Obesity is associated with increased serum TSH level , independent of thyroid function

Objective: To reinvestigate the relationship between circulating TSH levels and adiposity in a cohort of obese people, who have normal thyroid function. Methods: Retrospective cross-sectional analysis was carried out on 226 euthyroid obese or overweight female patients. Thirty-nine female lean and euthyroid subjects (BMI <25 kg/m2) were included in the study group. TSH, free thyroxine (FT4), free triiodothyronine (FT3), fasting plasma levels of insulin and glucose, homeostasis model assessment (HOMA) for insulin resistance (HOMA-IR) and insulin secretion (HOMA-b cell), body weight, height, body mass index (BMI) and waist circumference were assessed. Results: Serum TSH levels were higher in the obese than in the lean subjects. In the study group (lean and obese subjects), there was a significant positive correlation between serum TSH and body weight (r = 0.231, p <0.001), BMI (r = 0.270, p <0.001), waist circumference (r = 0.219, p = 0.001), fasting insulin (r = 0.201, p = 0.002) and HOMA-IR (r = 0.201, p = 0.002); there was no correlation between serum FT4 and any of the parameters. A multivariate linear regression analysis revealed that only BMI (p = 0.012, 95% CI = 0.01–0.08) contributed significantly to the variance of TSH. Conclusions: This study strongly supports existing, but contradictory evidence that serum TSH levels are positively correlated with the degree of obesity and some of its metabolic consequences in overweight people with normal thyroid function.

The eminent endocrine role of the pituitary hormone thyrotropin is to regulate the function of the thyroid gland.The serum level of thyrotropin (TSH) is a reliable index of the biological activity of thyroid hormones.In turn, as prime regulators of energy balance, the contribution of thyroid hormones to the maintenance of body weight has been the subject of numerous clinical studies.Measurement of serum levels of TSH has been a consistent component of the clinical studies on the relationship between thyroid function and adiposity.The conclusion from some of these studies has been that weight gain increases serum levels of TSH; yet, others showed no relationship between TSH and body weight [1][2][3][4][5][6].
Evidence emerging that thyrotropin induces adipogenesis and adipokine production directly, independent of the mediating influence of thyroid hormones on energy balance, prompted us to reinvestigate the relationship between circulating TSH levels, body weight and the metabolic consequences of adiposity in a cohort of obese people, with normal thyroid function.Our results provide circumstantial evidence that the pituitary gland, via thyrotropin may indeed contribute to the evolution of obesity, independent of any involvement of the thyroid gland.

Subjects
Retrospective cross-sectional analysis was carried out on 350 obese or overweight patients referred to the Obesity Outpatient Clinic of the Department of Endocrinology and Metabolic Diseases at Pamukkale University Hospital in Denizli, Turkey between 2002 and 2003.Of these, 124 pa-tients were excluded for any of the following reasons: male gender, history of thyroid disease, thyroid autoantibody positivity, abnormal thyroid hormone levels, history of radioiodine treatment, being on treatment with thyroid hormone, antithyroid drugs or any drug that might affect evaluation of thyroid status.Together, 226 female patients were consid-ered eligible for this study.The mean age was 43 [12] years, the mean BMI 36 [6] kg/m 2 , and the duration of obesity 17±3 years.Twenty-nine carried the diagnosis of type 2 diabetes mellitus, and of these, 23 patients were on treatment with acarbose, metformine and/or sulphonylureas.Six of the diabetic patients were newly diagnosed.
We needed also subjects who had BMI <25 kg/m 2 to evaluate the effect of BMI on TSH level.We examined 100 volunteers of which 61 subjects were excluded for any of the following reasons: BMI ≥25 kg/m 2 , history of thyroid disease, thyroid autoantibody positivity, abnormal thyroid hormone levels, being on treatment with thyroid hormone, antithyroid drugs or any drug that might affect evaluation of thyroid status and 8 subjects desisted from giving blood for blood testing.Thirty-nine female subjects were considered eligible for this study.The mean age was 40 [9] years, the mean BMI 22 [2] kg/m 2 .
The subjects in the lean group gave written voluntary consent to participate in the study.
The study was reviewed and approved by the Human Subject Research Ethics Committee of the Pamukkale University Medical School.Informed consent was not required for retrospective analysis of records; the research database was not linkable to the subjects.

Study design
Serum levels of TSH, free thyroxine (FT4) and free triiodothyronine (FT3) were measured as indicators of thyroid function.As indicators of insulin action, fasting plasma levels of insulin and glucose were measured and submitted to homeostasis model assessment (HOMA) for insulin resistance (HOMA-IR) and insulin secretion (HOMA-b cell).

Anthropometrical measurements
Body weight (kg) and height (m) were measured while subjects wore light clothing and no shoes.BMI was calculated as weight divided by squared height (kg/m 2 ).Waist circumference was measured at the level of the umbilicus.

Statistical analyses
All statistical analyses were performed using SPSS 9.0 software (SPSS Inc).Data are presented as means±SD.Correlation between measured parameters was assessed using the analytical method of partial correlation, and was adjusted for age by linear regression analysis.Comparison of TSH levels between groups of lean, preobese, class I, class II and class III obese subjects were performed with one-way ANOVA test.Multivariate linear regression with TSH as the outcome was used to show that BMI was correlated with TSH having adjusted for covariates including age, waist circumference, fasting glucose, fasting insulin, HOMA-IR and HOMA-β cell.

Results
Anthropometric and clinical characteristics of the subjects are shown in table 1. Serum TSH levels were higher in the obese than in the lean subjects; no significant difference was found in serum FT4 levels between the two groups.Fasting plasma insulin levels and HOMA-IR were higher in the obese subjects.HOMA-b cell was similar in the lean and obese subject groups.
A multivariate linear regression analysis for the association between TSH and variables are shown in table 2. This analysis revealed that only BMI contributed significantly to the variance of TSH.This association was only slightly attenuated after additional adjustment for metabolic factors, including HOMA-IR, HOMA-β cell, fasting insulin and fasting glucose.

Discussion
Our study in a cohort of overweight people with normal thyroid function strongly supports the previously reported, but still contested positive correlation between serum TSH levels and the degree of obesity.The significant correlation is corroborated by the inclusion of non-obese healthy people.Since all TSH values in all patients were -by definitionwithin normal range, a study of this type can only be done with sufficient precision by using a highly sensitive third generation assay.The "normal range" of serum TSH is derived exclusively from its relationship with thyroid hormones in strictly euthyroid persons.One might assume that the interaction between TSH and adipose tissue occurs on a scale different from that involved in thyroid regulation.
Recent studies in man and other mammalian species have yielded convincing evidence that adipocytes and preadipocytes possess thyrotropin receptors [8][9][10].The signal generated by thyrotropin in adipocytes is mediated by the activation of cAMP-dependent protein kinase [4].Studies in vitro and in vivo demonstrate that the action of thyrotropin via its receptors in fat tissue induces differentiation of preadipocytes into adipocytes, and expansion of adipose tissue (adipogenesis) [8,12].Adipose tissue is a major endocrine gland, producing and releasing numerous adipokines, which have metabolic or inflammatory effects in other tissues, including the liver, muscle, pancreatic islet b-cells and the brain [13,14].Thyrotropin directly induces the synthesis and release of adipokines [15,16], some of which control appetite (eg leptin) by acting on the brain.
The well-documented expression of thyrotropin receptors and transduction of the thyrotropin signal in adipocytes warrant the consideration that the positive relationship we observed in our study between serum TSH and adiposity has biological significance.
Based on their observation that receptors for many of the pituitary hormones are expressed in adipose tissue [10] proposed that a "hypothalamic-pituitary-adipose axis" exists.The positive relationship between serum TSH and adiposity would be consistent with this concept in a "downstream" sense.Yet, such an axis would require a feedback system, and hence the positive relationship between serum TSH and adiposity could also be interpreted in the reverse.
The increase in fat mass, in particular the visceral fat, leads to increased production and release of many of the adipokines [7].Among the adipokines, extensive information is available on the inhibition of appetite by leptin, dominantly by acting on the hypothalamus.A positive correlation has been reported between serum levels of leptin and TSH [16,18,19].
Obesity, particularly visceral obesity, is associated with insulin resistance [20].Thus, almost twofold increase in HOMA-IR in the obese group was expected.The fact that in our study HOMA-IR was positively correlated with serum TSH in the study group (lean and obese subjects in combination) suggests that the role played by thyrotropin in adipogenesis has a metabolic consequence.The positive correlation between serum TSH and fasting plasma insulin and lack of correlation with HOMA-b cell could be interpreted as lack of an influence of thyrotropin signal upon pancreatic islets and thus upon the increases in insulin release to compensate for insulin resistance.
As expected, in a cohort selected for euthyroid state, mean serum T4 levels were well within the normal range and very similar among lean and obese subjects.Serum T3 levels, available only of the obese subjects, were also well within the normal range.No statistical relationship existed between serum T4 and any of the parameters of adiposity.This lack of a correlation coincident with a positive correlation between serum TSH and adiposity would be consistent with a direct interaction of thyrotropin and adipose tissue in the regulation of body weight.
Our study was not designed to prove or disprove a direct role of thyrotropin in adiposity.It was at best an observational study, taking advantage of the availability of structured clinical information on a reasonably large cohort of people in a wide spectrum of adiposity.It is not known at the present time whether increased TSH levels favour the deposition of fat or, on the contrary, whether excessive accumulation fatty tissue increases TSH secretion.Among the important statements is, eg the presence of TSH receptors in adipose tissue.On the other hand, since TSH is correlated with obesity and obesity leads by itself to insulin resistance, it is obvious that TSH must necessarily be correlated with insulin resistance.Therefore, no cause-effect relationship can be constructed.Moreover, one needs to be careful not to overinterpret the role of the minimally increased TSH itself on metabolic events.