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Radioimmunoassay (RIA) is a scientific method used to test antigens (for example, hormone levels in the blood) without the need to use a bioassay. It involves mixing known quantities of radioactive antigen (frequently labeled with gamma-radioactive isotopes of iodine attached to tyrosine) with antibody to that antigen, then adding unlabeled or "cold" antigen and measuring the amount of labeled antigen displaced.

Initially, the radioactive antigen is bound to the antibodies. When "cold" (unlabeled, quest) antigen is added, the two compete for antibody binding sites - at higher concentrations of "cold" antigen, more of it binds to the antibody, displacing the radioactive variant. The bound antigens are separated from the unbound ones.

One method of separation used initially was the use of a second antibody directed against the first for precipitation and centrifugation. The use of charcoal suspension for precipitation was extended but replaced later by Drs. Werner and Acebedo at Columbia University for RIA of T3 and T4.[1] An ultramicro RIA for human TSH was published in BBRC (1975) by Drs. Acebedo, Hayek et al.[2] The free hormone stays in the supernatant, the radioactivity of which is measured and a binding curve is plotted.

The technique is both extremely sensitive and specific, but it requires special precautions (because radioactive substances are used), sophisticated apparatus, and is expensive. The technique has been largely replaced by the ELISA method where instead of a radioactive signal, the antigen-antibody reaction is measured by colorometric signals.

The RIA was developed by Rosalyn Yalow and Solomon Aaron Berson in the 1950s.[3] Rosalyn Sussman Yalow received the 1977 Nobel Prize in Medicine for the development of the RIA for insulin. The precise measurement of minute amounts of such a hormone was a key event in endocrinology.


  1. Werner SC, Acebedo G, Radichevich I (1974). "Rapid radioimmunoassay for both T4 and T3 in the same sample of human serum". J. Clin. Endocrinol. Metab. 38 (3): 493–5. PMID 4815178.
  2. Acebedo G, Hayek A, Klegerman M, Crolla L, Bermes E, Brooks M (1975). "A rapid ultramicro radioimmunoassay for human thyrotropin". Biochem. Biophys. Res. Commun. 65 (2): 449–56. PMID 1148002.
  3. Yalow RS, Berson SA. Immunoassay of endogenous plasma insulin in man. J Clin Invest 1960;39:1157-75. PMID 13846364.

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