The relation between thyroid function and human regulatory nonshivering thermogenesis.

By a standardized exposure to cold air under simultaneous vasodilation, it was possible to elucidate the chemical factors responsible for heat production independent of muscular activity in seven patients with thyrotoxicosis and in nine patients with hypothyroidism. The results showed that hyperthryroid patients have a thermoregulatory reserve equal to normal persons, as they were able to increase their oxygen consumption significantly by an average of 0.035 mmol . kg-1 . min-1 (16%) (range: 0.014--0.064). The plasma concentration of non-esterified fatty acids (NEFA) increased significantly by an average of 0.675 mmol/l (64%) (range: 0.015--2.220). The hypothyroid patients also increased their oxygen consumption significantly by an average of 0.032 mmol . kg-1 . min-1 (30) (range: -0.016--0.066), but in this group the normal fat mobilization was inhibited as no rise in plasma NEFA was seen. Administration of exogenous thyroxine restored thermoregulation to the level and pattern seen in normothyroid persons, as the increase in plasma NEFA after treatment was on average 0.483 nnol/l (192%) (range: 0.342--0.733).