What Is a Radioactive Iodine Uptake Test?

Medically Reviewed by Sanjay Ponkshe on February 24, 2024
3 min read

A radioactive iodine uptake (RAIU) tests the function of your thyroid gland, which helps regulate your metabolism. It works by measuring how much radioactive iodine is absorbed by the thyroid gland, using a controlled amount of radioactive iodine in a specific time frame. 

A thyroid uptake test is another name that is used to describe an RAIU test. 

An RAIU is a nuclear test‌ that works by detecting molecular activity. Using radioactive material, the test can assess, diagnose, and treat certain diseases. It can also show if treatment is working in your body. Its results tend to be more accurate than other diagnostic tools, including exploratory surgery.

An RAIU test can produce results and invaluable information that is not easily accessible with other testing methods.

Make sure to speak with your doctor about any medications that you are taking.

You will need to fast for 8 hours before taking the radioactive iodine uptake test.

The first part of the test is swallowing radioactive iodine (I-131 or I-123) up to 24 hours before the test. Then the level of radioactivity in your thyroid gland will be measured at certain intervals hours later using a device called a gamma probe. The probe is placed over the area where the thyroid gland is located in your neck.

The original amount of thyroid hormone measured in the blood before the test is compared to the amount of radioactivity measured 6 to 24 after taking the radioactive iodine. This new amount is listed as a percentage of the original amount.

It takes about 30 minutes to complete each scan. While the scan is being done, you will be asked to sit on a clinic chair while the gamma probe is set on your neck. You should not feel any pain or discomfort at any time during this test. 

When the test is completely finished you can begin to eat as normal. 

Normal results. The following percentages show the average values according to the time of the test: 

6 hours from ingestion: 3% to 16%‌

24 hours from ingestion: 8% to 25%

Not all laboratories measure at the 6-hour mark. It is also important to note that there may be a change in results based on different ways that the laboratory processes the test. The amount of dietary iodine that is initially ingested can also affect the results. 

Abnormal results. If the results are abnormal, it means the following:


  • Goiter
  • Hyperthyroidism
  • Hashimoto’s thyroiditis (early)

If the test taken at 24 hours shows the amount of radioactivity to be greater than 35%, the reading is considered elevated. 


  • Subacute thyroiditis
  • Hypothyroidism
  • Excessive iodine ingestion

There haven't been any recorded side effects of an RAIU test.

There is a low amount of radiation exposure with the test. But if you are pregnant or breastfeeding, the RAIU is not recommended.  

It is not common to have an allergic reaction to an RAIU test. If they do happen, they are usually mild. Make sure to speak with your doctor about any issues you have experienced with previous nuclear medicine tests. It is also important to discuss any allergies that you have.