General and Comparative Endocrinology vol:36 issue:4 pages:598-603
In Rana temporaria acclimation to higher environmental temperatures results in an increased 125I uptake by the thyroid glands but only in males. Both male and female frogs have comparable excretion rates for 125I which increase with consecutive elevations of temperature. December frogs had the lowest thyroidal uptake rates and the increased uptake percentage following acclimation to higher temperature was less pronounced compared to October or January frogs. In males 125I uptake was more pronounced in skin, stomach, and liver, whereas in females up to 21.5% of iodine injected was accumulated in the ovaries. Thyroxine content of thyroids in December frogs was about five times as high as in March frogs. No triiodothyronine was detectable in thyroids of December frogs and in March frogs only minimal amounts could be found. In frogs tested in March, the thyroxine content of male thyroids at 22° was only half that at 4°, whereas the level of thyroxine remained unchanged at both temperatures in females. The results support the hypothesis that the lower 125I thyroid uptake in females is not caused by an accumulation of 125I in the ovaries but by competition at the thyroidal site with iodine released from the ovaries.