A thyroid uptake scan measures how actively your thyroid gland absorbs iodine, which directly reflects how hard the gland is working. It’s one of the most reliable ways to determine the cause of an overactive thyroid, distinguishing between conditions like Graves’ disease and thyroiditis that look similar on blood tests but require very different treatments. The test involves swallowing a small radioactive iodine capsule, then returning hours later so a specialized probe can measure how much of that iodine your thyroid has pulled from your bloodstream.
How the Test Works
Your thyroid is uniquely dependent on iodine to produce hormones. Specialized transporters on thyroid cells actively pull iodine from your blood and concentrate it to roughly 40 times the level found in your plasma. This natural iodine-trapping ability is what makes the uptake scan possible.
For the test, you swallow a capsule containing a small amount of radioactive iodine, most commonly iodine-123 (I-123). Your thyroid can’t distinguish this tracer from regular dietary iodine, so it absorbs the radioactive version the same way. A probe placed over your neck then detects the radiation coming from your thyroid, giving a precise measurement of how much iodine the gland has captured. The higher the percentage absorbed, the more actively your thyroid is working. In some cases, a different tracer called technetium-99m is used intravenously instead, which behaves similarly to iodine in the thyroid.
What Happens During the Appointment
The test usually requires two visits over 24 hours. At your first visit, you swallow the radioactive iodine capsule. That’s the only thing that happens at this point, and it takes just a few minutes.
You return 4 to 6 hours later for the first uptake measurement. You’ll lie on an exam table while a technologist holds a gamma probe near your neck. The probe doesn’t touch you and causes no pain. The actual scanning takes five minutes or less. Many facilities then have you come back again at the 24-hour mark for a second reading, since this later measurement gives the most accurate picture of total iodine absorption with less background interference.
If your provider also orders a thyroid scan (imaging) alongside the uptake measurement, a special camera captures a picture of your thyroid showing its shape, size, and which areas are most active. This combined “uptake and scan” is common, since the imaging adds useful information at no extra cost or radiation exposure beyond what’s already been given.
Why Your Doctor Ordered It
The most common reason for this test is to figure out why your thyroid hormone levels are elevated. Blood tests can confirm hyperthyroidism, but they can’t tell your doctor the underlying cause. That distinction matters because it changes the treatment plan entirely.
In Graves’ disease, the entire thyroid is overstimulated by abnormal antibodies, so it aggressively absorbs iodine. Uptake values in Graves’ disease are significantly elevated. In contrast, thyroiditis (inflammation of the thyroid) causes stored hormone to leak into the bloodstream, mimicking hyperthyroidism on blood tests. But because the thyroid isn’t actually producing new hormone, it has almost no need for iodine. Uptake in thyroiditis is extremely low, often near zero. This stark difference makes the uptake scan one of the best tools for telling these conditions apart.
The test can also help evaluate thyroid nodules. A nodule that absorbs more iodine than surrounding tissue is called a “hot” nodule and is almost always benign. A “cold” nodule that doesn’t absorb iodine needs further evaluation, since a small percentage of cold nodules turn out to be cancerous.
Preparing for the Test
Several things can interfere with your thyroid’s iodine absorption and skew results. Your provider will likely give you specific preparation instructions, and following them closely is important for an accurate test.
Low-Iodine Diet
You’ll typically need to follow a low-iodine diet for one to two weeks beforehand, limiting yourself to no more than 50 micrograms of iodine per day. This “starves” your thyroid of iodine slightly, so when you take the radioactive tracer, your gland absorbs it predictably. The diet is more restrictive than most people expect. Foods to avoid include:
- Dairy products: milk, cheese, yogurt, ice cream
- Seafood: ocean fish, shrimp, shellfish, seaweed, kelp
- Eggs
- Iodized salt and sea salt (and anything made with them, which includes most processed and restaurant food)
- Processed meats: bacon, ham, sausage, hot dogs, deli meats
- Soy products: soy sauce, soy milk, tofu
- Chocolate and molasses
- Canned fruits and vegetables (fresh or frozen are fine)
- White bread made with iodate dough conditioners (wheat or rye bread is generally okay)
- Vitamins or supplements containing iodine or kelp
- Red-dyed foods, pills, or capsules (red dye may contain iodine)
Fresh meat, rice, fresh fruits and vegetables, and drip or percolated coffee are all fine. The general rule: avoid restaurant food, read every label, and skip processed or frozen meals.
Medications to Discuss
Certain medications can dramatically affect iodine uptake. Thyroid medications, iodine-containing supplements, and some heart and contrast dye medications can all interfere. Your doctor will tell you which to pause and for how long before the test. Do not stop any medication on your own without guidance.
What Results Mean
Results are reported as the percentage of the radioactive iodine dose your thyroid absorbed at each time point. Normal 24-hour uptake generally falls between about 10% and 30%, though reference ranges vary slightly between labs and regions depending on local dietary iodine levels.
High uptake points to conditions where the thyroid is overproducing hormones. Graves’ disease produces the most dramatically elevated numbers, often well above the normal range, with the entire gland lighting up uniformly on imaging. A toxic nodule or toxic multinodular goiter also raises uptake, but the scan image shows concentrated activity in one or a few spots rather than the whole gland.
Low uptake in someone with high thyroid hormone levels is the hallmark of thyroiditis. It tells your doctor that the excess hormone in your blood is leaking from damaged or inflamed thyroid tissue, not being freshly manufactured. This pattern is also seen after excessive iodine intake or in people taking thyroid hormone medication they may not need.
Safety and Radiation Exposure
The radiation dose from a diagnostic thyroid uptake scan is small. I-123, the most commonly used tracer, delivers less radiation than I-131 and is preferred for routine diagnostic testing for that reason. The amount of radioactivity involved is far lower than what’s used in therapeutic radioactive iodine treatments for thyroid cancer or hyperthyroidism.
The test is not performed during pregnancy because any radioactive iodine can cross the placenta and affect the developing fetal thyroid. If you are breastfeeding, you’ll need to pump and discard breast milk for a period after the test (typically around 24 to 48 hours depending on the tracer used) since radioactive iodine is excreted in breast milk. Beyond pregnancy and breastfeeding, there are no absolute contraindications to the scan. Most people experience no side effects from the tracer capsule itself.