Associations between thyroid hormone levels and regional fat accumulation in euthyroid men

Weighing in on the role of brown adipose tissue for treatment of obesity

Brown adipose tissue (BAT) activation is an emerging target for obesity treatments due to its thermogenic properties stemming from its ability to shuttle energy through uncoupling protein 1 (Ucp1). Recent rodent studies show how BAT and white adipose tissue (WAT) activity can be modulated to increase the expression of thermogenic proteins. Consequently, these alterations enable organisms to endure cold-temperatures and elevate energy expenditure, thereby promoting weight loss. In humans, BAT is less abundant in obese subjects and impacts of thermogenesis are less pronounced, bringing into question whether energy expending properties of BAT seen in rodents can be translated to human models. Our review will discuss pharmacological, hormonal, bioactive, sex-specific and environmental activators and inhibitors of BAT to determine the potential for BAT to act as a therapeutic strategy. We aim to address the feasibility of utilizing BAT modulators for weight reduction in obese individuals, as recent studies suggest that BAT's contributions to energy expenditure along with Ucp1-dependent and -independent pathways may or may not rectify energy imbalance characteristic of obesity.

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

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

Waist Circumference and Subclinical Thyroid Dysfunction in a Large Cohort of Chinese Men and Women

OBJECTIVE

The association between subclinical thyroid dysfunction and waist circumference (WC) is still controversial, especially from the perspective of sex differences. We aimed to explore the impact of sex on this relationship in a large Chinese cohort.

METHODS

This cross-sectional study recruited 13,505 healthy participants (8,346 males, 5,159 females) who were enrolled in a health check program. Clinical data were collected. The association between subclinical thyroid dysfunction and WC of both sexes was analyzed separately after dividing WC into quartiles. Odds ratios (ORs) were calculated by binary logistic regression models, and linear regression analysis was also performed.

RESULTS

The prevalence rates of subclinical hyper-and hypothyroidism were significantly lower in males. Binary logistic regression models showed that WC in females with subclinical hypothyroidism had a detrimental effect with an OR of 1.011, but the effect disappeared when we included other covariates. The other ORs indicated no significant effects. The weak negative relationship between WC and thyrotropin was also indicated by linear regression analyses with very low R² values.

CONCLUSION

The current research did not show WC as a risk factor for subclinical thyroid dysfunction in either sex. Regional and ancestral origin differences may account for the variations with other studies.

ABBREVIATIONS

ALT = alanine aminotransferase; BMI = body mass index; FT3 = free triiodothyronine; FT4 = free thyroxine; TG = triglycerides; TSH = thyroid-stimulating hormone; UA = uric acid; WC = waist circumference.

FAT DISTRIBUTION AND METABOLIC PROFILE IN SUBJECTS WITH HASHIMOTO'S THYROIDITIS

CONTEXT

Previous studies have associated overt/subclinical hypothyroidism and obesity but have failed to confirm a causative relationship between them. Confusion is even more for subjects with Hashimoto's Thyroiditis (HT).

OBJECTIVE

In this study, we aimed to evaluate the fat distribution and metabolic profile of subjects with euthyroid HT as well as to establish an appropriate cut-off level of TSH for the development of metabolic syndrome (Mets) in both groups.

PATIENTS AND METHODS

All subjects were euthyroid whether under levothyroxine replacement or not. We recruited 301 volunteers (99 with HT and 202 without thyroid autoimmunity). Together with some metabolic variables, we measured the waist circumference, hip circumference, neck circumference manually; the total body fat with a body composition analyzer; and the visceral fat/trunk fat percentage via abdominal bioelectrical impedance analysis.

RESULTS

A significant positive correlation was established between TSH levels and insulin, fasting plasma glucose, HOMA-IR and body mass index (r=0.28; p<0.001; r=0.27; p<0.05: r=0.32; p<0.001: r=0.13; p<0.05 respectively). The prevalence of Metabolic Syndrome (Mets) was comparable in HT and control groups (27.3% vs. 30.7%; p>0.05). The prevalence of Mets was similar when HT subjects using levothyroxine or HT subjects with accompanying thyroid nodules were taken into consideration. Similarly, anthropometric and metabolic parameters were similar in both the HT group and the control group.We were unable to establish the TSH cut-off level by ROC analysis with desired sensitivity and specificity (AUC: 0.563 with 95% C.I. p=0.35; standard error 0.76).

CONCLUSIONS

Although weight gain is frequently encountered in subjects with HT, such subjects with thyroid function tests in the euthyroid range have a similar prevalence of Mets and similar metabolic and anthropometric measurements compared to subjects without autoimmunity.

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

BACKGROUND

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

OBJECTIVE

We determined an association between serum TSH levels and VAT.

METHODS

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

RESULTS

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

CONCLUSIONS

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

Changes in the before and after thyroxine treatment levels of adipose tissue, leptin, and resistin in subclinical hypothyroid patients

BACKGROUND

Subclinical hypothyroidism (SH) occurs when serum thyroid stimulating hormone (TSH) concentrations are raised and serum thyroid hormone concentrations are normal. The effect of SH on the proinflammatory adipose cytokine releasing visceral adipose tissue (VAT) is not clear. The aim of this study is to identify the difference between the pre and posttreatment levels of VAT, leptin, and resistin in SH patients.

METHODS

There were 51 SH patients and 43 age- and gender-matched healthy subjects included in the study. Thyroid functions, biochemical tests, leptin, resistin, and visceral and subcutaneous fat measurements were made. The measurements were repeated in the SH group in the third month following L-thyroxin treatment.

RESULTS

Initially, high sensitivity C-reactive protein, carotid artery intima-media thickness (mm), leptin, and resistin levels were significantly higher in the SH group compared to the controls, while the other parameters were similar. While no correlation was observed between TSH levels and adipokines, a positive correlation was detected between waist circumference and leptin levels (r = 0.549, p < 0.01). Visceral adipose tissue was positively correlated to age, waist circumference, and leptin levels, but negatively correlated to free thyroxin (T4) levels (r = 0.419, p = 0.009; r = 0.794, p < 0.01; r = 0.515, p < 0.01 and r = - 0.416, p = 0.009, respectively). A significant decrease was observed in VAT volume, leptin, and resistin levels of SH patients following levothyroxine treatment. Conclusion The reduced VAT volume, leptin, and resistin levels in SH patients following treatment may support the idea that TSH affects adipose tissue functions.

A low serum free triiodothyronine level is associated with epicardial adipose tissue in peritoneal dialysis patients

AIM

Cardiovascular disease is a major cause of mortality in dialysis patients. Epicardial adipose tissue (EAT) has been proposed as a cardiovascular risk marker in this population. Subclinical hypothyroidism and low free triiodothyronine (fT3) levels are associated with EAT in patients without chronic renal failure. The aim of this study was to investigate the relationship between EAT and low free T3 levels in peritoneal dialysis (PD) patients.

METHODS

A total of 125 prevalent PD patients were enrolled in this cross-sectional study. The epicardial fat thickness (EFT) was measured by echocardiography, and the endothelial function was assessed by flow mediated dilatation (FMD). Thyroid function tests were performed by an enzyme immunoassay.

RESULTS

The mean age of the patients was 51 ± 13, and the time on PD was 36 months. The mean EFT was 6.7 ± 2.9 mm. The EFT correlated positively with the patient age, systolic blood pressure (BP), mean BP, high sensitivity C-reactive protein (hs-CRP) level and body mass index (BMI), and negatively with the fT3 level and FMD. The median fT3 value was 2.53, and patients were divided according to their serum fT3 values (within the normal range and below the reference level). Compared with patients in the low fT3 group, the subjects in the normal fT3 group had higher serum albumin levels and FMD, but a lower BMI, plasma fasting glucose level, EFT, TSH level, hs-CRP level, low density lipoprotein (LDL) cholesterol level and mean BP in office measurements, and both the diastolic BP and mean BP by ambulatory blood pressure measurement. A multivariate linear regression analysis showed that the EFT was predicted by the hs-CRP and fT3 levels.

CONCLUSION

Low free T3 levels are associated with the epicardial fat thickness in PD patients. Further studies are needed to evaluate the pathogenesis and to support these findings.