Seasonal changes in circulating levels of thyroid hormones are not dependent on the age in Djungarian hamsters Phodopus sungorus.

1. Plasma levels of total and free thyroxine (T4; FT4) and triiodothyronine (T3; FT3) were measured by radioimmunoassay in adult and senescent Djungarian hamsters at different times of the year. 2. Seasonal changes of both hormones were found in adult and senescent hamsters. 3. However, except for total thyroxine, the patterns were different in both groups of hamsters. 4. These results suggest, that in Djungarian hamsters age-related changes of thyroid function do not affect the secretory activity of the thyroid gland rather than the phase of the seasonal cycle.