[The inhibition of the thyroid and calcitonin-producing functions of the rat thyroid gland in weightlessness].

Histological, immunochemical and morphometric techniques were used to assess the functional activity of the thyroid parenchyma and calcitonin-producing C-cells of the thyroid gland (TG) in rats after 13 days on board Spacelab-2, in 5-6 hrs., and 14 days following the return from space flight. For the first time the fact of inhibition of the thyroid parenchyma and C-cells functioning during space flight was ascertained. Similarity of changes in the TG activity in and shortly after the space flight suggests that the morphological criteria used in 5 hrs. post flight give quite good insight into the character of changes occurring during microgravity. Two weeks of readaptation to the Earth's gravity result in recovery of the calcitonin production whereas the thyroid parenchyma displays postflight hyperactivity even after 14 days of return.