Human cold air habituation is independent of thyroxine and thyrotropin

Abstract

Thyroxine (T4) is required in species possessing brown adipose tissue (BAT) for the maintenance of cold tolerance and adaptation. In humans, who possess negligible quantities of BAT, the importance of T4 has not been demonstrated. We studied the effects of decreased serum T4 and thyrotropin (TSH) on human cold habituation after repeated cold air exposures. Eight men (T3+) received a single daily dose of triiodothyronine (T3; 30 micrograms/day), and another eight men (T3-) received a placebo. All 16 normal thyroid men underwent a standardized cold air test (SCAT) under basal conditions in January and again in March after eighty 30-min 4.4 degrees C air exposures (10/wk). Measurements of basal metabolic rate (BMR), O2 consumption (VO2), mean arterial pressure (MAP), plasma norepinephrine (NE), serum TSH, free and total T4, and free and total T3 were repeated before and after 8 wk of exposure. TSH, free T4, and total T4 were 50% lower for T3+ than for T3- subjects. Total and free T3 were not different between groups. BMR was unchanged after habituation, whereas the cold-stimulated VO2, MAP, and NE were significantly reduced for all subjects in March. The relationship between VO2 and NE (r2 = 0.44, P less than 0.001) during the initial SCAT was unchanged with habituation. We suggest that human cold habituation is independent of major changes in circulating T4 and TSH.

The pituitary-thyroid axis in healthy men living under subarctic climatological conditions

“…These findings are difficult to reconcile, since we observed neither seasonal changes in serum total T 3 nor any relationships between TSH and thyroid hormones. An experimental study in which healthy subjects were repeatedly exposed to cold air and in which free T 3 was found to be decreased with no changes in other thyroid hormone or TSH levels (Hesslink et al 1992) supports our present observation. We point out that other seasonal changes in outdoor activities, luminosity, caloric intake, body weight or serum albumin do not explain our findings, since we did not find any significant changes in these parameters during our study.…”

Homeostatic Responses to Prolonged Cold Exposure: Human Cold Acclimatization

“…As indicated in the table, the different approaches appear to yield different acclimation patterns. Brief (1 hour or less) daily exposures to cold air, repeated over a two-week period reportedly blunt the shivering response to cold (habituation) but have no effect on body temperature changes during the exposure (4,27,62). In studies employing longer exposure durations and a longer acclimation period (14,42,43,50), both reduced shivering and more pronounced declines in body temperature during cold exposure have been reported; hence, this pattern of acclimation is termed hypothermic habituation.…”

Thermoregulatory and Immune Responses During Cold Exposure: Effects of Repeated Cold Exposure and Acute Exercise

“…In previous studies involving cold exposures, shivering habituation has developed over the course of a number of days or weeks of repeated cold exposure (12,47,114). The interesting possibility arises that even as few as one or two cold exposures may be a sufficient stimulus for adaptation to cold to begin.…”