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

BackgroundThyroid hormones (TH) may influence glucose metabolism. Hyperthyroid subjects have higher insulin secretion rates when compared with euthyroid individuals.ObjectiveTo 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.MethodsTSH, 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.ResultsFT3 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 (β=0.09, P=0.01; β=0.16, P=0.03; and β=0.24, P=0.0007 respectively). No associations were found for TSH or FT4.ConclusionFT3 was associated with several measurements of insulin secretion in euthyroid individuals with NGT. T3 concentrations may play a role in the regulation of insulin secretion.