Triiodothyronine: a link between the insulin resistance syndrome and blood pressure?

Evaluation of Thyroid Function and Its Relation to Glycemic Status in Pregnant Women With Gestational Diabetes Mellitus

Background Gestational diabetes mellitus (GDM) is a prevalent complication during pregnancy that can lead to adverse outcomes for both the mother and the fetus. It also increases the likelihood of developing type 2 diabetes mellitus (T2DM) later in life. Thyroid hormones play an essential role in regulating growth and metabolism and often coexist with diabetes mellitus (DM), affecting glucose metabolism. Pregnant women with GDM frequently exhibit thyroid issues, impacting insulin secretion and beta-cell function. Aim This study aims to assess thyroid function and glycemic status in pregnant women with and without GDM and to evaluate the correlation between thyroid function and glycemic status in pregnant women with GDM. Methods This prospective case-control study was conducted over two months at a tertiary care hospital in Puducherry, India. It included 60 cases (pregnant women with GDM, blood glucose > 140 mg/dL per DIPSI guidelines) and 60 age- and parity-matched controls. Blood samples were collected, centrifuged, and analyzed for blood glucose and serum thyroid levels (FT3, FT4, thyroid-stimulating hormone (TSH)) using the Cobas e-411 autoanalyzer through an electrochemiluminescence assay. Results Serum plasma glucose levels were significantly higher in cases (159.25 ± 16.22 mg/dL) compared to controls (101.6 ± 17.30 mg/dL) (p < 0.05). FT3 levels were higher in cases (3.98 ± 4.18) compared to controls (2.87 ± 0.54) (p = 0.04). The FT3/FT4 ratio was also higher in cases (3.99 ± 4.927) than in controls (2.70 ± 0.58) (p = 0.04). No significant differences were found in FT4 or TSH levels between the groups. Correlation analysis revealed no significant correlations between plasma glucose levels and thyroid function parameters. Conclusion Pregnant women with GDM showed significantly higher plasma glucose levels, FT3 levels, and FT3/FT4 ratio compared to normal pregnant women. These findings suggest an association between altered thyroid function, particularly higher FT3 levels and the FT3/FT4 ratio, and GDM.

Thyroid functions and insulin resistance in pregnant Sudanese women

BACKGROUND

The thyroid function test (free triiodothyronine [FT3], free thyroxine [FT4], and thyroid-stimulating hormone [TSH]) is one of the key determinant of glucose homeostasis by regulating the balance of insulin. Thyroid dysfunction alters glucose metabolism, leading to insulin resistance (IR). This study aimed to assess the association between thyroid function and IR in pregnant Sudanese women.

METHOD

A cross-sectional study was conducted in Saad Abuelela Hospital, Khartoum-Sudan, from January to April 2021. Obstetric/sociodemographic characteristics were gathered through questionnaires. Serum TSH, FT3, FT4, fasting plasma glucose (FPG), and fasting insulin levels were measured and evaluated, and IR was estimated using the homeostatic model assessment for insulin resistance (HOMA-IR) equation.

RESULTS

In total, the study included 127 pregnant women with a median age of 27.0 years (interquartile range [IQR] 23.0‒31.2) and a median gestational (IQR) age of 25.0 (IQR 25.0‒27.0) weeks. The medians (IQRs) of the TSH, FT3, and FT4 were 1.600 (1.162‒2.092) IU/ml, 2.020(1.772‒2.240) nmol/l, and 10.70 (9.60‒11.90) pmol/l, respectively. The median (IQR) of the FPG and fasting blood insulin level was [69.0 (62.00‒78.00) mg/dl] and [5.68(2.99‒11.66) IU/ml], respectively. The median (IQR) of the HOMA-IR level was 0.9407 (0.4356‒2.1410). There was a positive correlation between HOMA -IR and FT3 levels (r = 0.375; P < 0.001) and a negative correlation with FT4 levels (r= -0.312; P < 0.001). Also, a significant positive correlation was found between fasting insulin levels and FT3 levels (r = 0.438; P < 0.001) and a negative correlation with FT4 levels (r= -0.305; P < 0.001).

CONCLUSIONS

This study indicated that FT3 has positive correlation with HOMA-IR, while FT4 has negative correlation among healthy pregnant women without a history of thyroid dysfunction. This may indicate screening of euthyroid pregnant women for thyroid dysfunction and IR. Further studies are needed.

Association of Early Pregnancy Free and Total Triiodothyronine With the Subsequent Risk of Gestational Diabetes Mellitus

OBJECTIVE

To estimate the association of free triiodothyronine (FT3) and total triiodothyronine (TT3) in early pregnancy and subsequent gestational diabetes mellitus (GDM) risk and define appropriate TT3 thresholds for GDM screening.

METHODS

This investigation is a hospital-based cohort study of pregnant women submitted to a universal thyroid function test before 24 weeks of gestation. GDM was diagnosed according to a 75-g oral glucose tolerance test. The association of maternal high FT3 and TT3 levels in early pregnancy with the risk of GDM was estimated using logistic regression. The potential nonlinear association was probed by the restricted cubic spline curve method.

RESULTS

A total of 27 184 pregnant women and 3073 GDM cases were included in the analysis. FT3 and TT3 were associated with an increased subsequent risk of GDM in a nonlinear fashion. The adjusted odds ratios were 1.59 (95% confidence interval, 1.50-1.68) and 2.80 (95% confidence interval, 2.46-3.18) for FT3 and TT3 continuous levels, respectively. Associations were strong in euthyroid women, showed heterogeneity in women with mild thyroid dysfunction, and lacked in patients with overt hypothyroidism and hyperthyroidism. The TT3 thresholds of 1.5 and 2.0 ng/mL between 7 and 12 weeks of gestation and 1.6 and 2.1 ng/mL for 13 to 23 weeks of gestation effectively distinguished the subsequent risk of GDM.

CONCLUSION

The increased FT3 and TT3 levels in early pregnancy were associated with a subsequent higher risk of GDM. These findings provide measures for early detection and potential prevention of GDM.

Circulating free triiodothyronine concentration is positively associated with β-cell function in euthyroid patients with obesity and type 2 diabetes

OBJECTIVE

To investigate the relationship between thyroid hormone concentrations and β-cell function in euthyroid patients with obesity and type 2 diabetes.

METHODS

We performed a single-center cross-sectional study of 254 patients with type 2 diabetes mellitus aged ≥40 years. The participants were allocated to an obesity group or non-obesity group on the basis of their body mass index (BMI). Their β-cell function was assessed by measuring C-peptide concentration during a 75-g oral glucose tolerance test (OGTT); and their serum free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone concentrations were measured.

RESULTS

The serum FT3 concentration and the C-peptide concentrations at five time points of the OGTT were significantly higher in the obesity group than in the non-obesity group. FT3 was positively associated with the β-cell function of the obesity group, but not that of the non-obesity group, in multiple linear regression analysis, after adjustment for potential confounding factors. Serum FT3 concentration was also significantly associated with indices of obesity (BMI, waist circumference, body fat percentage, fat mass, fat mass/height² and visceral fat area).

CONCLUSIONS

Obesity-associated high serum FT3 concentrations might affect β-cell function in euthyroid patients with obesity and type 2 diabetes.

Effects of thyroid hormones in women with gestational diabetes

PURPOSE

This study aims to determine the effect of serum free triiodothyronine (fT3), free thyroxine (fT4), thyroid-stimulating hormone (TSH) and fT3/fT4 ratio on the gestational diabetes.

METHODS

This multicenter study was conducted by analyzing retrospectively 495 pregnant women who admitted to the Tepecik training and Research Hospital, and Kocaeli Derince training and Research Hospital between January 2017 and December 2019. Seventy-eight pregnant women diagnosed with Gestational Diabetes Mellitus (GDM) and 82 pregnant women without GDM diagnosis were included in the study. The relationship between thyroid markers (TSH, fT3, fT4 and fT3/fT4) and gestational diabetes was determined using logistic regression analysis.

RESULTS

Advanced age, low fT4 and high TSH levels were higher in GDM group, p < .05. Results of the logistic regression analysis revealed that the gestational week (OR 20.291, 95% CI 3.132-131-449, p = .002], fasting plasma glucose (OR 48,551, 95% CI 9.223-255.582, p < .001), HbA1c (OR 13,671, 95%CI 2.977-62.781, p =.001) and fT3/fT4 (OR 29.934, 95% CI 3.558-251836, p = .002) were the risk factors of GDM.

DISCUSSION

Advanced gestational week, fasting blood glucose, HbA1c and fT3/fT4 ratio are among the risk factors for the development of the gestational diabetes.

Early Prediction of Gestational Diabetes Mellitus in the Chinese Population via Advanced Machine Learning

CONTEXT

Accurate methods for early gestational diabetes mellitus (GDM) (during the first trimester of pregnancy) prediction in Chinese and other populations are lacking.

OBJECTIVES

This work aimed to establish effective models to predict early GDM.

METHODS

Pregnancy data for 73 variables during the first trimester were extracted from the electronic medical record system. Based on a machine learning (ML)-driven feature selection method, 17 variables were selected for early GDM prediction. To facilitate clinical application, 7 variables were selected from the 17-variable panel. Advanced ML approaches were then employed using the 7-variable data set and the 73-variable data set to build models predicting early GDM for different situations, respectively.

RESULTS

A total of 16 819 and 14 992 cases were included in the training and testing sets, respectively. Using 73 variables, the deep neural network model achieved high discriminative power, with area under the curve (AUC) values of 0.80. The 7-variable logistic regression (LR) model also achieved effective discriminate power (AUC = 0.77). Low body mass index (BMI) (≤ 17) was related to an increased risk of GDM, compared to a BMI in the range of 17 to 18 (minimum risk interval) (11.8% vs 8.7%, P = .09). Total 3,3,5'-triiodothyronine (T3) and total thyroxin (T4) were superior to free T3 and free T4 in predicting GDM. Lipoprotein(a) was demonstrated a promising predictive value (AUC = 0.66).

CONCLUSIONS

We employed ML models that achieved high accuracy in predicting GDM in early pregnancy. A clinically cost-effective 7-variable LR model was simultaneously developed. The relationship of GDM with thyroxine and BMI was investigated in the Chinese population.

Relationship between Free Thyroxine and Islet Beta-cell Function in Euthyroid Subjects

Thyroid hormones have a specific effect on glucose-induced insulin secretion from the pancreas. We aimed to investigate the association between euthyroid hormones and islet beta-cell function in general population and non-treated type 2 diabetes mellitus (T2DM) patients. A total of 5089 euthyroid participants (including 4601 general population and 488 non-treated T2DM patients) were identified from a cross-sectional survey on the prevalence of metabolic diseases and risk factors in East China from February 2014 to June 2016. Anthropometric indices, biochemical parameters, and thyroid hormones were measured. Compared with general population, non-treated T2DM patients exhibited higher total thyroxine (TT4) and free thyroxine (FT4) levels but lower ratio of free triiodothyronine (T3):T4 (P<0.01). HOMA-β had prominently negative correlation with FT4 and positive relationship with free T3:T4 in both groups even after adjusting for age, body mass index (BMI) and smoking. When analyzed by quartiles of FT4 or free T3:T4, there were significantly decreased trend of HOMA-β going with the higher FT4 and lower free T3:T4 in both groups. Linear regression analysis showed that FT4 but not FT3 and free T3:T4 was negatively associated with HOMA-β no matter in general population or T2DM patients, which was independent of age, BMI, smoking, hypertension and lipid profiles. FT4 is independently and negatively associated with islet beta-cell function in euthyroid subjects. Thyroid hormone even in reference range could play an important role in the function of pancreatic islets.

A Longitudinal Study of Thyroid Markers Across Pregnancy and the Risk of Gestational Diabetes

CONTEXT

T3 is the biologically active thyroid hormone involved in glucose metabolism. The free T3 (fT3)/free T4 (fT4) ratio, a marker indicating conversion of fT4 to fT3, is also implicated in glucose homeostasis.

OBJECTIVE

To examine associations of fT3 and the fT3/fT4 ratio with gestational diabetes mellitus (GDM).

DESIGN

In a case-control study, thyroid markers (fT3, fT4, TSH) were measured and the fT3/fT4 ratio was derived across four visits in pregnancy, including first (gestational weeks 10 to 14) and second (weeks 15 to 26) trimester. Conditional logistic regression adjusting for thyroid autoimmunity status and major GDM risk factors estimated trimester-specific associations of thyroid markers with subsequent GDM risk.

SETTING

Twelve US clinical centers.

PARTICIPANTS

One hundred seven GDM cases and 214 non-GDM controls from a multiracial pregnancy cohort of 2802 women.

MAIN OUTCOME MEASURES

GDM diagnosis ascertained from medical records.

RESULTS

Both fT3 and the fT3/fT4 ratio were positively associated with GDM: adjusted OR (95% CI) comparing the highest vs lowest fT3 quartile was 4.25 (1.67, 10.80) at the first trimester and 3.89 (1.50, 10.10) at the second trimester. Similarly, the corresponding risk estimates for the fT3/fT4 ratio were 8.63 (2.87, 26.00) and 13.60 (3.97, 46.30) at the first and second trimester, respectively. Neither TSH nor fT4 was significantly associated with GDM.

CONCLUSIONS

Higher fT3 levels, potentially resulting from de novo synthesis or increased fT4 to fT3 conversion, may be an indicator of GDM risk starting early in pregnancy.

Association Between Thyroid Hormones, Thyroid Antibodies, and Cardiometabolic Factors in Non-Obese Individuals With Normal Thyroid Function

BACKGROUND

Hypothyroidism is an important risk factor for cardiovascular diseases, and autoimmune thyroiditis (AIT) is the leading cause of hypothyroidism. Recent studies showed that even AIT patients with euthyroidism still had an increased number of early atherosclerotic lesions. However, the precise mechanism is not yet known. This study aimed to investigate the association of thyroid function, thyroid autoimmunity, and cardiometabolic risk factors in non-obese AIT patients with euthyroidism.

METHODS

A total of 5,608 non-obese individuals including 1,402 AIT patient and 4,206 sex-, age-, and body mass index (BMI)-matched healthy controls were recruited.

RESULTS

The AIT patients had significantly lower free T3 and free T4 levels, and higher TSH, antithyroid peroxidase antibodies (TPOAb) and TgAb levels. The elevated levels of high sensitivity C reactive protein (hsCRP) and homeostasis model assessment of insulin resistance (HOMA-IR) were observed in the AIT patients than the controls [hsCRP: 0.65 (0.27-1.33) vs. 0.20 (0.03-0.74) mg/L; HOMA-IR: 2.78 ± 1.60 vs. 2.33 ± 1.49; all P < 0.05]. Thyroid function was not associated with metabolic parameters and inflammatory makers, while the TPOAb titer was positively associated with the HOMA-IR and hsCRP levels after adjustment for confounding factors (all P < 0.05). Multivariate regression analysis demonstrated that the TPOAb level was an independent influencing factor for the HOMA-IR and hsCRP levels (HOMA-IR: β = 0.058, P < 0.05; hsCRP: β = 0.108, P < 0.05).

CONCLUSION

The TPOAb level is associated with HOMA-IR and hsCRP levels independently of thyroid function in non-obese individuals. Mild deviation of thyroid function within the normal range, chronic inflammation, and insulin resistance may be the links between AIT and atherosclerosis in the non-obese population.

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.

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.

Association between thyroid hormones, thyroid antibodies and insulin resistance in euthyroid individuals: A population-based cohort

AIM

The association between insulin resistance and thyroid function in euthyroid subjects has not yet been clarified. This study aimed to investigate the association between thyroid function within the normal reference range and insulin resistance in participants of the Tehran Thyroid Study (TTS).

METHODS

This cross-sectional study was conducted within the framework of the TTS. Of 5786 subjects aged ≥ 20 years, 2758 euthyroid subjects free of thyroid disorders, diabetes, chronic kidney disease and cardiovascular disease, and not taking steroids and lipid-lowering agents, were included. Serum concentrations of free thyroxine (FT4) and TSH were measured. The homoeostasis model assessment index for insulin resistance (HOMA-IR) was used to evaluate IR.

RESULTS

On linear regression analysis, a negative association was found between serum FT4 levels and HOMA-IR in the model with age, smoking and physical activity (B = -0.09, P < 0.001) and in the WC-adjusted model with age, smoking and physical activity for men (B = -0.06, P < 0.01). In addition, there was a positive association between serum TSH levels and HOMA-IR in both models [with age, smoking and physical activity (B = 0.07, P = 0.006), and age, smoking, physical activity and adjusted for WC (B = 0.05, P = 0.01)] that was not more significant on logistic regression analysis. In women, neither serum FT4 nor TSH levels were associated with HOMA-IR; the prevalence of IR decreased from 27.2 to 19.1 with increasing tertiles of FT4 only in men (P = 0.01). No significant differences were observed in HOMA-IR and its components between thyroid peroxidase antibody (TPOAb)-negative and -positive groups. Also, it was found that metabolically healthy but obese (MHO) subjects had higher levels of TSH than individuals who were MONW (metabolically obese but normal weight; P < 0.01).

CONCLUSION

Low FT4 was independently associated with IR in healthy euthyroid Iranian men.

Positive association of free triiodothyronine with pancreatic β-cell function in people with prediabetes

AIM

To analyse the effects of thyroid hormones on β-cell function and glucose metabolism in people with prediabetes who are euthyroid.

METHODS

A total of 111 people who were euthyroid underwent 75-g oral glucose tolerance tests, of whom 52 were assigned to the normal glucose tolerance and 59 to the prediabetes groups. Homeostatic model assessment of β-cell function, insulinogenic index and areas under the curve for insulin and glucose were evaluated as indices of pancreatic β-cell function.

RESULTS

In both groups, BMI, fasting insulin, homeostasis model assessment ratio and HDL cholesterol correlated significantly with all indices of pancreatic β-cell function. Free triiodothyronine correlated positively with all insulin secretion indices in the prediabetes group. Multiple linear regression analysis showed that free triiodothyronine was an independent variable that had a positive correlation with all indices of β-cell function in the prediabetes group. By contrast, no such correlation was found in the normal glucose tolerance group.

CONCLUSIONS

Free triiodothyronine is associated with both basal and glucose-stimulated insulin secretion in people with prediabetes who are euthyroid; therefore, the regulation of insulin secretion by thyroid hormones is a potentially novel therapeutic target for the treatment of diabetes.

Body composition and metabolic parameters are associated with variation in thyroid hormone levels among euthyroid young men

OBJECTIVE

Thyroid disorders affect metabolism and body composition. Existing literature has been conflicting on whether this is also the case for thyroid hormone levels within the euthyroid range. Therefore, we have investigated the relationship between thyroid hormone concentrations and body composition together with metabolic parameters in a population of healthy euthyroid men.

METHODS

Healthy male siblings (n=941, 25-45 years, median BMI 24.6) were recruited in a cross-sectional, population-based study; a history or treatment of thyroid disease and thyroid autoimmunity were exclusion criteria. Body composition and muscle cross-sectional area were assessed by dual-energy X-ray absorptiometry and peripheral quantitative computed tomography. Total (triiodothyronine (T(3); TT(3)) thyroxine and (T(4); TT(4))) and free thyroid hormones (FT(3) and FT(4)), TSH, and reverse T(3) (rT(3)) and thyroid-binding globulin (TBG) were determined using immunoassays.

RESULTS

BMI was positively associated with (F)T(3) (P<0.0001). Whole body fat mass displayed positive associations with TT(3) and with (F)T(4) and TBG (P≤0.0006). Positive associations were further observed between leptin and (F)T(3), TT(4), and TBG (P≤0.0002). Inverse associations between lean mass and muscle cross-sectional area and (F)T(3), (F)T(4), and TBG were observed (P≤0.0003). Higher levels of (F)T(3) and TBG were associated with lower insulin sensitivity, assessed by homeostatic model assessment of insulin resistance (IR; P≤0.0001). No associations between TSH and body composition or metabolic parameters were seen.

CONCLUSION

We show that a less favorable body composition (with higher fat and lower muscle mass and accompanying higher leptin concentrations) and IR are associated with higher thyroid hormone levels in healthy young men with well characterized euthyroidism.

Hyperinsulinemia and insulin resistance is associated with low T₃/T₄ ratio in pre diabetic euthyroid Pakistani subjects

OBJECTIVE

To investigate the relationship of thyroid hormones in glucose homeostasis in impaired glucose-tolerant subjects with normal thyroid functions.

METHODS

Cross-sectional analysis was carried out in (n=260) impaired glucose-tolerant (IGT) and normal glucose-tolerant (NGT) subjects. Thyrotropin (TSH), total triiodothyronine (TT₃), total thyroxin (TT₄) free T₃ (fT₃), free T₄ (fT₄), and insulin were assessed by enzyme-linked immunoassays (ELISA). Fasting plasma glucose (FPG) and HbA1c were measured by glucose oxidase and low-pressure cation exchange chromatography. Homeostasis model of assessment (HOMA-IR) was employed to assess the level of insulin resistance; fT₃/fT₄ ratio was calculated. Anthropometric measurement and habits were recorded.

RESULTS

Marked hyperinsulinemia and insulin resistance were observed in IGT subjects. Serum TT₃ and fT₃ levels were significantly low in the IGT as compared to normal glucose-tolerant (NGT) controls. TT₄ and TSH were higher in IGT subjects as compared to control subjects. There was a significant positive correlation of TSH with BMI only in the control group (r=0.351; P<0.05). Correlation of insulin with TT₃, fT₃,and TSH was significant (P<0.05) in IGT subjects. A significant low fT₃/fT₄ ratio was observed in IGT subjects as compared to NGT subjects (P<0.01). In multiple regression analysis, TSH, TT₄ and fT₃ contributed significantly to the variance of fasting insulin and insulin resistance in IGT subjects.

CONCLUSION

Hyperinsulinemia and insulin resistance are associated with low T₃/T₄ ratio in pre-diabetic euthyroid Pakistani subjects.

Low serum free triiodothyronine levels mark familial longevity: the Leiden Longevity Study

BACKGROUND

The hypothalamo-pituitary-thyroid axis has been widely implicated in modulating the aging process. Life extension effects associated with low thyroid hormone levels have been reported in multiple animal models. In human populations, an association was observed between low thyroid function and longevity at old age, but the beneficial effects of low thyroid hormone metabolism at middle age remain elusive.

METHODS

We have compared serum thyroid hormone function parameters in a group of middle-aged offspring of long-living nonagenarian siblings and a control group of their partners, all participants of the Leiden Longevity Study.

RESULTS

When compared with their partners, the group of offspring of nonagenarian siblings showed a trend toward higher serum thyrotropin levels (1.65 vs157 mU/L, p = .11) in conjunction with lower free thyroxine levels (15.0 vs 15.2 pmol/L, p = .045) and lower free triiodothyronine levels (4.08 vs 4.14 pmol/L, p = .024).

CONCLUSIONS

Compared with their partners, the group of offspring of nonagenarian siblings show a lower thyroidal sensitivity to thyrotropin. These findings suggest that the favorable role of low thyroid hormone metabolism on health and longevity in model organism is applicable to humans as well.

Free triiodothyronine plasma concentrations are positively associated with insulin secretion in euthyroid individuals

BACKGROUND

Thyroid hormones (TH) may influence glucose metabolism. Hyperthyroid subjects have higher insulin secretion rates when compared with euthyroid individuals.

OBJECTIVE

To evaluate the association between TH concentrations and insulin secretion in euthyroid, healthy Pima Indian adults (n=55, 29+/-7 years, females/males 36/19) with normal glucose tolerance (NGT) admitted to a Clinical Research Unit.

METHODS

TSH, free thyroxine (FT4), 3,5,3'-L-tri-iodothyronine (FT3), and fasting plasma insulin (FPI) concentrations were measured in fasting plasma samples, percentage of body fat (%BF) by dual energy x-ray absorptiometry (DXA), acute insulin response (AIR), and incremental area under the curve (AUC) of insulin in response to a 25 g intravenous glucose tolerance test (IVGTT) and 75 g oral glucose tolerance test (OGTT) respectively and insulin action (M) during an euglycemic clamp.

RESULTS

FT3 concentrations were associated with FPI, AIR, and insulin AUC both before (r=0.33, P=0.01; r=0.29, P=0.03; and r=0.35, P=0.008 respectively) and after adjustment for age, sex, %BF, glucose (fasting concentrations or glucose AUC), and M (beta=0.09, P=0.01; beta=0.16, P=0.03; and beta=0.24, P=0.0007 respectively). No associations were found for TSH or FT4.

CONCLUSION

FT3 was associated with several measurements of insulin secretion in euthyroid individuals with NGT. T3 concentrations may play a role in the regulation of insulin secretion.

Thyroid function is associated with components of the metabolic syndrome in euthyroid subjects

CONTEXT

Thyroid disease and the metabolic syndrome are both associated with cardiovascular disease.

OBJECTIVE

The aim of this study was to explore the hypothesis that thyroid function, in euthyroid subjects, is associated with components of the metabolic syndrome, including serum lipid concentrations and insulin resistance.

METHODS

A total of 2703 adult inhabitants of a middle-sized city in The Netherlands participated in this cross-sectional study. Subjects who were not euthyroid were excluded, as were subjects taking thyroid medication, medication for diabetes, and subjects for whom medication data were not available (n = 1122). Homeostasis model assessment for insulin resistance (HOMA-IR) (mU*mmol/liter2) was calculated as fasting insulin (mU/liter) times fasting glucose (mmol/liter) divided by 22.5. The metabolic syndrome was defined according to National Cholesterol Education Program's Adult Treatment Panel III criteria.

RESULTS

After adjustment for age and sex, free T4 (FT4) was significantly associated with total cholesterol [standardized beta (beta) = -0.059; P = 0.014], low-density lipoprotein cholesterol (beta = -0.068; P = 0.004), high-density lipoprotein cholesterol (beta = 0.100; P < 0.001), and triglycerides (beta = -0.102; P < 0.001). Both FT4 and TSH were significantly associated with HOMA-IR (beta = -0.133; P < 0.001 and beta = 0.055; P = 0.024, respectively). Median HOMA-IR increased from 1.42 in the highest tertile of FT4 to 1.66 in the lowest tertile of FT4. FT4 was significantly related to four of five components of the metabolic syndrome (abdominal obesity, triglycerides, high-density lipoprotein cholesterol, and blood pressure), independent of insulin resistance.

CONCLUSIONS

We have demonstrated an association between FT4 levels within the normal reference range and lipids, in accordance with the earlier observed association between (sub)clinical hypothyroidism and hyperlipidemia. Moreover, low normal FT4 levels were significantly associated with increased insulin resistance. These findings are consistent with an increased cardiovascular risk in subjects with low normal thyroid function.

Association between ghrelin and cardiovascular indexes in healthy obese and lean men

OBJECTIVE

Obesity is an increasingly common condition and is associated with excess morbidity and mortality, including clinical and subclinical cardiac dysfunction. The role of hormones involved in energy homeostasis, including ghrelin and leptin, in cardiovascular function remains incompletely understood. Therefore, we sought to evaluate the association between systemic ghrelin and leptin levels with indexes of cardiovascular structure and function.

DESIGN AND MEASUREMENTS

We measured serum ghrelin and leptin levels in 25 clinically healthy obese men and 25 lean controls, using commercially available immunoassays. We also assessed right and left ventricular structure and function using cardiac magnetic resonance imaging. We then investigated the association between serum ghrelin and leptin levels with cardiac indexes, using univariate and multivariate analysis.

RESULTS

There was an independent association between serum ghrelin levels and height-adjusted right ventricular mass (r = -0.324, P = 0.026), right ventricular end-diastolic volume (r = -0.363, P = 0.017) and right ventricular end-systolic volume (r = -0.398, P = 0.009) as well as right ventricular ejection fraction (r = 0.317, P = 0.050). There was no significant association between serum ghrelin and indexes of left ventricular structure or function. We also identified an association between serum leptin levels and resting heart rate (r = 0.391, P = 0.002). There was an association between serum leptin and height-adjusted left ventricular mass on univariate, but not on multivariate, analysis.

CONCLUSIONS

Serum ghrelin is associated with right ventricular cardiovascular indexes and serum leptin is associated with resting heart rate. These associations indicate a close interaction between the endocrine and cardiovascular systems in obesity.