The radioactive iodine uptake test is a type of scan used in the diagnosis of

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans, it is a butterfly-shaped gland located in the neck below the Adam's apple. It consists of two connected lobes. The lower two thirds of the lobes are connected by ...

problems, particularly

hyperthyroidism Hyperthyroidism is a endocrine disease in which the thyroid gland produces excessive amounts of thyroid hormones. Thyrotoxicosis is a condition that occurs due to elevated levels of thyroid hormones of any cause and therefore includes hyperth ...

. 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 still be cancerous.
ThyCa: Thyroid Cancer Survivors' Association, Inc., Radioactive Iodine (RAI). The patient swallows Isotopes of iodine#Notable radioisotopes, a radioisotope of

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

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 A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the Electron excitation, excitation effect of incident radiation on a Scintillation (physics), scintillating material, and detecting the resultant li ...

. The dose is typically 0.15–0.37 MBq (4–10 μCi) of ¹³¹I iodide, or 3.7–7.4 MBq (100–200 μCi) of ¹²³I iodide. The RAIU test is a reliable measurement when using a dedicated probe with a reproducibility of 1 percent and a 95%-least-significant-change of 3 percent. 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

, such as dairy products and seafood.M. Sara Rosenthal. ''The Thyroid Sourcebook.'' McGraw-Hill, 2008. Page 140. Low uptake suggests

thyroiditis Thyroiditis is the inflammation of the thyroid gland. The thyroid gland is located on the front of the neck below the laryngeal prominence, and makes hormones that control metabolism. Thyroiditis is a group of disorders that all cause thyroi ...

, high uptake suggests

Graves' disease Graves' disease, also known as toxic diffuse goiter or Basedow's disease, is an autoimmune disease that affects the thyroid. It frequently results in and is the most common cause of hyperthyroidism. It also often results in an enlarged thyro ...

,WebMD article
on RAIU test. and unevenness in uptake suggests the presence of a

nodule Nodule may refer to: * Nodule (geology), a small rock or mineral cluster * Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, ...

. ¹²³I has a shorter

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''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. Furthermore, ¹²³I emits

gamma radiation A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

, while ¹³¹I emits gamma and

beta radiation A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and � ...

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Contraindications

The test is inappropriate for patients who are pregnant or breastfeeding.

Additional images

File:Thyroid scintigraphy.jpg, Thyroid scintigraphy

References

{{Medical imaging Diagnostic endocrinology Nuclear medicine Thyroid disease Thyroidological methods