Evidence that the pacemaker controlling activity rhythms is shortened in male thyroparathyroidectomized (TPX) rats: similarities to the effects of estradiol in females.

The experiments described here demonstrate that TPX and TX male rats display shorter free-running activity rhythms and higher activity levels compared to intact males. Neither effect could be duplicated in TPX females. The patterns in activity period, range of entrainment, and levels of activity observed in TPX males closely resembles that reported in intact females or OVX females treated with estradiol. These data suggest the possibility of a common mechanism linking circadian alterations produced by estradiol and thyroid state. This hypothesis needs to be tested, as does the question of whether thyroid hormones or calcitonin are responsible for the TPX and TX results. If thyroid hormones prove to be the causal agents, opposing effects of thyroid hormones and estradiol on adrenergic receptors provide a possible common mode of action for these agents on the circadian system. Disparate results from rat and hamster studies concerning the influence of thyroid state on activity rhythms suggest the possibility of species differences in this level of circadian organization. Thus, caution should be exercised in extrapolating results to other species, including humans.