentrations of thyroid peroxidase antibody (TPOAb), free T4 (FT4), thyrotropin (TSH), glucose, and lipid profiles were measured after 3, 6 and 9 years of follow-up. Cox proportional hazard models were used to evaluate associations of different phenotypes with the incidence of thyroid autoimmunity, adjusted for age, sex, FT4, and TSH.

RESULTS

Highest and lowest incidence rates of TPOAb positivity were observed among metabolically unhealthy, non-abdominally obese (MUNAO) [8.78 (7.31-10.55) per 1000 person-years of follow-up] and metabolically unhealthy abdominally obese (MUAO) [4.98 (3.88-6.41) per 1000 person-years of follow-up] phenotypes. Considering the metabolically healthy non-abdominal obese (MHNAO) individuals as reference, none of metabolically healthy abdominally obese (MHAO), MUNAO, and MUAO phenotypes were associated with increased risk of developing TPOAb positivity. Compared to individuals with high WC, the incidence rate (95%CI) of TPOAb positivity was higher among those with normal WC: 8.44 (7.13-10.0) vs 5.11 (4.01-6.51) per 1000 person-years, respectively. Higher WC was not associated with incident TPOAb positivity.

CONCLUSION

There was no significant association between baseline abdominal obesity phenotype status and development of TPOAb positivity over 9 years of follow-up.

Characteristics of Serum Thyroid Hormones in Different Metabolic Phenotypes of Obesity

Aim: Metabolically healthy obese (MHO) individuals have attracted broad attention. We aimed to investigate the characteristics of serum thyroid hormones in different metabolic phenotypes of obesity. Methods: The study included 1,023 community-based euthyroid subjects (age range: 27-81 years), of whom 586 were women. Fat% was detected by a bioelectrical impedance analyzer. Two definitions of obesity were applied as follows: (1) fat% ≥ 25% for men and ≥ 30% for women; (2) body mass index (BMI) ≥ 25 kg/m². According to the diagnostic criteria for metabolic syndrome by the Chinese Diabetes Society, metabolically unhealthy was defined as two or more components of metabolic syndrome, excluding waist circumference. Serum-free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) levels were measured by electrochemiluminescence immunoassay. Results: The proportions of obesity defined by fat% and BMI were 41.3 and 27.1%, respectively. The proportion of metabolically unhealthy was 41.6%. After adjusting for age and gender, regardless of the definitions based on fat% or BMI, FT3 was positively related to both the MHO and the metabolically unhealthy obese (MUO) phenotypes [MHO: odds ratio (OR)s = 1.676 based on fat% and 2.055 based on BMI; MUO: ORs = 1.818 based on fat% and 1.526 based on BMI; all P < 0.05]; FT4 was negatively related to the MUO phenotype (ORs = 0.870 based on fat% and 0.849 based on BMI, all P < 0.05); FT3/FT4 was also positively related to both the MHO and the MUO phenotypes (MHO: ORs = 1.678 based on fat% and 2.825 based on BMI; MUO: ORs = 2.866 based on fat% and 2.883 based on BMI; all P < 0.05); and TSH was positively related to the metabolically unhealthy non-obese phenotype (ORs = 1.329 based on fat% and 1.321 based on BMI, all P < 0.01). Conclusions: In euthyroid population, both the MHO and the MUO phenotypes were characterized by increased FT3 and FT3/FT4 levels.

Thyroid Function and the Risk of Non-Alcoholic Fatty Liver Disease in Morbid Obesity

BACKGROUND

An association between hypothyroidism and the risk of Non-alcoholic Fatty Liver Disease (NAFLD) has been suggested. This association remains to be elucidated in patients with morbid obesity.

AIM

To evaluate the association between thyroid function and parameters of liver function and hepatic scores in patients with morbid obesity.

METHODS

Patients with morbid obesity followed in our center between January 2010 and July 2018 were included. The ones without evaluation of liver and thyroid functions were excluded. Fatty Liver Index (FLI) and BARD scores were used as predictors of hepatic steatosis and fibrosis, respectively.

RESULTS

We observed a positive association between TSH and both BARD (OR 1.14; p = 0.035) and FLI (OR 1.19; p = 0.010) in the unadjusted analysis. We found a negative association between free triiodothyronine levels and BARD (OR 0.70; p<0.01) and a positive association between free triiodothyronine levels and FLI (OR 1.48; p = 0.022). Concerning liver function, we found a positive association between total bilirubin and free thyroxine levels (β = 0.18 [0.02 to 0.35]; p = 0.033) and a negative association between total bilirubin and free triiodothyronine levels (β = -0.07 [-0.14 to -0.002]; p = 0.042).

CONCLUSION

Higher levels of TSH and free triiodothyronine may be associated with a higher risk of NAFLD, particularly steatosis, in patients with morbid obesity.

Organophosphate pesticides exposure in pregnant women and maternal and cord blood thyroid hormone concentrations

BACKGROUND

Animal studies suggest that organophosphate (OP) pesticides exposure affects thyroid function, but evidence in humans remains sparse and inconclusive. Gestational exposure is of particular interest, since thyroid hormone is essential for fetal brain development. OP pesticides are able to cross the placental and blood-brain barrier and may interfere with fetal development processes regulated by thyroid hormone.

OBJECTIVE

To investigate the association of gestational OP pesticides exposure during pregnancy with maternal and cord blood thyroid hormone concentrations.

METHODS

This study was embedded within Generation R (Rotterdam, the Netherlands), a prospective population-based birth cohort. Mother-child pairs with OP pesticides assessment and maternal (N = 715) or cord blood (N = 482) thyroid hormone measurements were included. OP pesticides exposure was assessed at <18, 18-25, and >25 weeks gestation by measuring six urinary dialkylphosphate (DAP) metabolites. Thyroid stimulating hormone (TSH) and free thyroxine (FT4) were measured in maternal and cord blood. Maternal measures also included total thyroxine (TT4) and TPO antibodies (TPOAbs). To study the association of creatinine-adjusted DAP metabolite concentrations with thyroid function and TPO antibodies, multivariable linear regression models including relevant confounders were used.

RESULTS

There was no association of DAP metabolites with maternal TSH, FT4, TT4 or TPOAb concentrations during pregnancy. Similarly, there was no association of DAP metabolites with cord blood TSH or FT4. Results did not change when DAP concentrations were analyzed at individual time points or as mean gestational exposure.

CONCLUSION

Gestational OP pesticides exposure, as assessed by repeatedly measured urinary DAP metabolite concentrations in an urban population, was not associated with maternal or cord blood thyroid hormone concentrations. These findings do not support a mediating role for serum thyroid hormone availability in the relation of early life exposure to low levels of OP pesticides with child neurodevelopment. However, disruption of the thyroid system at tissue level cannot be excluded. In addition, this is one of the first studies on this subject and measurement error in DAP metabolites might have resulted in imprecise estimates. Future studies should use more urine samples to increase precision and should investigate specific OP pesticide metabolites.

Trunk fat and leg fat in relation to free triiodothyronine in euthyroid postmenopausal women

BACKGROUND

A high level of free triiodothyronine (FT3) within the reference range may be a potential metabolic risk marker. However, the relationship between different fat depots and FT3 has remained unclear.

OBJECTIVE

We aimed to explore the relationships between segmental fat distribution and FT3 in euthyroid middle-aged and elderly men and postmenopausal women.

METHODS

A total of 891 subjects (394 men and 497 women) were enrolled. A bioelectrical impedance analyzer was used to measure total, trunk, arm and leg fat mass (FM) and fat percentage (fat%). The leg fat mass to trunk fat mass ratio (LTR) was calculated to evaluate the relative distribution of leg fat compared with that of trunk fat. Thyroid hormones were measured by electrochemical luminescence immunoassay.

RESULTS

FT3 in men did not change significantly with increases in LTR quartiles, while FT3 in women decreased significantly (P for trend = 0.004). In multivariate linear regression analysis, multiple metabolic and cardiovascular risk factors were adjusted. The LTR was negatively related to FT3 in women (P < 0.05). After further mutual adjustment for trunk fat and leg fat parameters, trunk FM and fat% were positively related to FT3, while leg FM and fat% were negatively related to FT3 in women (all P < 0.05).

CONCLUSIONS

In euthyroid postmenopausal women, trunk fat was positively correlated with FT3, whereas leg fat was negatively correlated with FT3. Our findings supported that a high level of FT3 within the reference range was related to adverse fat distribution.

Combination Thyroid Hormone Replacement; Knowns and Unknowns

Hypothyroidism is common throughout the world and readily diagnosed with thyroid function tests. Management should be straightforward but appears not to be the case. Thyroid hormone replacement with levothyroxine monotherapy is the standard treatment which is effective in the majority of cases. However, 10-15% of patients established on levothyroxine do not feel their health is entirely restored and some patients prefer the addition of liothyronine. Proponents of liothyronine argue that the ratio of T3 and T4 hormones is substantially altered on T4 monotherapy and therefore both hormones may be needed for optimal health. This remains controversial as clinical trials have not demonstrated superiority of combination therapy (levothyroxine and liothyronine) over levothyroxine monotherapy. There is now a pressing need for further studies and in particular randomized controlled trials in this area. To help design and facilitate dedicated trials and better understand thyroid hormone replacement, this review summarizes the evidence where there is established knowledge and agreement (knowns) and areas where research is lacking (unknowns). Agreements include the extent of dissatisfaction with levothyroxine monotherapy, biases in testing for hypothyroidism and prescribing levothyroxine, as well as variable thresholds for prescribing levothyroxine and challenges in liothyronine dosing. The review will also highlight and summarize the unknowns including the long-term safety profile of liothyronine, and potential biomarkers to identify individuals who might benefit most from combination therapy.

The association between thyroid function and incidence of metabolic syndrome in euthyroid subjects: Tianjin chronic low-grade systemic inflammation and health cohort study

OBJECTIVE

Thyroid hormones (THs) are primarily responsible for the regulation of energy homeostasis and metabolism. However, few prospective studies have assessed the association between THs and metabolic syndrome (MetS) in a general population. We therefore designed a cohort study to examine whether serum TH levels within the reference range are predictive factors for developing MetS in adults.

DESIGN

Prospective cohort study.

PARTICIPANTS

A prospective study (n = 6119) was performed in Tianjin, China. Participants without a history of MetS were followed up for 1 to 3 years with a median follow-up duration of 2 years.

MEASUREMENTS

Serum free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) levels were measured by chemiluminescence immunoassay. MetS was defined in accordance with the criteria of the American Heart Association scientific statements of 2009. THs, TSH levels and MetS were assessed yearly during the follow-up. Adjusted Cox proportional hazards regression models were used to assess the associations between FT3, FT4 and TSH quintiles and MetS.

RESULTS

The incidence of MetS was 17.7% (96 per 1000 person-years). In the final multivariate models, the hazard ratios (95% confidence interval) for MetS across serum FT3 quintiles were 1.00 (reference), 1.03 (0.84, 1.25), 1.14 (0.94, 1.38), 1.09 (0.90, 1.32) and 1.33 (1.11, 1.61), respectively (P for trend <.01). However, no significant associations between FT4, TSH and MetS were observed.

CONCLUSIONS

This population-based prospective cohort study suggests that increased serum FT3 level, rather than FT4 and TSH, is an independent predictor for developing MetS in euthyroid subjects.

Association Between Thyroid Function and Development of Different Obesity Phenotypes in Euthyroid Adults: A Nine-Year Follow-Up

BACKGROUND

Considering inconsistent and conflicting data on associations of thyroid function, within the reference range, with anthropometric measures and metabolic syndrome, this study aimed to investigate the relationship between thyroid function and different obesity phenotypes over nine years of follow-up.

METHODS

This study was conducted on 1938 individuals from an ongoing population-based cohort study, the Tehran Thyroid Study. Participants were categorized into four obesity phenotypes based on body mass index and metabolic status. To investigate the associations of thyrotropin and free thyroxine (fT4) with incidence of different obesity phenotypes across the study period, a multivariate approach based on a generalized estimating equation method was used.

RESULTS

At baseline, individuals with the metabolically healthy normal weight (MHNW) phenotype had higher serum fT4 levels (1.2 ± 0.16 ng/dL vs. 1.14 ± 0.14 ng/dL, 1.16 ± 0.14 ng/dL, and 1.17 ± 0.15 ng/dL in metabolically healthy obese [MHO], metabolically unhealthy normal weight, and metabolically unhealthy obese individuals, respectively). The results of the generalized estimating equation analysis after multivariate adjustment for age, sex, smoking, physical activity, education level, thyroid peroxidase antibody status, and homeostasis model assessment-insulin resistance showed that each 1 ng/dL increment in fT4 levels within the reference range was accompanied with a 1.65-fold [confidence interval (CI) 1.09-2.5] increase of developing the MHNW phenotype during 9.2 years of follow-up. Moreover, each 1.0 ng/dL increment in fT4 within the reference range was associated with a 50% decreased risk of developing the MHO phenotype (odds ratio = 0.50 [CI 0.32-0.76]). Meanwhile, a significant positive association was found between serum thyrotropin levels and development of the metabolically unhealthy normal weight phenotype (odds ratio = 1.22 [CI 1.01-1.48]).

CONCLUSIONS

Serum fT4 concentrations within the reference range are associated with the development of some obesity phenotypes, including the MHNW and MHO phenotypes, after consideration of potential confounders.

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.

Childhood Thyroid Function Reference Ranges and Determinants: A Literature Overview and a Prospective Cohort Study

BACKGROUND

Reported cutoffs for childhood thyrotropin (TSH) and free thyroxine (fT4) reference ranges vary widely, and knowledge on the determinants of childhood thyroid function is sparse. This study aimed to summarize the existing studies on thyroid function reference ranges in children. Furthermore, the objective was to investigate the determinants of childhood TSH and fT4 concentration in a population based-prospective cohort.

METHODS

First, to identify studies on childhood thyroid reference ranges, The National Library of Medicine's PubMed, Embase, Ovid Medline, Web of Science, and Google Scholar databases were systematically searched. Second, in a non-selected sample of 4273 children (median age 6.0 years, range 4.9-9.1 years) from the cohort, the associations of age, sex, anthropometric characteristics, ethnicity, maternal education, and time and season at venipuncture were studied with TSH and fT4 concentrations. The study also investigated to what extent between-individual variations in the determinants of TSH and fT4 could influence the calculation of reference ranges.

RESULTS

Published reference ranges for TSH and fT4 differ per age range and within age ranges (cutoffs low TSH: 0.13 to >1 mIU/L; high TSH: 2.36 to >10 mIU/L; low fT4: 7.0 to >10 pmol/L; high fT4: 15.5 to >30 pmol/L). In the present cohort, weight, sex, and ethnicity were determinants of TSH (p ≤ 0.03) and fT4 concentrations (p ≤ 0.01), and height and time at venipuncture were determinants of TSH only (p < 0.0001). The between-individual variation depending on clinical determinants for TSH ranged between 0.64 and 0.96 mIU/L (total population 0.87 mIU/L) for the lower limit and 4.30 and 5.62 mIU/L (total population 5.20 mIU/L) for the upper limit, whereas for fT4, the lower limit ranged between 13.6 and 14.2 pmol/L (total population 13.8 pmol/L) and the upper limit ranged between 20.2 and 23.0 pmol/L (total population 20.8 pmol/L).

CONCLUSIONS

Considerable differences exist in the reported reference ranges for childhood TSH and fT4 across and within age ranges and assays. The present cohort shows only a minimal association between TSH and fT4, suggesting that the hypothalamus-pituitary-thyroid axis remains unaffected by thyroid interfering factors. Various determinants of TSH and fT4 in children were identified, which accounted for a considerable variation of reference range cutoffs.

Thyroid status and mortality in nonagenarians from long-lived families and the general population

The relationship between thyroid status and longevity has been investigated extensively. However, data on thyroid status and survival in old age is scarce. In this study we investigated associations of different parameters of thyroid status with mortality in nonagenarians, and whether these associations were different in nonagenarians from long-lived families than in nonagenarians from the general population. In total, 805 nonagenarians from the Leiden Longevity Study and 259 nonagenarians from the Leiden 85-plus Study were followed up to collect mortality data. At baseline, levels of thyrotropin (TSH), free thyroxine (fT4) and free triiodothyronine (fT3) were measured. In nonagenarians from long-lived families and from the general population, associations between thyroid parameters and mortality were similar. We found no interaction between study population and parameters of thyroid status on mortality (P-values>0.70). The results from both studies were combined to derive generalizable associations. Hazard ratios (HRs) for the highest compared to lowest tertiles were determined, resulting in TSH HR 0.91 (P=0.25), fT4 HR 1.22 (P=0.02), fT3 HR 0.74 (P=1.31e-4), and fT3/fT4 HR 0.66 (P=5.64e-7). In conclusion, higher fT3/fT4 ratios, higher levels of fT3, and lower levels of fT4 were associated with lower mortality rate in nonagenarians and independent of familial longevity status.

Childhood thyroid function, body composition and cardiovascular function

OBJECTIVE

The cardiovascular system is a known target for thyroid hormone. Early-life cardiovascular alterations may lead to a higher risk of cardiovascular disease in adulthood. Little is known about the effects of thyroid hormone on cardiovascular function during childhood, including the role of body composition in this association.

DESIGN

Population-based prospective cohort of children (n = 4251, median age 6 years, 95% range: 5.7-8.0 years).

METHODS

Thyroid-stimulating hormone (TSH) and free thyroxine (FT4) concentrations were measured to assess thyroid function. Left ventricular (LV) mass was assessed with echocardiography. Arterial stiffness was assessed with carotid-femoral pulse wave velocity (CFPWV). Systolic and diastolic blood pressure (BP) was measured. Body composition was assessed by dual-energy X-ray absorptiometry scan.

RESULTS

FT4 was inversely associated with LV mass (P = 0.002), and with lean body mass (P < 0.0001). The association of FT4 with LV mass was partially mediated through variability in lean body mass (55% mediated effect). TSH was inversely associated with LV mass (P = 0.010), predominantly in boys. TSH was positively associated with systolic and diastolic BP (both P < 0.001). FT4 was positively associated with CFPWV and diastolic BP (P < 0.0001, P = 0.008, respectively), and the latter association attenuated after adjustment for CFPWV.

CONCLUSIONS

At the age of 6 years, higher FT4 is associated with lower LV mass (partially through effects on lean body mass) and with higher arterial stiffness, which may lead to higher BP. Our data also suggest different mechanisms via which TSH and FT4 are associated with cardiovascular function during early childhood.

Free triiodothyronine/free thyroxine ratio rather than thyrotropin is more associated with metabolic parameters in healthy euthyroid adult subjects

OBJECTIVE

The interrelation between TSH, thyroid hormones and metabolic parameters is complex and has not been confirmed. This study aimed to determine the association of TSH and thyroid hormones in euthyroid subjects and the relationship between thyroid function and metabolic risk factors. Furthermore, this study examined whether thyroid function has predictive power for metabolic syndrome.

DESIGN

This is a cross-sectional study that included subjects in a medical health check-up programme at a single institution.

PATIENTS

The study included 132 346 participants (66 991 men and 65 355 women) aged over 18 years who had TSH, free T4 (FT4) and free T3 (FT3) levels within the institutional reference ranges.

MEASUREMENTS

Thyrotropin, FT4, FT3 and metabolic parameters including height, weight, waist circumference, blood pressure, serum levels of total cholesterol, triglyceride, high-density lipoprotein cholesterol, insulin and glucose were measured.

RESULTS

There was a positive association between FT3/FT4 ratio and TSH in both men and women after adjusting for age, body mass index, smoking status and menopausal status (in women). The FT3/FT4 ratio and TSH were positively associated with risk of metabolic syndrome parameters including insulin resistance. The FT3/FT4 ratio had a greater predictive power than TSH for metabolic syndrome in both men and women.

CONCLUSIONS

Thyrotropin levels were positively associated with FT3/FT4 ratio within the euthyroid range. The higher FT3/FT4 ratio is associated with increased risk of metabolic syndrome parameters and insulin resistance. FT3/FT4 ratio has a better predictive power for metabolic syndrome than TSH.

Maturation in Serum Thyroid Function Parameters Over Childhood and Puberty: Results of a Longitudinal Study

CONTEXT

Serum thyroid hormone levels differ between children and adults, but have not been studied longitudinally through childhood.

OBJECTIVE

To assess changes in thyroid-stimulating hormone (TSH) and thyroid hormone levels over childhood and their interrelationships.

DESIGN

Cohort study.

SETTING

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

PARTICIPANTS

A total of 4442 children who had thyroid function measured at age 7, and 1263 children who had thyroid function measured at age 15. Eight hundred eighty-four children had measurements at both ages.

MAIN OUTCOME MEASURES

Reference ranges for TSH, free tri-iodothyronine (FT3), free thyroxine (FT4), their longitudinal stability, and interrelationships.

RESULTS

Children at age 7 years had a higher FT3 [6.17 pmol/L, standard deviation (SD) 0.62] than children at age 15 (5.83 pmol/L, SD 0.74); P < 0.0001 with 23.2% of children at age 7 having FT3 above the adult reference range. Higher FT3 levels at age 7 in boys (P = 0.0001) and girls (P = 0.04) were associated with attainment of a more advanced pubertal stage at age 13. TSH was positively associated with FT3 at age 7 and age 15 even after adjusting for confounders. In contrast, TSH was negatively associated with FT4.

CONCLUSIONS

There are substantial changes in TSH and thyroid hormone levels over childhood, in particular for FT3, which appear to relate to pubertal readiness. Our data provide increased insight into the evolution of the pituitary-thyroid axis over childhood and may have implications for determining optimal ranges for thyroid hormone replacement in children.

Causal relationship between obesity and serum testosterone status in men: A bi-directional mendelian randomization analysis

CONTEXT

Obesity in men is associated with low serum testosterone and both are associated with several diseases and increased mortality.

OBJECTIVES

Examine the direction and causality of the relationship between body mass index (BMI) and serum testosterone.

DESIGN

Bi-directional Mendelian randomization (MR) analysis on prospective cohorts.

SETTING

Five cohorts from Denmark, Germany and Sweden (Inter99, SHIP, SHIP Trend, GOOD and MrOS Sweden).

PARTICIPANTS

7446 Caucasian men, genotyped for 97 BMI-associated SNPs and three testosterone-associated SNPs.

MAIN OUTCOME MEASURES

BMI and serum testosterone adjusted for age, smoking, time of blood sampling and site.

RESULTS

1 SD genetically instrumented increase in BMI was associated with a 0.25 SD decrease in serum testosterone (IV ratio: -0.25, 95% CI: -0.42--0.09, p = 2.8*10-3). For a body weight reduction altering the BMI from 30 to 25 kg/m2, the effect would equal a 13% increase in serum testosterone. No association was seen for genetically instrumented testosterone with BMI, a finding that was confirmed using large-scale data from the GIANT consortium (n = 104349).

CONCLUSIONS

Our results suggest that there is a causal effect of BMI on serum testosterone in men. Population level interventions to reduce BMI are expected to increase serum testosterone in men.

Causal inference in obesity research

Obesity is a risk factor for a plethora of severe morbidities and premature death. Most supporting evidence comes from observational studies that are prone to chance, bias and confounding. Even data on the protective effects of weight loss from randomized controlled trials will be susceptible to confounding and bias if treatment assignment cannot be masked, which is usually the case with lifestyle and surgical interventions. Thus, whilst obesity is widely considered the major modifiable risk factor for many chronic diseases, its causes and consequences are often difficult to determine. Addressing this is important, as the prevention and treatment of any disease requires that interventions focus on causal risk factors. Disease prediction, although not dependent on knowing the causes, is nevertheless enhanced by such knowledge. Here, we provide an overview of some of the barriers to causal inference in obesity research and discuss analytical approaches, such as Mendelian randomization, that can help to overcome these obstacles. In a systematic review of the literature in this field, we found: (i) probable causal relationships between adiposity and bone health/disease, cancers (colorectal, lung and kidney cancers), cardiometabolic traits (blood pressure, fasting insulin, inflammatory markers and lipids), uric acid concentrations, coronary heart disease and venous thrombosis (in the presence of pulmonary embolism), (ii) possible causal relationships between adiposity and gray matter volume, depression and common mental disorders, oesophageal cancer, macroalbuminuria, end-stage renal disease, diabetic kidney disease, nuclear cataract and gall stone disease, and (iii) no evidence for causal relationships between adiposity and Alzheimer's disease, pancreatic cancer, venous thrombosis (in the absence of pulmonary embolism), liver function and periodontitis.

Higher free triiodothyronine is associated with non-alcoholic fatty liver disease in euthyroid subjects: the Lifelines Cohort Study

OBJECTIVE

Overt hypothyroidism confers an increased risk of non-alcoholic fatty liver disease (NAFLD). The liver plays a crucial role in the metabolism of cholesterol and triglycerides; thyroid hormones interact on hepatic lipid homeostasis. Thyroid function within the euthyroid range affects a number of health issues, including atherosclerosis development and biochemical markers of increased cardiovascular risk. However, the association of thyroid hormones with NAFLD in euthyroid subjects has not been unequivocally established. We therefore determined associations of thyroid hormone parameters with NAFLD among euthyroid subjects.

METHODS

The study was conducted in the Lifelines Cohort Study, a population-based cohort study of participants living in the North of the Netherlands. Only euthyroid subjects (thyroid-stimulating hormone (TSH) 0.5-4.0mU/L, free thyroxine (FT4) 11-19.5pmol/L and free triiodothyronine (FT3) 4.4-6.7pmol/L) older than 18years were included. Exclusion criteria were participants with excessive alcohol use, known hepatitis or cirrhosis, liver functions ≥ three times the upper limit, current cancer, non-white ancestry, previous or current use of thyroid medication and current use of lipid or glucose lowering medication. A priori defined liver biochemistry, thyroid function parameters and metabolic syndrome (MetS) were studied. NAFLD was defined by using the validated Fatty Liver Index (FLI); FLI≥60 was categorized as NAFLD. A P<0.01 was considered significant.

RESULTS

FLI≥60 was found in 4274 (21.1%) of 20,289 individuals (62.1% male, median age 46years) with increased prevalence of MetS (P<0.0001). In age- and sex-adjusted analysis FLI≥60 was independently associated with a higher FT3 (OR 1.34, 95% CI 1.29-1.39, per SD increment, P<0.0001) and a lower FT4 (OR 0.73, 95% CI 0.70-0.75, P<0.0001) but not by TSH. The strongest association was found for the FT3/FT4 ratio (OR 1.44, 95% CI 1.39-1.49, P<0.0001). These associations remained similar after additional adjustment for the presence of MetS. In subjects with enlarged waist circumference, TSH and FT4 were lower while FT3 was higher, resulting in an increased FT3/FT4 ratio (P<0.0001).

CONCLUSIONS

Euthyroid subjects with suspected NAFLD are characterized by higher FT3, lower FT4 and higher FT3/FT4 ratio, probably consequent to central obesity.

Thyroid hormones and changes in body weight and metabolic parameters in response to weight loss diets: the POUNDS LOST trial

BACKGROUND

The role of thyroid hormones in diet-induced weight loss and subsequent weight regain is largely unknown.

OBJECTIVES

To examine the associations between thyroid hormones and changes in body weight and resting metabolic rate (RMR) in a diet-induced weight loss setting.

SUBJECTS/METHODS

Data analysis was conducted among 569 overweight and obese participants aged 30-70 years with normal thyroid function participating in the 2-year Prevention of Obesity Using Novel Dietary Strategies (POUNDS) LOST randomized clinical trial. Changes in body weight and RMR were assessed during the 2-year intervention. Thyroid hormones (free triiodothyronine (T3), free thyroxine (T4), total T3, total T4 and thyroid-stimulating hormone (TSH)), anthropometric measurements and biochemical parameters were assessed at baseline, 6 months and 24 months.

RESULTS

Participants lost an average of 6.6 kg of body weight during the first 6 months and subsequently regained an average of 2.7 kg of body weight over the remaining period from 6 to 24 months. Baseline free T3 and total T3 were positively associated, whereas free T4 was inversely associated, with baseline body weight, body mass index and RMR. Total T4 and TSH were not associated with these parameters. Higher baseline free T3 and free T4 levels were significantly associated with a greater weight loss during the first 6 months (P<0.05) after multivariate adjustments including dietary intervention groups and baseline body weight. Comparing extreme tertiles, the multivariate-adjusted weight loss±s.e. was -3.87±0.9 vs -5.39±0.9 kg for free T3 (P_trend=0.02) and -4.09±0.9 vs -5.88±0.9 kg for free T4 (P_trend=0.004). The thyroid hormones did not predict weight regain in 6-24 months. A similar pattern of associations was also observed between baseline thyroid hormones and changes in RMR. In addition, changes in free T3 and total T3 levels were positively associated with changes in body weight, RMR, body fat mass, blood pressure, glucose, insulin, triglycerides and leptin at 6 months and 24 months (all P<0.05).

CONCLUSIONS

In this diet-induced weight loss setting, higher baseline free T3 and free T4 predicted more weight loss, but not weight regain among overweight and obese adults with normal thyroid function. These findings reveal a novel role of thyroid hormones in body weight regulation and may help identify individuals more responsive to weight loss diets.

BMI as a Modifiable Risk Factor for Type 2 Diabetes: Refining and Understanding Causal Estimates Using Mendelian Randomization

This study focused on resolving the relationship between BMI and type 2 diabetes. The availability of multiple variants associated with BMI offers a new chance to resolve the true causal effect of BMI on type 2 diabetes; however, the properties of these associations and their validity as genetic instruments need to be considered alongside established and new methods for undertaking Mendelian randomization (MR). We explore the potential for pleiotropic genetic variants to generate bias, revise existing estimates, and illustrate value in new analysis methods. A two-sample MR approach with 96 genetic variants was used with three different analysis methods, two of which (MR-Egger and the weighted median) have been developed specifically to address problems of invalid instrumental variables. We estimate an odds ratio for type 2 diabetes per unit increase in BMI (kg/m(2)) of between 1.19 and 1.38, with the most stable estimate using all instruments and a weighted median approach (1.26 [95% CI 1.17, 1.34]). TCF7L2(rs7903146) was identified as a complex effect or pleiotropic instrument, and removal of this variant resulted in convergence of causal effect estimates from different causal analysis methods. This indicated the potential for pleiotropy to affect estimates and differences in performance of alternative analytical methods. In a real type 2 diabetes-focused example, this study demonstrates the potential impact of invalid instruments on causal effect estimates and the potential for new approaches to mitigate the bias caused.

Effects of thyrotrophin, thyroid hormones and thyroid antibodies on metabolic parameters in a euthyroid population with obesity

OBJECTIVE

To investigate whether thyroid function in the euthyroid range and thyroid autoimmunity status would affect metabolic measures in individuals with obesity.

DESIGN

Cross-sectional.

PATIENTS

We retrospectively evaluated 5300 consecutive obese (BMI ≥30 kg/m(2) ) subjects attending the Obesity Outpatient Clinic. Subjects with overt or subclinical thyroid disease, diabetes mellitus, chronic disease or using any medication were excluded. After exclusion, 1275 euthyroid [TSH values >0·4 and <4·5 μIU/ml, free triiodothyronine (FT3), free thyroxine (FT4) in the normal reference range] obese subjects (aged 18-65 years) were eligible for the study.

MEASUREMENTS

The physical and biochemical records of the subjects at first admission to the obesity outpatient clinic were examined.

RESULTS

Eighty-three per cent (n = 1063) of the study population were women. Antithyroid peroxidase (anti-TPO) positivity was 14%, and antithyroglobulin (anti-TG) positivity was 15%. TSH was 1·8 μIU/ml (1·3-2·4) in antibody-negative subjects and 2·1 μIU/ml (1·4-2·9) in antibody-positive subjects. Neither TSH nor thyroid antibody positivity was associated with insulin resistance (IR) and atherogenic dyslipidaemia after adjustment for confounders. FT3 was positively associated with IR (P < 0·001) and atherogenic dyslipidaemia (P = 0·03); however, this association lost its significance after adjustment for age, gender and BMI. FT4 was negatively associated with IR and this association remained even after adjustment for confounders (P < 0·001).

CONCLUSION

TSH and thyroid antibody positivity were not related with metabolic measures. Low-normal FT4 had an inverse association with HOMA-IR even after adjustment for confounders.