Abstract: A description of the radioimmunoassay method for the determination of reverse T₃ in human serum was given. The antibody to rT₃ was raised in rabbits by immunizing the animals with hemocynin conjugated reverse T₃.¹²⁵Ⅰ-labeled rT₃ with a specific radioactivity of 500-560 μCi/μg was obtained by chloramine-T iodination method. 8-anilino-1-naphthalene-sulfonic acid was used as an agent to inhibit the binding capacity of serum TBG to rT₃. Polyethylene glycol was utilized for separation of the antigen-antibody complex from the free antigen.
Normal serum values of rT₃ detected in 126 healthy persons, including men and women, were found to be 37.3±6.3 ng/dl （X±SD） with a range of 26—58 ng/dl.
Recovery tests expressing the accuracy of the method were determined to be 93.5—105% with a mean value of 100.8%.
Precision as the variation coefficients of interassay and intraassay were 2.4% and 6.8% respectively. The least detectable amount was 6.25 ng/dl of rT₃. The cross reaction of T₄ to anti-rT₃ serum was only 0.077% of the binding reactivity of rT₃ to antirT₃ serum and that of T₃ was still lower. Even when the concentration of T₃ was up to 8,000 ng/dl, no cross reaction was observed.