Studies on hypothermia and thyroid function in the obese (ob/ob) mouse

Abstract

The fall in body temperature of obese mice treated with L-triiodothyronine (T3; 300 ng/day) was less rapid than in untreated obese mice. In the lean mouse, hypothyroidism enhanced the fall of body temperature in the cold. Circulating levels of thyrotropin (TSH) were the same in lean and obese mice at 28°C (Ta) and both groups showed a similar rise in TSH after exposure to 4°C (Ta). The rise in TSH after injecting thyrotropin-releasing hormone (TRH; 1 or 5 μg/100 g) was significantly less in the obese mice. The 24-hr radioactive iodine uptake was lower in the obese mouse regardless of diet. Pretreatment with TSH increased the size of the thyroid gland, the uptake of radioactive iodine, the concentration of thyroxine and triiodothyronine in the serum, but did not alter the rapid decline in body temperature of the obese mouse in the cold. The circulating concentrations of triiodothyronine and thyroxine were similar in lean and obese mice. The response to exogenous triiodothyronine was examined in hypothyroid lean and obese mice by injecting L-triiodothyronine (50, 150, or 500 ng/day) for days. Thyroid weight was highest in the hypothyroid mice but decreased to comparable weights in lean and obese mice at each increasing dose of T3. Circulating concentrations of T3 were raised to similar levels by this treatment. The fall in TSH was also comparable. However, not even the highest dose of T3 in the obese mice prevented the fall in body temperature as much as the lowest dose of T3 in the lean mice. These studies show that the thyroid-pituitary axis of the obese mouse is essentially normal and suggest that the hypothermia of the obese mouse is not due to hypothyroidism.