Thyroid hormone maintains normal circadian rhythm of blood corticosterone levels in the rat by restoring the release and synthesis of ACTH after thyroidectomy.

The role of thyroid hormone in maintenance of the circadian adrenocortical rhythm was studied in the rat. The amplitude of the circadian rhythm of blood corticosterone levels gradually decreased with time after thyroidectomy, and by 5 weeks the rhythm disappeared. However, the circadian peak elevation constantly occurred just before the onset of the dark period until 4 weeks. Three weeks after thyroidectomy, daily overall treatment with thyroxine (T4) or 3,5,3'-triiodothyronine (T3) completely restored the amplitude of the circadian adrenocortical rhythm to the previous level within 2 weeks. Thyroidectomy did not affect the circadian rhythm in water intake. However, thyroidectomy resulted in a loss of significant difference of plasma adrenocorticotrophin (ACTH) levels between the morning and the evening, by decreasing the evening levels. Similarly, pituitary ACTH content was decreased by thyroidectomy. Replacement of T4 completely restored the decreased ACTH levels to the previous ones. These results suggest that thyroid hormone plays an important role in maintenance of the normal amplitude in circadian adrenocortical rhythm in the rat, by affecting ACTH synthesis.