Radioactive Iodine Uptake Test – Indications, Procedures, Results

Last updated: February 8, 2026 Medically reviewed by: RxHarun Orthopedic Specialist Doctor Reading time: 4 min read

Iodine uptake test; RAIU

Radioactive iodine uptake (RAIU) tests thyroid function. It measures how much radioactive iodine is taken up by your thyroid gland in a certain time period.

A similar test is the thyroid scan . The 2 tests are commonly performed together, but they can be done separately.

How the Test is Performed

The test is done in this way:

You are given a pill that contains a tiny amount of radioactive iodine. After swallowing it, you wait as the iodine collects in the thyroid.
The first scan is usually done 4 to 6 hours after you take the iodine pill. Another scan is usually done 24 hours later. During the scan, you lie on your back on a table. A device called a gamma probe is moved back and forth over the area of your neck where the thyroid gland is located.
The probe detects the location and intensity of the rays given off by the radioactive material. A computer displays images of the thyroid gland.

The test takes about 30 minutes.

How to Prepare for the Test

Follow instructions about not eating before the test. You may be told not to eat after midnight the night before your test.

Your health care provider will tell you if you need to stop taking medicines before the test that may affect your test results. DO NOT stop taking any medicine without first talking to your provider.

Tell your provider if you have:

Diarrhea (may decrease absorption of the radioactive iodine)
Had recent CT scans using intravenous or oral iodine-based contrast (within the past 2 weeks)
Too little or too much iodine in your diet

How the Test will Feel

There is no discomfort. You can eat beginning about 1 to 2 hours after swallowing the radioactive iodine. You can go back to a normal diet after the test.

Why the Test is Performed

This test is done to check thyroid function. It is often done when blood tests of thyroid function show that you may have an overactive thyroid gland.

Normal Results

These are normal results at 6 and 24 hours after swallowing the radioactive iodine:

At 6 hours: 3% to 16%
At 24 hours: 8% to 25%

Some testing centers only measure only 24 hours. Values may vary depending on the amount of iodine in your diet. Normal value ranges may vary slightly among different labs. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

Higher-than-normal uptake may be due to an overactive thyroid gland. The most common cause is Graves disease.

Other conditions can cause some areas of higher-than-normal uptake in the thyroid gland. These include:

An enlarged thyroid gland that contains nodules producing too much thyroid hormone ( toxic nodular goiter )
An single thyroid nodule that is producing too much thyroid hormone (toxic adenoma)

These conditions often result in normal uptake, but the uptake is concentrated into a few (hot) areas while the rest of the thyroid gland does not take up any iodine (cold areas). This can only be determined if the scan is done along with the uptake test.

Lower-than-normal uptake may be due to:

Factitious hyperthyroidism (taking too much thyroid hormone medicine or supplements)
Iodine overload
Subacute thyroiditis (swelling or inflammation of the thyroid gland)
Silent (or painless) thyroiditis
Amiodarone (medicine to treat some kinds of heart disease)

Risks

All radiation has possible side effects. The amount of radiation in this test is very small, and there have been no documented side effects. The amount of iodine used is less than the amount in a normal diet.

People with an allergy to iodine in the diet or to shellfish may not be able to have this test. You may be able to have this test if you have an allergy to the iodine in the contrast dye.

Women who are pregnant or breastfeeding should not have this test.

Talk to your provider if you have concerns about this test.

Considerations

The radioactive iodine leaves your body through your urine. You may need to take special precautions, such as flushing twice after urinating, for 24 to 48 hours after the test. Ask your provider or the radiology/nuclear medicine team performing the scan about taking precautions.

Frequently Asked Questions

Is this article a replacement for a doctor?

No. It is educational content only. Patients should consult a qualified clinician for diagnosis and treatment.

When should I seek urgent care?

Seek urgent care for severe symptoms, rapidly worsening condition, breathing difficulty, severe pain, neurological changes, or any emergency warning sign.

References

Add references, clinical guidelines, textbooks, journal articles, or trusted medical sources here. You can edit this area later with a custom field named _rx_references.

Written by Dr. Harun Ar Rashid, MD - Arthritis, Bones, Joints Pain, Trauma, and Internal Medicine Specialist

Dr. Md. Harun Ar Rashid, MPH, MD, PhD, is a highly respected medical specialist celebrated for his exceptional clinical expertise and unwavering commitment to patient care. With advanced qualifications including MPH, MD, and PhD, he integrates cutting-edge research with a compassionate approach to medicine, ensuring that every patient receives personalized and effective treatment. His extensive training and hands-on experience enable him to diagnose complex conditions accurately and develop innovative treatment strategies tailored to individual needs. In addition to his clinical practice, Dr. Harun Ar Rashid is dedicated to medical education and research, writing and inventory creative thinking, innovative idea, critical care managementing make in his community to outreach, often participating in initiatives that promote health awareness and advance medical knowledge. His career is a testament to the high standards represented by his credentials, and he continues to contribute significantly to his field, driving improvements in both patient outcomes and healthcare practices. Born and educated in Bangladesh, Dr. Rashid earned his BPT from the University of Dhaka before pursuing postgraduate training internationally. He completed his MD in Internal Medicine at King’s College London, where he developed a special interest in inflammatory arthritis and metabolic bone disease. He then undertook a PhD in Orthopedic Science at the University of Oxford, conducting pioneering research on cytokine signaling pathways in rheumatoid arthritis. Following his doctoral studies, Dr. Rashid returned to clinical work with a fellowship in interventional pain management at the Rx University School of Medicine, refining his skills in image-guided joint injections and minimally invasive pain-relief techniques.