Relationship between Free Thyroxine and Islet Beta-cell Function in Euthyroid Subjects

Thyroid hormones have a specific effect on glucose-induced insulin secretion from the pancreas. We aimed to investigate the association between euthyroid hormones and islet beta-cell function in general population and non-treated type 2 diabetes mellitus (T2DM) patients. A total of 5089 euthyroid participants (including 4601 general population and 488 non-treated T2DM patients) were identified from a cross-sectional survey on the prevalence of metabolic diseases and risk factors in East China from February 2014 to June 2016. Anthropometric indices, biochemical parameters, and thyroid hormones were measured. Compared with general population, non-treated T2DM patients exhibited higher total thyroxine (TT4) and free thyroxine (FT4) levels but lower ratio of free triiodothyronine (T3):T4 (P<0.01). HOMA-beta had prominently negative correlation with FT4 and positive relationship with free T3:T4 in both groups even after adjusting for age, body mass index (BMI) and smoking. When analyzed by quartiles of FT4 or free T3:T4, there were significantly decreased trend of HOMA-beta going with the higher FT4 and lower free T3:T4 in both groups. Linear regression analysis showed that FT4 but not FT3 and free T3:T4 was negatively associated with HOMA-beta no matter in general population or T2DM patients, which was independent of age, BMI, smoking, hypertension and lipid profiles. FT4 is independently and negatively associated with islet beta-cell function in euthyroid subjects. Thyroid hormone even in reference range could play an important role in the function of pancreatic islets.