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Women's Health

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Radioactive Iodine Uptake Test

How It Feels

You may find it uncomfortable to lie still with your head tipped backward.


There is a slight chance of damage to cells or tissue from radiation, including the low levels of radiation used for this test. But the chance of damage from the radiation is usually very low compared with the benefits of the test.

This test is not done for pregnant women because of the chance of exposing the baby (fetus) to radiation. This test is also not recommended for breast-feeding women or young children.


A radioactive iodine uptake (RAIU) test uses a radioactive tracer and a special probe to measure how much tracer the thyroid gland absorbs from the blood. The radioactive tracer used in this test is iodine. An RAIU test is done to check for thyroid gland problems, such as hyperthyroidism.

Radioactive thyroid scan and radioactive iodine uptake test (RAIU)

The amount of radioactive tracer in the thyroid gland is normal. An RAIU test measures the amount of tracer taken up by the thyroid gland at certain times after the tracer is given. The measured amount of radioactive tracer in the thyroid gland at each one of these times is at normal levels.


The test shows either more or less uptake of tracer than normal in the thyroid gland. If hyperthyroidism is present, abnormal test results may mean certain conditions are present.

  • A low uptake of tracer by the thyroid gland may mean that hyperthyroidism is caused by inflammation of the thyroid gland (thyroiditis), taking too much thyroid medicine, or another rare condition.
  • A high uptake of tracer spread evenly in the thyroid gland may mean that hyperthyroidism is caused by conditions such as Graves' disease.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Taking thyroid medicine.
  • Eating foods with iodine, such as shellfish, iodized salt, or kelp.
  • Having other tests using contrast materials in the past 4 weeks.

What To Think About

WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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