radioimmunoassay

Radioimmunoassay (RIA) was the breakthrough method that first made it possible to measure hormones present in vanishingly small amounts. The trick is a competition: a known quantity of the hormone tagged with a radioactive label is mixed with antibody, and the patient's unlabeled hormone competes with it for the limited antibody binding sites. The more hormone the patient has, the more radioactive tracer gets bumped off the antibody.

By measuring how much radioactivity stays bound versus how much is displaced, the lab back-calculates the patient's hormone concentration from a standard curve. Developed by Rosalyn Yalow and Solomon Berson in the late 1950s for insulin, RIA was so transformative that it earned Yalow a Nobel Prize and opened the entire field of quantitative endocrinology.

RIA is extremely sensitive but carries practical burdens: it needs radioactive reagents, special handling and disposal, and licensing. For these reasons most laboratories have shifted to non-radioactive equivalents that use enzymes, fluorescence, or chemiluminescence as the reporter instead of a radioisotope, retaining the competitive or sandwich principle while removing the radiation hazard.