Radioimmunoassay of 3-L-Monoiodothyronine: Application in Normal Human Physiology and Thyroid Disease*

A RIA for 3-monoiodothyronine (3T1) using tritiated ligand, has been validated for measurement of extracted serum. The mean euthyroid level of serum 3T1 was 2.9 +/- 1.7 ng/dl. In thyrotoxic patients, the mean serum 3T1 was significantly higher: 6.2 +/- 3.9 ng/dl (P less than 0.001) and in hypothyroid patients, the mean level was 2.1 +/- 1.5 ng/dl, lower but not significantly lower than the mean euthyroid level (P greater than 0.05). The mean serum level in severely ill patients was 2.5 +/- 1.5 ng/dl (P greater than 0.05) and in cord blood was significantly higher at 9.1 +/- 2.6 ng/dl (P less than 0.001). The appearance of 3T1 within 5 min in two normal volunteers after iv injection of 3,3'-diiodothyronine was followed over 2 1/2 h. Thus 3T1 is a normal constituent of human plasma and is derived from 3,3'-diiodothyronine. Its concentrations appear to be severalfold higher than those reported for 3'T1.