Effects of phenotype, feeding condition and cold exposure on thyrotropin and thyroid hormones of obese and lean mice

The effects of acute cold exposure on rectal temperature (Tr) and circulating thyrotropin (TSH), thyroxine (T4) and triiodothyronine (T3) levels were examined in fed and food-deprived obese (ob/ob) and lean (?/+) C57BL/6 mice. At 23 degrees C, obese mice had lower body temperatures but higher TSH, T4 and T3 values than lean mice while male mice of both phenotypes had similar body temperatures and higher levels of all three hormones than females. Obese mice became severely hypothermic during 4 h cold exposure (8 degrees C) although TSH and T4 concentrations declined equally in obese and lean mice and T3 values were unaffected by cold. Male and female mice exhibited similar Tr responses to cold, while males continued to have higher values of TSH and thyroid hormones than females. When allowed food during cold exposure, both obese and lean mice displayed higher Tr although obese mice remained hypothermic. Thyroid hormones in all groups were increased by feeding but only male mice exhibited increased TSH values. These data show that the acute feeding, metabolic and thermogenic responses of mice to low ambient temperatures are not closely associated with altered systemic levels of TSH, T4 or T3. Furthermore, since ob/ob mice did not display lower hormone levels or defective hormone responses to cold or feeding, the data suggest that their apparent hypothyroidism is largely independent of hormone availability to target tissues.