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Effects of Fever and Hyperthermia on Thyroid Function

Veterans Administration Medical Center and University of Minnesota, Minneapolis, Minnesota

Correspondence: For reprints contact: Rex B. Shafer, MD, Nuclear Medicine Service (115), V.A. Medical Center, 54th Street & 48th Avenue South, Minneapolis, MN 55417.

ABSTRACT

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 µg E. coli endotoxin, and during heat-induced hyperthermia. All rabbits receiving endotoxin developed fever with peaks at one hour (T = 1.1°C) and three hours (T = 1.4°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 in 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 enhanced clearance of T³. Heat-induced hyperthermia, except for slight decrease in T⁴ at 6 and 24 hr, had little effect on thyroid hormones.