General and Comparative Endocrinology vol:57 issue:2 pages:266-273
Annual variations in plasma concentration and thyroid gland content of thyroid hormones were studied in male Rana ridibunda during 2 consecutive years together with in vitro capacity of the thyroid gland to release thyroxine (T4) following stimulation with bovine thyrotropin (b-TSH). A low thyroidal content of triiodothyronine (T3) (2–4 ng) and T4 (300–400 ng) is found during winter and increases rapidly after hibernation to maximal values of ±50 ng T3 and 2000–2500 ng T4 in May–July. Plasma levels of T3 vary between 10 and 15 pg/ml for all months studied except for a low of 5.4 and 5.7 pg/ml in December and March, respectively. Plasma concentrations of T4 are low in winter (100–200 pg/ml) but increase in January, reaching maximal levels in February–March (700–1000 pg/ml). In the first year levels decrease again until August (last observation month) to about 250 pg/ml, whereas in the second year this decrease is more sudden, reaching winter values of 100–200 pg/ml as early as May–June. Thereafter a second increase in plasma concentrations of T4 is observed again with maximal values of 800 pg/ml obtained in October. This biphasic circannual pattern, from two different observations, is also found in the in vitro T4 release capacity of the thyroid glands following stimulation with 100 mU of b-TSH. The maximal release capacity of the thyroids is present in March and July–August. Minimal release capacities are present in winter but also in April–June. It is concluded that the thyroid glands become activated at the end of hibernation, reaching full-size activity prior to reproduction. During the process of reproduction they are almost resistant to any TSH stimulation, but thereafter, a second reactivation occurs, perhaps in relation to gonadal restoration processes.