Circannual oscillations of function compared with morphometric changes in the thyroid gland of the Wistar-rat.

To investigate biological rhythms of the thyroid gland circannual oscillations of thyroxine (T 4 ), triiodothyronine (T 3 ), and thyrotropin (TSH) were compared in serum samples of untreated young male Wistar-rats with the circannual changes of thyroid weights and with the relative proportion of colloid, epithelium, and interstitium of the thyroids. Our results represent an example for the concordance between functional and morphometrical changes in the course of circannual oscillations. Furthermore, these data confirm our earlier results describing higher T 4 -levels in the winter time(short-day) and lower serum titers in the summer time (long-day)