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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# 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.

## In detail

- 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.

**Also called**
Radioactive tracers, Competitive immunoassay, Half-life

## Clinical notes

- 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).

## Pathways

- [Thyroid hormone synthesis](https://durgical.com/en/biochemistry/pathways/thyroid-hormone-synthesis/)

## Related conditions

- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Thyroid nodule](https://durgical.com/en/conditions/thyroid-nodule/)
- [Thyroid cancer (papillary)](https://durgical.com/en/conditions/thyroid-cancer/)

## Related

- [Spectrophotometry and colorimetry (Beer–Lambert law)](https://durgical.com/en/biochemistry/topics/spectrophotometry-and-colorimetry/)
- [Chromatography](https://durgical.com/en/biochemistry/topics/chromatography/)
- [Iodine](https://durgical.com/en/biochemistry/topics/iodine/)
- [ELISA (enzyme-linked immunosorbent assay)](https://durgical.com/en/biochemistry/topics/elisa/)
- [Thyroxine (T4)](https://durgical.com/en/biochemistry/molecules/thyroxine/)

## Test yourself

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.

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

Sources: [Rosalyn Yalow – Facts - NobelPrize.org](https://www.nobelprize.org/prizes/medicine/1977/yalow/facts/) (NobelPrize.org (Nobel Prize Outreach), 2026); [Radioactive iodine uptake: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003689.htm) (MedlinePlus, US National Library of Medicine, 2026); [Thyroid scan: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003829.htm) (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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