Radioactive iodine uptake test
Encyclopedia
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.
The patient swallows 100-200 microcuries of radioactive iodine (131I sodium iodide) in the form of capsule or fluid, and its absorption by the thyroid is studied after 6 hours and after 24 hours with the aid of a scintillation counter
.
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. Low uptake suggests thyroiditis
, high uptake suggests Graves' disease
, and unevenness in uptake suggests the presence of a nodule.
The test is inappropriate for patients who are pregnant or breastfeeding.
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. Also, I-123 emits gamma radiation, while I-131 emits gamma and beta radiation.
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
problems, particularly hyperthyroidism
Hyperthyroidism
Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...
. 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.
The patient swallows 100-200 microcuries of radioactive iodine (131I sodium iodide) in the form of capsule or fluid, and its absorption by the thyroid is studied after 6 hours and after 24 hours with the aid of a scintillation counter
Scintillation counter
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
.
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
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
, such as dairy products and seafood. Low uptake suggests thyroiditis
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.-Classification:...
, high uptake suggests Graves' disease
Graves' disease
Graves' disease is an autoimmune disease where the thyroid is overactive, producing an excessive amount of thyroid hormones...
, and unevenness in uptake suggests the presence of a nodule.
The test is inappropriate for patients who are pregnant or breastfeeding.
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. Also, I-123 emits gamma radiation, while I-131 emits gamma and beta radiation.