Triiodothyronine and thyroxine in urine. I. Measurement and application

Abstract

Urinary triiodothyronine (T3) and thyroxine (T4) were measured by RIA, and T4 was also measured by competitive protein binding (CPB). pH 1-hydrolysable conjugates were 48% of total urinary T3, and enzyme- or pH 1-hydrolysable conjugates were 55% and 61% of total urinary T4. The mean unconjugated T3 excretion was 34.3 ng/h (0.99 mug T3/g creatinine) in normal subjects (no day-night rhythm found), 1.56 mug/g in late pregnancy, 0.82 mug/g in neonates (1-12 days), and was also unchanged in persons with high or low thyroxine-binding globulin (TBG). In thyrotoxicosis, mean T3 excretion was 281 ng/h, no values being in the normal range. In primary hypothyroidism it was 18.3 ng/h, but over half the values were in the normal range. The mean urinary unconjugated T4 was 82.2 ng/h (1.37 mug T4/g creatinine) in normal subjects, 1.6 mug/g in neonates, and unchanged in persons with high or low TBG, except that in pregnancy high values were compatible with increases protein excretion. Apparently increased day-time T4 excretion compared with night-time excretion may also be due to changes in protein excretion rate. The mean T4 in thyrotoxicosis was 337 ng/h (12% of values in the normal range) and 32.8 ng/h in primary hypothyroidism (over half the normal range). All the assays, especially that of T4 by CPB gave readings which were incorrect with protein concentrations above 100 mg/l. Urinary T3 and T4 assays for clinical purposes have few practical advantages over serum assays, despite the relationship of urine T3 and T4 to serum unbound levels.