Effects of fever and hyperthermia on thyroid function.

Thyroid hormones in man are affected by acute and chronic febrile states. To define these acute changes, we used a previously described rabbit model. Serum levels of T3, rT3, and T4 were measured at 0, 2, 4, 6, and 24 hr following injection of 1 microgram E. coli endotoxin, and during heat-induced hyperthermia. All rabbits receiving endotoxin developed fever with peaks at one hour (delta T = 1.1 degrees C) and three hours (delta T = 1.4 degrees C); they then defervesced to base levels at 6 hr. Similar temperature elevations occurred with heat-induced hyperthermia. Results show that endotoxin-induced fever produces changes similar to those reported during infections to man, and more rapidly than previously recognized. These include a prompt decrease in T3, reciprocal rise in rT3, and an initially reduced T4 that rebounds above basal levels. These findings may represent suppressed TSH release, alteration of peripheral monodeiodination of T4 from T3 to rT3, or enchanced clearance of T3. Heat-induced hyperthermia, except for slight decrease in T4 at 6 and 24 hr, had little effect on thyroid hormones.