Variations of rat thyroid activity during exposure to high environmental temperature (34 degrees C). Relation between hypothalamic pituitary and thyroid hormone levels.

Changes in thyroid activity and variations in the hypthalamo-pituitary-thyroid hormone levels were examined in rats exposed to heat (34 degrees C)for3 weeks. Thyroid activity evaluated histologically (epithelium/colloid ratio, nuclear size) by radioiodine exploration (24 hrs 125 I uptake, ratio of mono- to di-125 iodotyrosines - MIT/DIT, ratio of tri- to tetra-125 iodothyronines-T3/T4, and plasma 125I-T4 and assay of plasma T4, evolves in a triphasic manner. 1.a depression phase between day 0 and day 2.5. 2. a rebound of thyroid activity between day 2.5 and day 9.3 a stabilization of thyroid parameters from day 9 to day 24. These results indicate adaptation of thyroid function to heat after 3 weeks. In phase i, plasma TSH )MeKenzie bioassay) fell to undectable levels concurrent with a 50% decrease in hypothalamic TRH (in vitro assay). Plasma TSH peaked on day 4.5, fell on day 9.5 and returned progressively to initial levels. Hypothalamic TRH returned to initial levels after 6.5 days. The rapid and simultaneous decrease in hypothalamic TRH, plasma TSH, plasma T4 and thyroid activity by the 36th hour of heat exposure (34 degrees C) suggests initiation at the hypothalamic level. In the secound phase, the rebound in thyroid activity is presumably due to the peak in circulating TSH in ralation to the marked decrease in plasma T4. The oscillations of phase 2 and the stabilization of all the thyroid parameters in phase 3 may be the reflection of an apparent discrepancy remains between a low plasma T4 and a normal or subnormal plasma TSH. A modification in the "set point" for the control of TSH secretion is discussed.