Influence of Thyroid Hormone Level on Insulin Action in Human Adipose Tissue

The relationship between the levels of circulating thyroid hormones and the action of insulin on adipose tissue was investigated in 6 hypothyroid patients and 6 hyperthyroid patients, all untreated, and 8 healthy control subjects. All were matched for age, body weight, and fat cell size. Gluteal s.c. adipose tissue was used. The insulin receptor number in isolated adipocytes was increased by 70% in hypothyroidism and decreased by 40% in hyperthyroidism. The sensitivities of the effects of insulin on lipolysis and glucose oxidation were increased fourfold in hypothyroidism and decreased fivefold in hyperthyroidism. The maximum insulin-induced glucose oxidation (insulin responsiveness) was inhibited by 60% in hypothyroidism and enhanced by 180% in hyperthyroidism. The thyroid hormone concentration was significantly correlated with insulin receptor number (r = -0.72), insulin responsiveness (r = 0.71), and insulin sensitivity (r = -0.75). It is suggested that thyroid hormones regulate the effect of insulin on adipose tissue, which occurs at the receptor and postreceptor levels of insulin action.