OBJECTIVE. Leptin, the obese gene product, is secreted exclusively by adipocytes and is thought to act as a lipostatic signal that regulates body weight homeostasis. We previously reported that thyroid hormone is one of the up-regulating factors of leptin in vitro. T3, at physiological concentrations, stimulates leptin mRNA expression and leptin secretion by 3T3-L1 adipocytes. The aim of this study was to explore the role of thyroid hormone in the regulation of leptin in humans. DESIGN AND PATIENTS. A total of 59 non-obese women aged 38·4 + 1·8 years (mean · SEM) were studied: 19 patients with hyperthyroidism, 17 patients with hypothyroidism, and 23 normal control subjects. The correlation between serum leptin concentrations and body mass index (BMI) was analyzed, and serum leptin levels were compared among the three groups. MEASUREMENTS. Serum leptin concentrations were measured by radioimmunoassay. RESULTS. Serum leptin concentrations after logarithmic transformation were correlated significantly (P<0·05) with BMI in the hyperthyroid (r -0·46), the hypothyroid (r=0·84), and normal (r- 0·63) groups. Even though age, body weight, and BMI were similar in all groups, serum leptin levels in the hypothyroid patients (5·30 + 1·12 μg/l) were significantly (P<0·05) lower than in the hyperthyroid and normal groups (6·87 + 0·66 and 6·58 ± 0·68 μg/l, respectively). CONCLUSIONS. These results indicate that thyroid hormone may play an important role in the appropriate secretion of leptin in humans.
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