Free thyroid hormone assays have broadened the repertoire in thyroid diagnosis. At elevated binding protein levels they provide more reliable information on the metabolic state than do total hormone determinations; yet under estrogen contraception free T4 may yield test specifically falsely too high data. As a rule, there is accordance between free hormone concentrations and the indices, being equivalent in establishing the free thyroid hormone fraction. In this light the Thyroid Section of the German Endocrinologic Society recommended in 1986 to consider determinations of free or total hormone concentrations with calculation of the indices as being of equal value. In borderline situations the TRH test, the hypersensitivity TSH assay as well as checkups establishing the course of development of the disorder are substantiated.