Effect of endotoxin fever on plasma clearance of thyroxine and tri-iodothyronine: concise communication.

Endotoxin-induced fever in rabbits causes a rapid decrease in serum T3, a reciprocal rise of 3,3',5'-tri-iodothyronine (rT3), and a less marked reduction in T4 with a rebound at 24 hr. To answer the question whether the alteration in hormone levels was the result of a decrease in T3 or T4 production and release or of an increase in T3 and T4 metabolism, we measured the disappearance of [125I]T4 and [125I]T3 during endotoxin-induced fever and externally applied heat. Results showed no significant difference in disappearance of [125I]T4 or [125I]T3. This suggests that the rapid change in T3 levels associated with endotoxin fever is due to an inhibition of thyroid production and peripheral conversion, and not to increased metabolism of hormones.