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 (T3).

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 T3 plays a major role in the maintenance of energy and lipid balance. Our results also indicate that an inverse relationship exists between T3 and leptin serum concentrations, and that this relationship is not only the result of changes in body fat stores induced by changed T3 concentrations.