Fat balance and serum leptin concentrations in normal, hypothyroid, and hyperthyroid rats

OBJECTIVE: To study the influence of thyroid hormones on the relationship between serum leptin and fat mass, as well as on energy and macronutrient balance. DESIGN: Rats with different thyroid states were obtained by 7 and 15 days of treatment with the antithyroid drug propylthiouracil or with triiodothyronine (T-3). MEASUREMENTS: Energy balance, macronutrient balance and serum leptin concentrations. RESULTS: In hypothyroid rats we found a decrease in metabolizable energy (ME) intake and energy expenditure together with an increase in lipid gain/lipid intake ratio and a decrease in protein gain/protein intake ratio. Consequently, body lipid percentage significantly increased compared to euthyroid rats. Hyperthyroid rats first increased energy expenditure and later ME intake, so that increased metabolism was balanced by increased intake, and energy gain was similar to that found in euthyroid rats. CONCLUSION: These results indicate that T-3 plays a major role in the maintenance of energy and lipid balance. Our results also indicate that an inverse relationship exists between T-3 and leptin serum concentrations, and that this relationship is not only the result of changes in body fat stores induced by changed T-3 concentrations.