3,5,3'-Triiodothyronine regulates insulin level in the circulation following glucose ingestion in humans.

To clarify the relationship between the 3,5,3'-triiodothyronine (T3) response and the insulin response after glucose ingestion in humans, serum T3, immunoreactive insulin (IRI), C-peptide immunoreactivity (CPR), nonesterified fatty acid (NEFA) and plasma glucose levels were measured in 11 nonobese inpatients with normal glucose tolerance after oral 75 g glucose tolerance test. Ingestion of the glucose solution significantly increased serum T3 levels from 120.1 +/- 4.1 ng/dl to 132.5 +/- 4.4 ng/dl (P less than 0.01) at 60 min after glucose ingestion and it continued to increase after 3 h. Serum IRI and CPR levels peaked at 60 min (P less than 0.01, respectively) after glucose ingestion and they also continued to increase after 3 h. Plasma glucose levels peaked at 30 min (P less than 0.01) after glucose ingestion and then decreased to basal level after 3 h. Negative correlation was found between sigma T3 level and sigma IRI level (r = -0.75), and between sigma T3 level and sigma IRI/sigma glucose ratio (r = -0.88). Negative correlation was also found between basal T3 level and sigma IRI level (r = -0.82). No correlation was found between sigma T3 level and sigma CPR level. Positive correlation was found between sigma glucose level and sigma dT3 level (r = 0.74), and between sigma dglucose level and sigma dT3 level (r = 0.81). The findings suggest that T3 regulates insulin level in the circulation after glucose ingestion and the increase in serum T3 levels after glucose ingestion is necessary for the glucose removal from the circulation in humans.