Acute and prolonged effects of insulin-induced hypoglycemia on the pituitary-thyroid axis in humans

Secretory activity of the pituitary-thyroid axis and thyroid hormone metabolism show characteristic changes in response to different stressors often referred to as the euthyroid sick syndrome. Hypoglycemia is an acute metabolic stressor inducing various neuroendocrine responses, the effects of which on pituitary-thyroid secretory activity so far have been entirely neglected. We performed stepwise hypoglycemic and euglycemic clamps each lasting 6 hours in 30 healthy men. To assess the potential influence of hyperinsulinemia on pituitary-thyroid hormone release, 2 different rates of insulin infusion were used for the clamps. During the hypoglycemic clamps, serum thyroid-stimulating hormone (TSH) concentration decreased in comparison to the euglycemic condition on average by 28% ± 4% (P < .001), while serum concentration of free triiodothyronin (fT₃), free thyroxine (fT₄), and thyroxine-binding globulin (TBG) remained unchanged. The effect did not depend on the rate of insulin infusion. To assess the prolonged effect of acute hypoglycemia on pituitary-thyroid-secretory activity, serum TSH and thyroid hormone concentrations were subsequently measured in another 15 healthy men before and 18 hours after 2 consecutive hypoglycemic clamps together lasting about 270 minutes. Compared with values before the hypoglycemic clamps, serum levels of TSH, fT₃, and fT₄ were found to be still reduced (by 44% ± 6%, 12% ± 2%, and 10% ± 1%, respectively) 18 hours after the last hypoglycemic episode (P < .001 for all comparisons). The observed hormonal changes after hypoglycemia were not accompanied by any change in resting energy expenditure (REE). Data indicate acute as well as prolonged inhibitory influences of hypoglycemia on pituitary-thyroid secretory activity. The pattern of changes suggests that hypoglycemia exerts its influence primarily at a central, ie, pituitary and/or hypothalamic, site of the axis.