Propylthiouracil, but not other antithyroid treatments, lengthens hamster circadian period.

Two experiments were performed to evaluate the role of the thyroid gland as a mediator of circadian rhythms in the hamster. In experiment 1, the antithyroid drug propylthiouracil (PTU) lengthened the circadian period (tau), increased thyroid weight, and eliminated detectable thyroxine (T4) and triiodothyronine (T3) from blood. A low-iodine diet greatly reduced T4 levels but had no effect on T3 or tau. Treatment with 500 microCi of 131I failed to alter any parameter of physiology or rhythmicity measured. In this experiment, some animals in the low-iodine and PTU groups had greatly reduced testes sizes, and testes size was inversely correlated with change in tau. In experiment 2, T4 and T3 levels detected 11 wk after surgical thyroidectomy were significantly less than those found in sham-operated animals, but concentrations of the two hormones varied widely across the thyroidectomized group. Thyroidectomy did not increase tau either 4 or 11 wk after surgery, nor was there evidence from individuals that level of thyroid function was associated with change in tau. The results from these experiments suggest that diminished thyroid function is not causal of lengthened circadian period.