Effects of temperature, photoperiod, and daily torpor on thyroid function in the white-footed mouse, Peromyscus leucopus

Abstract

The biological half-life of thyroidal radioiodide and proportions of thyroid-iodoamino acids were determined in Peromyscus leucopus after 3 weeks' treatment with one of several constant ambient temperatures ranging from 7 to 35° and prior exposure to long (16:8 LD) or short (8:16 LD) photoperiod. Similarly, the biological half-life of radioiodide was determined in mice exposed to temperature cycles of 16 hr at 22° and 8 hr at 7° or 16 lit at 35° and 8 hr at 22° and prior treatment with long photoperiod. The mean radioiodide half-life of mice kept at 7° was 9.4 days which was significantly longer than the pooled radioiodide half-life of 5.6 days exhibited by mice in all other temperature-photoperiod regimes. It is suggested that this long radioiodide half-life was due to the occurrence of daily torpor in the experimental animals. Mice monitored for daily torpor also exhibited significant reductions in the radioiodide release rate. Thyroid activity was not depressed in these animals on days during which they remained euthermic. The daily radioiodide release rate was positively correlated with the minimum body temperature of torpor and negatively correlated with the duration of torpor. It is suggested that this decrease in thyroid activity was a result of a direct effect of body temperature on thyroid function. Percentages of thyroid hydrolysate components were similar throughout all treatments.