[en] Radioisotope functional investigations of the thyroid gland were carried out with weaned lambs and pigs. The ₁₃₁I captation capacity of the thyroid gland was studied to characterize the inogranic phase of iodine exchange. The evaluation of the hormone producing function of the thyroid gland required the determination of the level of the protein-bound ₁₃₁ I (PB ₁₃₁ I), T³-binding capacity of the thyroxin-binding globulin (TBG) and the ratio of the T³-binding capacity of TBG and the free thyroxine (T⁴) calculated as the FT⁴ index.Established was the species peculiarity of the ₁₃₁ I captation capacity of the thyroid gland in weaned lambs and pigs in the early phases of investigation. At the second hour the percent captation was calculated as 7.2+-0.8 and 1.11+-0.21%, respectively. The values of PB ₁₃₁ I, T³-binding capacity of TBG and FT⁴ of pigs varied within the limits of 1.20+-0.05%, 0.605+-0.012, and 0.976+-0.015 and of weaned lambs they were 0.954+-0.009%, 0.912+-0.017 and 0.890+-0.03. (authors)