[en] The aims of the present investigation were as follows: to determine the level of thyroid hormones and thyrotrophin (TSH) at various times after induction of graft versus host reaction (GVHR); to study the degree of ¹²⁵ I uptake by thyroid gland tissue at the same times of the GVHR, and to determine correlation between the hormone levels and weight of the gland in the animals and also the body weight of the recipients. Serum levels of tri-iodothyronine (T₃), thyroxine (T₄), and TSH were determined by radioimmunoassay. ¹²⁵ /SUB I/ was injected intraperitoneally in a dose of 3-4 microCi/100 g body weight. During the development of a systemic GVHR marked inhibition of thyroid function was discovered