Radioisotopes and radioimmunoassay (RIA) !

Radioisotopes are unstable atoms that give off radiation and can be traced. Radioimmunoassay used them to measure tiny amounts of hormones.

Updated 2026-11-01

Key facts

Isotopes have the same number of protons but different neutrons; radioactive ones decay, emitting α, β or γ radiation, with a fixed physical half-life.
Common tracers: ³H and ¹⁴C (metabolic research), ³²P (nucleic acids), ¹²⁵I (RIA), ¹³¹I (thyroid uptake and therapy), ⁹⁹ᵐTc (scans), ¹⁸F-fluorodeoxyglucose (PET).
RIA (Yalow and Berson; Nobel Prize 1977): a competitive assay; a fixed amount of radiolabelled antigen competes with the patient's unlabelled antigen for a limited amount of antibody.
In RIA, the more antigen in the sample, the less labelled antigen stays bound: bound radioactivity is inversely related to the concentration, read from a standard curve.
Non-radioactive immunoassays (ELISA, chemiluminescence) now replace most RIAs, with the same antibody principle and no radiation waste.

1. Tracers follow the same chemistry as the normal atom, so they reveal pathways (for example ¹⁴C-labelled glucose turning into CO₂ or fat).
2. Half-life sets the use: short-lived ⁹⁹ᵐTc (about 6 h) suits diagnostic scans; ¹³¹I (about 8 days) suits thyroid therapy.
3. RIA steps: mix antibody + labelled antigen + sample → incubate → separate bound from free antigen (second antibody, charcoal, solid phase) → count radioactivity → read the standard curve.
4. RIA was the first method sensitive enough (pico- to nanomolar) to measure insulin, thyroid hormones, cortisol, ACTH and drugs in blood.
5. Immunoradiometric assay (IRMA) is a 'sandwich' variant with labelled antibody: there, signal rises with concentration.
6. Radiation safety: shielding, distance, short exposure time and correct waste disposal; isotopes are avoided in pregnancy when possible.

Radioactive iodine uptake and thyroid scans separate Graves disease (high, diffuse uptake) from thyroiditis (low uptake).
¹³¹I destroys overactive thyroid tissue in Graves disease and residual tissue after surgery for thyroid cancer.
FDG-PET shows tissues with high glucose uptake, such as many tumours (the Warburg effect).

Think of the answer, then tap the question to see it.

In a radioimmunoassay, how does bound radioactivity change as the patient's hormone level rises?

It falls: the unlabelled hormone competes with the labelled one for the antibody, so less label is bound.

Who developed radioimmunoassay?

Rosalyn Yalow and Solomon Berson; Yalow received the 1977 Nobel Prize for it.

Why is technetium-99m useful for diagnostic scans?

It emits γ rays that a camera detects and has a short half-life of about 6 hours, giving little radiation dose.

How does an IRMA differ from a classic RIA?

IRMA is a sandwich assay with labelled antibody in excess; the signal rises with antigen concentration instead of falling.

Which isotope is used to treat hyperthyroidism and thyroid cancer remnants?

Iodine-131, which the thyroid concentrates and which destroys the tissue with β radiation.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Rosalyn Yalow – Facts - NobelPrize.org (NobelPrize.org (Nobel Prize Outreach), 2026); Radioactive iodine uptake: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Thyroid scan: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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