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

OBJECTIVE: To study the influence of thyroid hormones on the relationship b etween serum leptin and fat mass, as well as on energy and macronutrient ba lance. DESIGN: Rats with different thyroid states were obtained by 7 and 15 days o f treatment with the antithyroid drug propylthiouracil or with triiodothyro nine (T-3). MEASUREMENTS: Energy balance, macronutrient balance and serum leptin concen trations. RESULTS: In hypothyroid rats we found a decrease in metabolizable energy (M E) intake and energy expenditure together with an increase in lipid gain/li pid intake ratio and a decrease in protein gain/protein intake ratio. Conse quently, body lipid percentage significantly increased compared to euthyroi d rats. Hyperthyroid rats first increased energy expenditure and later ME i ntake, so that increased metabolism was balanced by increased intake, and e nergy gain was similar to that found in euthyroid rats. CONCLUSION: These results indicate that T-3 plays a major role in the maint enance of energy and lipid balance. Our results also indicate that an inver se relationship exists between T-3 and leptin serum concentrations, and tha t this relationship is not only the result of changes in body fat stores in duced by changed T-3 concentrations.