The MCR of diiodotyrosine (DIT) was determined by measuring serum DIT concentrations by RIA after a single injection of 200 micrograms DIT and noncompartmental analysis. Comparison of the stable DIT method with the tracer DIT technique in dogs yielded good agreement of measured DIT MCRs. The mean (+/- SD) MCR and blood production rate of DIT were 122 +/- 29 L/day X 70 kg and 24.2 +/- 12.7 nmol/day X 70 kg (10.5 micrograms/day X 70 kg), respectively, in 10 normal subjects. Urinary DIT was measured by RIA after its immunoprecipitation from urine. Acid hydrolysis had no effect on measured urinary DIT concentrations, suggesting the presence of predominantly unconjugated DIT. Mean urinary DIT excretion was 1.23 +/- 0.43 (+/- SD) nmol/24 h (533 ng/24 h) or 0.108 +/- 0.048 nmol/nmol creatinine in 32 normal individuals. In patients with defective thyroidal iodine metabolism, urinary DIT was extremely elevated, ranging from 1.2-17.7 nmol/mmol creatinine. Comparison of normal production and excretion rates suggests that about 5% of the daily extrathyroidal DIT turnover is excreted in the urine unchanged or in a DIT-like form.