Objective: To examine whether obesity, body fat distribution and insulin resistance have an independent effect on serum TSH and free thyroid hormones (FT3 and FT4) in a cohort of euthyroid women, represented by overweight and obese patients.
Design and patients: A total of 201 women, aged 18-68 years, with body mass index (BMI) > or = 25.0 kg/m(2) and TSH levels < 4.0 mU/l were investigated.
Measurements: Fasting TSH, FT3, FT4, insulin, glucose, and serum lipid concentrations, and the level of insulin resistance, estimated by the homeostasis model assessment for insulin resistance (HOMA-IR). Waist circumference was measured as an indirect parameter of central fat accumulation.
Results: FT3 was directly associated with BMI (P < 0.01) and waist circumference (P < 0.01), and negatively correlated with age (P < 0.001). FT4 was negatively associated with HOMA-IR (P < 0.05) and fasting insulin levels (P < 0.05). TSH was positively correlated with waist circumference (P < 0.05) and negatively associated with age (P < 0.05). When multiple regression analysis was performed with FT3 as the dependent variable, and waist circumference, HOMA-IR, blood pressure levels and serum lipid concentrations as independent variables, FT3 maintained an independent association only with waist circumference (positive, P < 0.05) and age (negative, P < 0.001). When multiple regression analysis was performed with TSH as the dependent variable, and the above parameters as independent variables, TSH maintained an independent association only with waist circumference (positive, P < 0.05) and age (negative, P < 0.05). By contrast, when multiple regression analysis was performed with FT4 as the dependent variable, FT4 did not maintain an independent association with any of the independent parameters.
Conclusions: Progressive central fat accumulation is associated with an increase in both FT3 and TSH serum levels, independently of insulin sensitivity, metabolic parameters and blood pressure. These results suggest that (1) progressive central fat accumulation is associated with a parallel increase in FT3 levels, possibly as an adaptive thermogenic phenomenon, and (2) the control of TSH secretion by free thyroid hormones is possibly impaired in obesity.