Seasonal Shifts in Thyroid Function in the Cotton Rat (Sigmodon hispidus)

Cotton rats (Sigmodon hispidus) were captured in southwestern Missouri in summer and winter, several thyroid parameters were measured, and utilization rates of thyroxine ( T4) and triiodothyronine ( T3) were calculated. In addition, resting metabolic rates were measured seasonally. Animals captured in winter had significantly lower concentrations of serum T4 and greater volumes of distribution than summer-caught rats. The resultant utilization of T4 was lower in winter, while the thyroxine efficiency (oxygen consumed per thyroxine used) was greater. The latter suggests an improved T4-to- T3 deiodination and is consistent with the markedly higher serum T3 observed in winter. Winter cotton rats also had shorter T3 half-lives, greater volumes of T3 distribution, and higher T3 utilization rates. Because resting metabolism was not significantly higher, it is concluded that, while winter may stimulate an increased T4-to- T3 deiodination, the excess T3 is removed from circulation before affecting metabolic rate.