RÉSUMÉ
BACKGROUND: Leptin, an adipocyte derived hormone, and thyroid hormone have similar effects on energy homeostasis, such that a shortage of both hormones is associated with decreased energy expenditure and increased body weight. Therefore, for the maintenance of energy homeostasis may require a close interaction between leptin and thyroid hormone. This study was performed to investigate the change in plasma leptin levels relating to short-term thyroid manipulation causing no significant change in body weight. METHODS: Hypothyroidism was induced by surgical thyroidectomy and hyperthyroidism by subcutaneous injection of 50 g of L-T3/100 g body weight/day, for 5 days, in 6~8 weeks old male Wistar rats. Body weights and food intakes were monitored daily until sacrifice. Plasma samples were collected, and the thyroid stimulating hormone (TSH), free triiodothyronine (T3) and leptin levels measured. The plasma leptin levels in rats with hypothyroidism and hyperthyroidism were compared with those of body weights at death and food intakes during the study, atched controls. RESULTS: The rats treated with L-T3 consumed equal amount of food as freely fed, rats but their final body weights were significantly lower (L-T3 treated 220.0 +/- 1.8 vs. freely fed 226.0 +/- 2.0 g, p<0.05). There was no difference in food intake during study, and final body weight, between the thyroidectomised rats and their paired controls (thyroidectomised 220.4 +/- 1.7 vs. paired 223.9 +/- 4.7 g, P=NS). Plasma leptin levels in the L-T3 treated rats were significantly lower than those in freely fed rats (L-T3 treated 1.7 +/- 0.1 vs. freely fed 4.8 +/- 0.2 ng/ml, p<0.005). Conversely, the thyroidectomised rats had higher plasma leptin levels, compared to those of their paired controls (thyroidectomised 4.8 +/- 0.3 vs. paired 1.7 +/- 0.1 ng/ml, p<0.005). CONCLUSION: The Plasma leptin levels in the rats were decreased by short term hyperthyroidism, while they were increased by short term hypothyroidism. These findings suggest that thyroid hormones may affect the production or secretion of leptin