Unconjugated thyroxine and triiodothyronine in urine: influence of age, sex, drugs and thyroid function.

The 24 h urinary excretion (dU) of T4 and T3, determined by radioimmunoassays using extraction and separation on Sephadex columns, is higher in men than in women (mean dU-T4 and dU-T3 +/- 2 SD: 2.6 +/- 0.9 vs. 2.2 +/- 1.0 and 2.5 +/- 1.0 vs. 2.0 +/- 0.8 nmol, respectively). The excretion of T3 decreases with age, but expressed relatively to dU-creatinine there is no effect of age and sex. Children have increased T4 and T3 to creatinine excretion ratios and neonates excrete predominantly T4. Oestrogens, acute salicylate loading and diurnal variation yielded results, which were at variance with the hypothesis that dU-T4 and dU-T3 mirror corresponding free hormone concentrations in serum, and there was only a weal positive correlation to total serum hormones in thyrotoxicosis as well. General clinical use of urine T4 and T3 is obviated by poor diagnostic discrimination and inherent analytical and interpretative disadvantages.