Endocrine Responses to Heat and Cold Stress

Abstract

This review focuses on the endocrine responses to thermal stimuli during passive heat or cold exposure, with particular reference to the relation of these responses to the changes in the body core temperature (Teare). Mild to moderate hyperthermia (<1°C rise in Teare) induces the release of growth hormone and prolactin (PRL). Moderate hypothermia (la_2°C fall in Teore) suppresses PRL release. A positive correlation between plasma PRL and Teare suggests some role for PRL in thermoregulation. Hypothermia activates the hypothalamo-pituitary-thyroid (HPT) axis and releases thyrotropin-releasing hormone, thyroid-stimulating hormone (TSH), and thyroid hormones and increases the metabolic rate. Enhancement of extrathyroidal production of triiodothyronine (T3) from thyroxine (T4) may precede the TSH response to cold. Both severe hyperthermia and hypothermia (1°_3°C changes in Teare) activate the hypothalamo-pituitary-adrenal (HPA) axis and the sympathetic nervous system, resulting in release of corticotropin-releasing factor, adreno-corticotropic hormone, cortisol, and norepinephrine. The responses in the HPT axis and the HPA axis are not apparent in humans, as they are in rats, probably owing to the larger body mass of humans. Hyperther-mia stimulates the renin-angiotensin-aldosterone system and the release of arginine vasopressin (AVP) and atrial natriuretic peptide, but this might be due to nonthermal factors. Diuresis due to suppression of AVP release is induced by cold. Gonadal response to thermal stimuli is possibly suppressive. The hormonal responses induced by thermal stress are mostly dependent on the change in Teare in humans; in small animals they are also dependent on the change in skin temperature. 228 I ntrod uction Body core temperature (Teare) may increase during physical exercise and passive heating (while sauna bathing, with head-out water immersion, and exposure to hot environments); it decreases during cold exposure. Changes in Teare induce various endocrine responses to maintain homeostasis of the body. Recently, local and whole-body hyperthermia therapy for cancer and some benign diseases and induced hypothermia treatment to protect the important organs from hypoxic damage have been developed and widely utilized. On these occasions, the endocrine responses induced by the changes in Teare should be considered. Hormonal responses and their relation to the immune system during exercise in the heat and passive hyper-thermia have been reviewed …