Serum T3 and T4 levels have been determined by a radioimmunoassay technique and the TRH test has been performed in 50 patients in whom hyperthyroidism could not be ruled out by the first clinical examination alone. Each patient was then further evaluated in order to establish the state of the thyroid function. The extent to which the determination of T3 or T4 could replace the TRH test in the routine diagnosis of hyperthyroisism was evaluated. The results showed that 26 of the 50 patients had normal thyroid function and 24 had hyperthyroidism. No patient in the normal groups and all but one in the hyperthyroid group had T3 levels above the upper normal limit (2 S.D.). Two of the patients in the normal group and 19 in the hyperthyroid group had T4 levels above the upper normal limit (2 S.D.). Twenty of the patients in the normal group showed a normal TSH response to TRH (increment is greater than 3.0 micronU/ml); the remaining 6 showed an impaired or absent response. Twenty of the hyperthyroid patients had no response and four had a slightly positive response to TRH. No hyperthyroid patient had a TSH response exceeding 3.0 micronU/ml. It is concluded that the determination of T3 is superior to both the determination of T4 and the TRH test for the laboratory discrimination between eu- and hyperthyroidism.