Changes in thyroid hormones in the serum and the thyroid gland of hibernating frogs, Rana temporaria L.

The hypothalamo-pituitary-gonadal axis spontaneously activates in hibernating frogs, Rana temporaria, under constant conditions (0-4 degrees, darkness). The hypothesis that the spontaneous hibernatory activation involves other regulatory processes preparing the frogs for breeding and posthibernatory activity was tested. The serum concentrations and glandular contents of thyroid hormones (THs) were determined during hibernation. It was shown that (i) in both sexes, serum thyroxine and triiodothyronine levels significantly increased in the middle of January (week 13/14, between day 92 and 98 of hibernation); (ii) the peak of THs blood concentration was accompanied by a slow decrease of free forms of THs and the bound forms of both hormones dropped rapidly over this period; (iii) after a decrease on day 111, the low level of serum THs (but higher than before the peak) was sustained to the end of hibernation; (iv) the thyroid content of free THs significantly rose after their "surge" into the blood, reached maximal values in the middle of February, and remained at this level to the end of hibernation (last week of March); (v) in spring, after spawning, the THs levels in glands and in serum were much lower than those at the end of hibernation or were not detectable; and (vi) the results were confirmed during the second season, when the material in the period of expected elevation of serum THs was collected every other day.