The cortisol fluctuations in plasma in relation to human regulatory nonshivering thermogenesis

It is known that many stressful situations are accompanied by an increase in plasma cortisol concentration. This study was undertaken to see if a standardized and moderate exposure to cold under simultaneous peripheral vasodilation, which is known to result in increased oxygen consumption and plasma concentration of noradrenaline, can be considered a stress, based on changes in plasma cortisol concentration. Since the nonshivering thermogenesis induced is dependent on normal thyroid function, nine of the 19 adult humans exposed to cold were normo-, four hyper-, and six hypothyroid. The cold exposure was carried out in every case between 9 a.m. and 11 a.m. Since the cortisol secretion reflects the circadian cycle, a further six normothyroid persons were exposure to cold between 5 a.m. and 7 a.m. The exposure in this study does not reflect any stress condition, as the plasma cortisol concentration during the investigations between 9 a.m. and 11 a.m. decreased significantly by mean values of 0.1 mmol/l (29%) (range: -0.07--0.2 mmol/l) in the normothyroid group, by 0.23 mmol/l (68%) (range: 0.09--0.44 mmol/l) in the hyperthyroid group, and by 0.16 mmol/l (29%) (range: 0.03--0.38 mmol/l) in the hypothyroid group. The normothyroid persons investigated between 5 a.m. and 7 a.m. showed a significant fall similar to that mentioned above. The differences in the results obtained between the normo- and dysthyroid groups, before and during the exposure, merely reflected variations known from non-stressful conditions.