The radioactive iodine uptake test is a type of scan used in the diagnosis of thyroid problems, particularly hyperthyroidism.

The RAIU test is also used as a follow-up to RAI therapy to verify that no thyroid cells survived, which could still be cancerous.

[1] The patient swallows a radioisotope of iodine in the form of capsule or fluid, and the absorption (uptake) of this radiotracer by the thyroid is studied after 4–6 hours and after 24 hours with the aid of a scintillation counter.

[3] The normal uptake is between 15 and 25 percent, but this may be forced down if, in the meantime, the patient has eaten foods high in iodine, such as dairy products and seafood.

[citation needed] 123I has a shorter half-life than 131I (a half day vs. 8.1 days), so use of 123I exposes the body to less radiation, at the expense of less time to evaluate delayed scan images.