Influence of thyroid status on serum immunoreactive leptin levels.

Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes. Serum leptin concentrations increase in correlation with the percentage of body fat, but besides that, little is known about the physiological actions of leptin in humans. The aim of this study was to assess the influence of hypo- and hyperthyroidism on serum leptin levels. Thirty-two patients (16 with hypothyroidism and 16 with hyperthyroidism) were studied before and after treatment with replacement doses of T4 (hypothyroid patients) or methimazole (hyperthyroid), when thyroid function was normal. Control serum for each group was obtained from healthy age-, sex-, and body mass index-matched subjects. Plasma leptin levels were measured by specific RIA. The mean leptin level in the hypothyroid patients was lower before treatment (4.7 +/- 0.7 microg/L) than that in the controls (8.6 +/- 1.4 microg/L; P < 0.02) and was lower than that during treatment with T4 and normalization of thyroid function in the same group of patients (6.3 +/- 0.8 microg/L; P < 0.05). Leptin levels in the hyperthyroid patients were similar before (7.2 +.0 1.1 microg/L) and after normalization of thyroid function following treatment with methimazole (6.2 +/- 1.1 microg/L) and were similar to the control value (8.8 +/- 1.4 microg/L). In conclusion, leptin levels are decreased in the hypothyroid patients and unchanged in hyperthyroidism. Whether decreased leptin levels may contribute to the decreased energy expenditure in patients with hypothyroidism merits further investigation.