Measurement of triiodothyronine in unextracted urine.

A radioimmunoassay procedure to measure triiodothyronine (T3) in unextracted urine is described, and the results are compared with those obtained with different assays by others. It is assumed that the radioimmunoassayable T3 in urine represents free T3. Euthyroid subjects excreted 1.0 +/- 0.3 mug (mean +/- S.D.) in 24 hours. Similar values were obtained in pregnant women, sick patients without thyroid disease, and euthyroid patients receiving antithyroid agents. Values were about 3 times normal in patients with hyperthyroidism and about one-third of normal in patients with hypothyroidism. Estrogen, salicylate, or diphenylhydantoin therapy did not significantly alter the values. Radioimmunoassayable T3 increased after acid hydrolysis, suggesting that a fraction of the T3 in urine is present as an acid-hydrolyzable conjugate. However, it cannot be excluded that at least part of the T3 found after hydrolysis derives from monodeiodination of T4. This hydrolyzable fraction was about 25 per cents of the total T3 in urine. Measurements of T3 in urine are reliable and easy to perform. These measurements have little significance as a clinical test of thyroid function, but the assay may allow a new approach to understanding thyroid hormone metabolism and action, provided that deiodination of T4 during the procedure can be rigorously controlled and that the urinary T3 conjugates can be determined quantitatively.