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

Objective To investigate the relationship between thyroid hormone concentrations and beta-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 beta-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 beta-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/height(2) and visceral fat area). Conclusions Obesity-associated high serum FT3 concentrations might affect beta-cell function in euthyroid patients with obesity and type 2 diabetes.