Effects of sauna and glucose intake on TSH and thyroid hormone levels in plasma of euthyroid subjects

The effect of sauna on thyroid function parameters and its modification by glucose was studied in young euthyroid male volunteers. A 30-minute stay in sauna resulted in an increase in plasma TSH; the response was exaggerated if glycemia had been increased by oral glucose intake at the beginning of the experiment. Plasma rT 3 also increased in sauna, this response was, however, blunted by the higher glycemia. TSH response to sauna was definitely present in young men (aged 20 to 25) and absent in middle-aged ones (50 to 55). To explore the mechanism of the effect of increased glycemia, TRH tests were performed and dopamine infusions were administered with and without glucose pretreatment. Increased glycemia did not affect TSH and T 3 response to TRH in young volunteers; however, 90 minutes after the administration, plasma rT 3 levels were significantly lower in glucose pretreated subjects than in those receiving TRH injections after water pretreatment. Simultaneous infusion of glucose prevented the inhibitory effect of dopamine infusion on plasma TSH. It was concluded that glucose directly modulates the effect of sauna on plasma TSH at a suprapituitary level, while the inhibiting effect of glucose on plasma rT 3 response to sauna and TRH is probably mediated by the insulin effect on thyroid hormone metabolism.