Radioimmunoassay of 3,3',-'-triiodo-L-thyronine (reverse T3) in human serum and its application in different thyroid states.

A radioimmunoassay for the measurement of 3.3',5'-triiodo-L-thyromine (reverse T3, rT3) has been developed. The known limitations of this technique have been overcome by the use of the biologically relevant L-compound for the production of highly specific antisera and for preparing the standard curve. The high sensitivity of the assay (lower limit of detection 20 ng/l serum) was obtained by using 125I-labelled rT3 of maximum specific radioactivity. Mean serum rT3 concentrations for various thyroid states were as follows: Normal subjects: 0.182 mug/l (0,280 nmol/l), hypothyroidism: 0.038 mug/l (0.058 nmol/l), hyperthyroidism: 0.522 mug/l (0.802 nmol/l), pregnants: 0.200 mug/l (0.307 nmol/l), newborn (cord serum): 2.11 mug/l (3.24 nmol/l). The method described should provide additional information with regard to the clarification of thyroxine metabolism.