immunoassay
An immunoassay measures a hormone by exploiting the immune system's most precise tool: the antibody. Antibodies are proteins that recognize one specific molecular shape and grip it tightly, like a key fitting a single lock. By manufacturing an antibody that latches onto the target hormone, scientists can fish that hormone out of a complicated mixture of blood and count how much they caught.
A common modern design is the 'sandwich' immunoassay: one antibody, fixed to a surface, captures the hormone, and a second antibody — carrying a reporter such as an enzyme, a fluorescent dye, or a glow-producing chemical — binds the other side. The amount of signal generated is proportional to how much hormone was present, so the machine reads light or color and converts it back into a concentration.
Immunoassays are fast, cheap, and automated, which is why they run the bulk of routine endocrine testing. Their main weakness is that the antibody can occasionally bind the wrong thing — a related molecule, a drug, or an interfering antibody in the patient's own blood — producing falsely high or low results. When a number does not fit the clinical picture, this cross-reactivity is one of the first culprits to consider.
For steroid hormones like testosterone and cortisol, liquid chromatography–mass spectrometry is increasingly preferred over immunoassay because it avoids antibody cross-reactivity and gives more accurate results at low concentrations.