Radiologic tests that use radioactive materials evaluate how the thyroid is functioning and create pictures of the gland to further pinpoint the cause of thyroid disorders. These tests may be recommended by your doctor to aid in proper diagnosis and treatment of thyroid disorders, such as hyperthyroidism and hypothyroidism.

RAIU Test

The thyroid uses iodine from food to make thyroid hormone. The RAIU test measures the amount of iodine the thyroid collects from the bloodstream. The test helps doctors evaluate how the thyroid is functioning and determine the cause of hyperthyroidism. The RAIU is not used to assess hypothyroidism.

For this test, the patient swallows a small amount of radioactive iodine in liquid or capsule form. After 4 to 6 hours and again at 24 hours, the patient returns to the testing center, where the doctor measures the amount of radioactive iodine taken up by the thyroid. The measurement is taken with a small device called a gamma probe, which resembles a microphone. The gamma probe is positioned near the patient’s neck over the thyroid gland. Measurement takes only a few minutes and is painless.

In the diagnosis of hyperthyroidism, a high RAIU reading usually indicates an overactive thyroid that produces too much thyroid hormone, as seen in Graves’ disease or toxic nodular goiter, an enlargement of the thyroid gland. A low RAIU reading suggests the thyroid is not overactive.

Thyroiditis may cause leakage of thyroid hormone and iodine out of the thyroid gland into the bloodstream, which can lead to high T4 levels. Because the thyroid is inflamed, it does not take up the radioactive iodine given as part of the RAIU test. Hyperthyroidism seen in Graves’ disease would be marked by high blood T4 and a high RAIU. In thyroiditis, temporary hyperthyroidism may exist because of the release of T4 into the blood, but the RAIU is low because of the inflammation. Temporary hyperthyroidism in thyroiditis is often followed by a period of hypothyroidism before the thyroid heals.

The radioactive compound used in the RAIU test is safe to ingest in the small amount given. Pregnant and nursing women should not undergo this test, however, because the radioactive material can travel across the placenta to the baby’s bloodstream or be transmitted to the baby via breast milk.

Thyroid Scan

A thyroid scan does not test thyroid function per se, but instead uses radioactive material to create a picture of the thyroid. The scan shows the size and shape of the thyroid and provides images of irregularities such as nodules. Nodules are tumors in the thyroid that can either be benign or cancerous and can sometimes produce excess thyroid hormone. The scan does not identify whether the nodules are benign or cancerous.

For the scan, radioactive iodine or radioactive technetium is either injected into the patient’s vein or swallowed in liquid or capsule form. The scan takes place 30 minutes after an injection or 6 to 24 hours after the radioactive substance is swallowed. The patient lies on an examining table for the scan, which takes about 30 minutes. A device called a gamma camera is suspended over the table or may be located within a large, doughnut-shaped machine that resembles a computerized tomography (CT) scanner.

The gamma camera detects the radioactive material and sends images to a computer that show how and where the radioactive substance has been distributed in the thyroid. Nodules that produce excess thyroid hormone—so-called “hot,” or toxic, nodules—show up clearly because they absorb more radioactive material than normal thyroid tissue. Graves’ disease shows up as a diffuse, overall increase in radioactivity rather than a localized spot.

Suggested Readings
Tests for Thyroid Disorders
Thyroid Disorders Fact Sheet
Hyperthyroidism
Diagnostic Tests for Overactive Thyroid
Hypothyroidism

Source: Information Courtesy of the National Endocrine and Metabolic Diseases Information Service, National Institute of Diabetes and Digestive and Kidney Diseases

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