MEASUREMENT OF THYROXINE AND TRIIODOTHYRONINE IN HUMAN URINE

Abstract

Urinary excretion of unconjugated Summary thyroxine (T 4) was 2·0 μg. per day (mean) in thirty-six normal subjects. The renal clearance of serum unbound T4 was 26 ml. per minute (mean) in ten subjects. Hydrolysis of conjugated T4 in urine, however, yielded a further 2·8 μg. per day (mean) by enzyme hydrolysis or 3·7 μg. per day by acid hydrolysis. Mean figures for percentages of unconjugated and conjugated thyroxine were 39% and 61% of total T4, respectively. The unconjugated fraction should reflect the prevailing serum level of unbound T4, and be a useful thyroid-function test. Urinary immunoassayable triiodothyronine (T3) was 0·8 μg. per day (mean) in thirty-eight normal subjects. This fraction was 52% (mean) of the total T3, which was no more than 1·4 μg. per day (mean), in nine subjects. It is probably close to the amount of unconjugated T3. Renal clearance of serum unbound T3 seemed to be greater than glomerular filtration-rate in some cases, raising the possibility of tubular excretion of T3.