The time course of changes in TRH responsiveness in man following a single dose of liothyronine

The serum thyrotropin (TSH) response to thyrotropin-releasing hormone (TRH, 100 mug i.v.) was evaluated prior to and at various times following the oral administration of single doses of liothyronine (100 mug) given at weekly intervals. The TSH response to TRH was mildly depressed when TRH was given 1 hr after liothyronine administration when the serum triiodothyronine (T3) concentration was strikingly elevated, was markedly reduced 16 and 24 hr after liothyronine, was essentially abolished 3 days after liothyronine when the serum T3 concentration was normal, and was normal 7 days after liothyronine administration. These findings suggest that the more prolonged suppression of TRH responsiveness, observed following the withdrawal of long-term excess endogenous or exogenous thyroid hormones, cannot be ascribed to the intrinsic duration of action of the hormone present at the time of withdrawal, but rather to the prolonged extent of the suppression itself.