Free triiodothyronine plasma concentrations are positively associated with insulin secretion in euthyroid individuals

Background: Thyroid hormones (TH) may influence glucose metabolism. Hyperthyroid subjects have higher insulin secretion rates when compared with euthyroid individuals. Objective: To evaluate the association between TH concentrations and insulin secretion in euthyroid, healthy Pima Indian adults (n=55, 29 +/- 7 years, females/males 36/19) with normal glucose tolerance (NGT) admitted to a Clinical Research Unit. Methods: TSH, free thyroxine (FT4), 3,5,3-L-tri-iodothyronine (FT3), and fasting plasma insulin (FPI) concentrations were measured in fasting plasma samples, percentage of body fat (%BF) by dual energy x-ray absorptiometry (DXA), acute insulin response (AIR), and incremental area under the curve (AUC) of insulin in response to a 25 g intravenous glucose tolerance test (IVGTT) and 75 g oral glucose tolerance test (OGTT) respectively and insulin action (M) during an euglycemic clamp. Results: FT3 concentrations were associated with FPI, AIR, and insulin AUC both before (r=0.33, P=0.01; r=0.29, P=0.03: and r=0.35, P=0.008 respectively) and after adjustment for age, sex, BF, glucose (fasting concentrations or glucose AUC), and M (beta=0.09, P=0.01; 0=0.16, P=0.03; and beta=0.24, P=0.0007 respectively). No associations were found for TSH or FT4. Conclusion: FT3 was associated with several measurements of insulin secretion in euthyroid individuals with NGT. T-3 concentrations may play a role in the regulation of insulin secretion.