Renal handling of 3,3',5'-triiodothyronine (reverse T3) compared to thyroxine and 3,5,3'-triiodothyronine in different thyroid function states in man

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Abstract
The renal clearance of 3,3'',5''-triiodothyronine (reverse T3, rT3) compared to thyroxine (T4) and 3,5,3''-triiodothyronine (T3) clearance were investigated. The urinary excretion of T4, T3 and rT3 was estimated by radioimmunoassay, serum unbound hormones (AFT4, AFT3, AFrT3) were measured, using ultrafiltration technique. In 27 euthyroid controls, the 24 h urinary T4 excretion was in median 1.7 nmol, T3 excretion 0.8 nmol and rT3 excretion 0.08 nmol. Serum AFT4 was in median 59 pmol/l, AFT3 7.9 pmol/l and AFrT3 2.2 pmol/l. Creatinine clearance was in median 93 ml/min. Median renal clearance of T4, T3 and rT3 were 26, 70 and 25 ml/min, and apparent tubular re-absorption was in average 77, 27 and 77%, respectively. In 18 hyperthyroid patients, urinary hormone excretion was highly increased and was found parallel to the increase in serum concentrations of free hormones. Clearance and percent tubular re-absorption of T4 and rT3 were equal to and not different from control values, but T3 clearance seemed to be increased to values higher than glomerular filtration rate. In 8 patients with hypothyroidism, low urinary hormone excretion was found. T4 and rT3 clearance did not differ from control values, but T3 clearance was reduced. Apparently T4, T3 and rT3 are excreted by glomerular filtration of free hormones and suggest that tubular transport mechanisms are involved. The hypothesis is put forward that the renal handling of the thyronines is influenced by the number of the outer phenolic ring I atoms.