Urine samples from 8 healthy subjects, from 16 patients with primary hypothyroidism and 8 patients with Graves' hyperthyroidism were pre-purified in SP-Sephadex-C-25 cation-exchange-chromatography, subjected to reverse phase high-pressure liquid chromatography (HPLC) with 0.01 M ammonium acetate pH 4 as a polar and propanol as a non-polar solvent with a 1%/min gradient and assayed in our TRH radioimmunoassay. Urine TRH-immunoreactivity levels were measured before and after 3 months of treatment with thyroxine or methimazole. The urine TRH-levels in healthy subjects were 5.5 +/- 1.4 ng/1 (mean +/- SEM, n = 8). In the hypothyroid patients, the urine TRH levels were 50.6 +/- 40 ng/1 before and 71.7 +/- 45.3 ng/1 after 3 months of treatment with thyroxine. These values did not significantly differ from those in healthy subjects. The large variations were due to highly elevated values in 3 patients. In 2 hypothyroid patients with initially high urine TRH values, 67 and 657 ng/1, urine TRH was measured 5 and 18 months later and was found to have decreased to 5 and 11 ng/1. In the hyperthyroid patients, urine TRH levels were 10.3 +/- 3.9 ng/1 before and 8.9 +/- 3.3 ng/1 after the treatment with methimazole and did not differ significantly from the levels in healthy subjects. After 3 months of treatment, the hyper- and the hypothyroid patients were euthyroid. Our results show, that, except in 2 hypothyroid patients, there does not appear to be any relationship between urine TRH levels and serum TSH or thyroid hormone levels in hypothyroid and hyperthyroid patients.