A diagnostic value of immunoradiometric TSH assay in the evaluation of the function of hypothalamo-hypophyseal-thyroid axis was investigated in this paper. Normal values of basal TSH and TSH response during TRH test were determined by the use of ROC analysis. It was found out that there was a highly significant correlation between basal TSH and the responses during TRH test and that the diagnostic value of basal TSH in the evaluation of function of thyroid axis was almost identical to the diagnostic value of responses during TRH test. This finding pointed to the fact that basal TSH determination could replace the application of TRH test in diagnostic of thyroid axis disturbances. Comparing diagnostic value of basal TSH with other tests which served for the confirmation of thyroid axis disturbances, this test proved to be dominant, the most specific and the most sensitive in respect to other applied tests and could be regarded as the first step in diagnostic scheme of thyroid disturbances.