Abstract and Introduction
Objective To investigate whether thyroid function in the euthyroid range and thyroid autoimmunity status would affect metabolic measures in individuals with obesity.
Patients We retrospectively evaluated 5300 consecutive obese (BMI ≥30 kg/m2) 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.
Changing lifestyle and nutritional habits had increased the prevalence of obesity by the end of the 20th century, such that the incidence has more than doubled worldwide since 1980. This statistic is perturbing because obesity is accompanied by several metabolic complications that increase mortality and morbidity in adulthood.[2,3]
Various causative or adaptive hormonal changes occur in obesity. The association between thyroid hormones and obesity has been particularly intriguing for researchers because slight changes in serum thyrotrophin (TSH) levels, even within the normal reference range, may cause weight gain and increase the risk of obesity. Several studies have reported an association between slightly elevated serum TSH levels and obesity.[4–10] This hormonal change was considered mainly adaptive, although this is still a matter of debate. Follow-up studies have shown that serum TSH levels normalize after weight loss due to a hypocaloric diet or bariatric surgery.[11–13] In addition, studies have supported the notion that obesity is a risk factor for thyroid autoimmunity. In this situation, an elevated TSH level is thought to be the causative factor in obesity; however, this hypothesis has not been confirmed by other studies.[5,14]
Thyroxine (T4) is the major secretory product of the thyroid gland and a precursor of the active form of the hormone, triiodothyronine (T3), which is mainly produced in peripheral tissues by deiodination of T4. Free T3 (FT3) has a critical role in temperature homoeostasis and is responsible for resting energy expenditure. Recent data regarding circulating concentrations of thyroid hormones in obese people have shown that FT3 is positively associated with body mass index (BMI), adiposity and feeding status.[16–18] On the other hand, studies have reported varying low, normal or high levels of free T4 (FT4) in obese subjects.[4,6,16–19]
The association between thyroid hormones within the euthyroid range and metabolic measures is not clear. It is apparent that thyroid hormones have significant roles in almost all aspects of carbohydrate and lipid metabolism. Although some studies reported that low-normal FT4 levels were associated with insulin resistance in euthyroid subjects with obesity,[4,10,17,20] other studies could not confirm this association.[7,21,22]
Therefore, in this study, we aimed to investigate whether thyroid function in the euthyroid range and thyroid autoimmunity status would affect metabolic measures in individuals with obesity.
Clin Endocrinol. 2016;85(4):616-623. © 2016 Blackwell Publishing