Effect of hyperthermia on the function of thyroid gland

Hyperthermia may be a consequence of environmental conditions, bacterial or viral infections and/or thyroid storm. This study investigates the acute effect of body temperature elevation on thyroid function and on its scintigraphy studies. Thyroid scintigraphy was performed on New Zealand White rabbits weighing approximately 3–3.5 kg. Each rabbit was injected with 115 MBq (3.1 mCi) technetium-99 m pertechnetate (^99mTc pertechnetate). Studies were performed using Gamma camera equipped with a low energy, high resolution, pinhole collimator interfaced with a computer. Static images were acquired 20 min after administration of the radiotracer. Two days later the same protocol was repeated for the same rabbit after increasing the body temperature by 2°C. The experiment was repeated again after a 2-day interval at 3°C, and then after another 2-day interval at 4°C. Plasma free thyroxine (FT₄), free triiodothyronine (FT₃) and thyroid stimulating hormone (TSH) were measured at control and at different hyperthermic temperatures (+2, 3, 4°C). We recorded isometric tension of rabbit thyroid artery strips in organ baths during stepwise temperature elevation. During hyperthermia the decrease in thyroid function and thyroid scintigraphy studies was proportional to body temperature elevation. The recording of isometric tension in rabbit thyroid artery strips in organ baths showed vasoconstriction during hyperthermia which is proportional to the heating temperature. Plasma FT₄ and FT₃ level were decreased while TSH levels were not affected by acute fever. Our results indicate that hyperthermia causes a transient decrease in thyroid gland function and scintigraphic patterns on radionuclide studies. Thus, body temperatures must be measured before radionuclide studies in order to ensure that interpretation of data is not influenced by hyperthermia.