Radioactive iodine uptake test

Radioactive iodine uptake test
Diagnostics
Thyroid scan with Iodine-123 for evaluation of hyperthyroidism.
ICD-9-CM	92.01
OPS-301 code	3-701

The radioactive iodine uptake test, or RAIU test, is a type of scan used in the diagnosis of thyroid problems, particularly hyperthyroidism. It is entirely different from radioactive iodine therapy (RAI therapy), which uses much higher doses to destroy cancerous cells. The RAIU test is also used as a follow up to RAI therapy to verify that no thyroid cells survived, which could potentially still be cancerous.^[1]

The patient swallows radioactive iodine in the form of capsule or fluid, and its absorption by the thyroid is studied after 4–6 hours and after 24 hours with the aid of a scintillation counter. The dose is typically 0.15–0.37 MBq (4–10 μCi) of ¹³¹I sodium iodide, or 3.7–7.4 MBq (100–200 μCi) of ¹²³I sodium iodide.^[2]

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.^[3] Low uptake suggests thyroiditis, high uptake suggests Graves' disease, and unevenness in uptake suggests the presence of a nodule.^[4]

I-123 has a shorter half life than I-131 (a half day vs. 8.1 days), so use of I-123 exposes the body to less radiation, but at the expense of less time to evaluate delayed scan images.^[5] Also, I-123 emits gamma radiation, while I-131 emits gamma and beta radiation.^[6]

Contraindications

The test is inappropriate for patients who are pregnant or breastfeeding.^[4]

Additional images

• Thyroid scintigraphy

References