To elucidate the mechanism of thyroid hormone-induced hyperinsulinemia, the 
acute and direct effect of thyroid hormone administration on insulin secretion 
was investigated in rats in vivo and in vitro. In the perfused rat pancreas, the 
addition of thyroxine (10 micrograms/dL) or 3,5,3'-triiodothyronine (150 ng/dL) 
to the perfusing medium did not affect insulin secretion. The administration of 
thyroxine (40 micrograms/kg, s.c.) in vivo increased the plasma insulin level 
from 11 +/- 2 microUnits/mL (mean +/- SD) to 30 +/- 7 microUnits/mL, while blood 
glucose and plasma glucagon were unchanged. This phenomenon was inhibited 
completely by the preadministration of oxprenolol hydrochloride (2 mg/kg, s.c.), 
and inhibited partly by the preadministration of metoprolol tartrate (35 mg/kg, 
s.c.). These results suggest that thyroid hormone induces hyperinsulinemia via 
beta-adrenergic stimulation in the rat.