Thyroid uptake scan: what happens on the day, and what the report says

Short answer: A thyroid uptake scan is really two tests that are often done together. You swallow a small dose of radioactive iodine. The uptake test then measures how much of it your thyroid takes in, as a percentage, usually 4 to 6 hours later and again at 24 hours, per MedlinePlus (reviewed January 2026). The scan takes pictures of where the iodine collects. NIDDK says the uptake test can help find the cause of an overactive thyroid.

If a scan just showed up on your order

If your doctor said "let's get an uptake and scan" and you left with a referral slip and no idea what happens next, this one is for you.

It's a reasonable thing to feel unsure about. The name has "radioactive" in it, MedlinePlus says you come back twice after swallowing the tracer, and the instructions can arrive on a sheet from a department you've never been to. You're allowed to want to know what you're walking into.

Uptake test, scan and ultrasound, side by side

The uptake test and the scan answer two different questions. RadiologyInfo, the patient site of the American College of Radiology and the Radiological Society of North America, puts it this way: the uptake "is a measurement of thyroid function, but does not involve imaging." The scan is the picture.

A thyroid ultrasound often comes up in the same conversation, so it's in the table too.

An ultrasound gives a picture of the gland, but RadiologyInfo says it can't measure how the gland is working. To check that, it says, your doctor may order a blood test or a radioactive iodine uptake test.

Why it gets ordered

MedlinePlus says the uptake test is often done "when blood tests of thyroid function show that you may have an overactive thyroid gland." So it often comes after the blood work.

What they're after is the cause. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says the uptake test "can help check thyroid function and find the cause of hyperthyroidism." An overactive thyroid has more than one possible cause. NIDDK's hyperthyroidism page names Graves' disease, overactive thyroid nodules and thyroiditis among them.

The scan helps tell Graves' from nodules by pattern. NIDDK's Graves' page puts it this way: "With Graves’ disease, the iodine shows up throughout the gland." With other causes such as nodules, it says, the iodine shows up in a different pattern.

It isn't always the first step. The American Thyroid Association (ATA) says a positive Graves' antibody test (TRAb or TSI) confirms Graves' without further testing. If that test is negative or unavailable, it says, "then your doctor should refer you to have a radioactive iodine uptake test (RAIU) to confirm the diagnosis." If you'd like the background on the two autoimmune conditions, see Graves' vs Hashimoto's.

If you have Hashimoto's, the diagnosing pages don't list this test. Neither NIDDK's nor the ATA's Hashimoto's page names an uptake test or scan. They list blood tests, thyroid antibodies and, sometimes, an ultrasound. The ATA's thyroiditis guide does list the uptake test for other kinds of thyroiditis, including postpartum thyroiditis. For postpartum thyroiditis, its table adds that the uptake test is "contraindicated if the hypothyroid woman is breast-feeding." If an overactive spell came up with your Hashimoto's, we wrote about the overactive phase.

Getting ready: what centers may ask about

Your instructions come from the team doing the test. MedlinePlus, RadiologyInfo and NIDDK each word the asks a little differently, so check every line against your own sheet.

• Anything with iodine in it. MedlinePlus asks you to tell your provider if you take anything that contains iodine: "This includes some medicines, including thyroid and heart medicines. Supplements such as kelp also contain iodine." RadiologyInfo adds seaweed, cough syrups and multivitamins to the list.
• Recent scans with contrast dye. MedlinePlus asks about a CT scan with iodine-based contrast in the past 2 weeks. RadiologyInfo, reviewed June 2026, asks about the last two months. The windows differ, so ask your center which one it uses.
• How far ahead, for iodine. NIDDK's thyroid tests page (last reviewed 2017) says your health care professional may ask you to avoid foods high in iodine, such as kelp, or medicines containing iodine for a week before the test. It's the only source here that gives a number.
• Food before the test. MedlinePlus says you may be told not to eat after midnight. RadiologyInfo says you may be told not to eat for several hours, because eating can affect the uptake reading.
• Your medicines. MedlinePlus says your provider will tell you whether to stop any. Its own line: "Do not stop taking any medicine without first talking to your provider."

Things to check on your center's sheet:

• Which medicines and supplements the center needs to know about.
• How far back it asks about contrast dye.
• When to stop eating, and when you can eat again.
• Which of the two tests you're booked for, or whether it's both.
• The time of each visit.

Your own questions for your doctor are yours to write down.

What happens on the day

MedlinePlus, RadiologyInfo and NIDDK describe the same basic order, starting with the tracer. Your center may run it a little differently.

1. You swallow the tracer. It's usually a pill or a liquid. The ATA says I-123 "is the usual form used to diagnose thyroid problems since it does not damage cells." MedlinePlus says you can eat again about 1 to 2 hours after swallowing it.
2. The first reading, a few hours later. MedlinePlus says it's usually done 4 to 6 hours after the pill. MedlinePlus has you lying on your back while a probe is moved over your neck. RadiologyInfo and NIDDK have you sitting in a chair, facing a probe in front of your neck. RadiologyInfo says each uptake reading takes five minutes or less.
3. The scan, if you're having one, on the same day or the next. MedlinePlus says the first scan is usually done 4 to 6 hours after the iodine pill and another 24 hours later. The ATA says you come back the next day for the scan. You lie on a table while a camera takes pictures of your neck. RadiologyInfo says the scanning itself takes 30 minutes or less.
4. The second reading, the next day. MedlinePlus says another measurement is usually done 24 hours later. Some centers only measure at 24 hours, it adds.
5. The report goes to your doctor. RadiologyInfo says a radiologist or nuclear medicine doctor reads the images and sends a report to the doctor who referred you.

Some scans use technetium instead of iodine, per MedlinePlus. NIDDK says the tracer can also be injected into a vein, and the scan then takes place about 30 minutes later.

How the report describes results

Your report will have your center's own numbers and wording. MedlinePlus and the ATA describe the reading and the picture in terms you may see on it.

The uptake number. MedlinePlus gives normal results of 5% to 15% at 6 hours and 10% to 30% at 24 hours. In the same breath it says values vary with the iodine in your diet and that "Normal value ranges may vary slightly among different labs." The range printed on your own report is the one that counts.

Higher uptake. MedlinePlus says higher-than-normal uptake may be due to an overactive thyroid gland, and that the most common cause is Graves' disease.

Lower uptake. MedlinePlus lists several possible causes, including subacute thyroiditis, silent (painless) thyroiditis, too much iodine, amiodarone, and taking too much thyroid hormone. The ATA's thyroid tests page links a low uptake with an underactive gland. The two pages don't list the same things, which is one more reason the reading belongs to your doctor.

The picture. MedlinePlus describes a normal scan as "an even gray color on the computer image without darker or lighter areas." MedlinePlus says nodules are often called hot or cold:

• Hot. "Nodules that are darker have taken up more iodine (often called a hot nodule)," per MedlinePlus. It says they can be overactive.
• Cold. "A nodule is usually lighter if it has not taken up the iodine (often called a cold nodule)," also per MedlinePlus.

What any of this says about you is your doctor's call. They read it next to your blood tests and your history, and that's the conversation to have.

Can it find thyroid cancer?

It isn't the main test for that. MedlinePlus lists checking for thyroid cancer among the scan's uses, but adds "rarely, since other tests are more accurate for this."

The ATA says nuclear scanning "is no longer considered a first-line method of evaluation" for thyroid nodules, because ultrasound and biopsy have proven so accurate. It says scanning still has a role for the rare nodules that cause an overactive thyroid. NIDDK says that if a nodule is found, you may have a fine needle aspiration biopsy to see whether it's cancerous. If your nodules were found on an ultrasound, we went through reading the ultrasound report.

Is it safe, and what about pregnancy?

MedlinePlus says the amount of radiation in the uptake test is very small and "there have been no documented side effects." The ATA says no special radiation precautions are needed after I-123. RadiologyInfo suggests drinking plenty of water afterward to help the tracer leave your body.

This test isn't radioactive iodine treatment. The ATA explains that treatment uses I-131 to destroy overactive thyroid tissue, and it comes with precautions for the people around you afterward. The diagnostic dose is a different thing.

Pregnancy and breastfeeding are the big exception. MedlinePlus is direct: "Women who are pregnant or breastfeeding should not have this test." RadiologyInfo says the scan and uptake aren't performed on patients who are pregnant and aren't recommended for breastfeeding women. NIDDK says you shouldn't have the test if you're pregnant or breastfeeding. If there's any chance you're pregnant, or you're breastfeeding, tell your doctor and the imaging team before the day. NIDDK says that for Graves', a Doppler ultrasound may be ordered "if radioactive iodine uptake is not a good option for you, such as during pregnancy or breastfeeding."

What to write down

MedlinePlus says you come back twice after swallowing the tracer, so there are several dates to keep, plus the blood work and notes from the weeks around them.

• The dates. When you swallowed the tracer, and when each reading and scan happened.
• What your center asked of you. Which foods, supplements or medicines you were asked about, and the dates they gave you. Write it as they said it.
• Your blood results from around that time. TSH, free T4 and any antibody tests, each with its date and the range your lab printed. Those are often what led to the scan in the first place. Our full thyroid panel explainer covers what those tests are.
• How you felt in the weeks before. Whatever you noticed, dated, in your own plain words.
• The exact words on the report. Your uptake numbers, the range printed beside them, and words like hot or cold. Then the questions you want to ask about them.
• Your own questions, in your words. Write them before the follow-up, so they don't get lost in the room.

Where Osmi fits

Osmi doesn't diagnose, and it doesn't read scans. What it keeps is the blood work around them.

Every blood result sits on its own card, shown against the range your own lab printed, with the day the blood was drawn. Photograph the paper, upload the PDF or type one number. Osmi shows what's changed since the last draw. Then your record goes onto one page you can share with your practitioner in a tap, with the three questions you wrote yourself.

Start with the result that came before the scan. That's the first card.

Questions women ask about thyroid uptake scans

What foods should I avoid before a thyroid uptake scan? Ask your center, because its sheet is the one that applies to you. NIDDK's thyroid tests page, last reviewed 2017, says your health care professional may ask you to avoid foods high in iodine, such as kelp, for a week before. RadiologyInfo lists kelp, seaweed, cough syrups and multivitamins among the things that contain iodine. MedlinePlus says you may be asked not to eat after midnight before the test.

How long does a thyroid uptake scan take? MedlinePlus says the test takes less than 30 minutes, but you come back twice after swallowing the tracer. The first reading is usually 4 to 6 hours later and the second about 24 hours later, it says. RadiologyInfo says each uptake reading takes five minutes or less, and the scan itself 30 minutes or less.

Is a thyroid uptake scan the same as radioactive iodine treatment? No. The scan uses a small diagnostic dose, and the ATA says I-123, the usual form for diagnosis, doesn't damage cells. Treatment uses I-131 to destroy overactive thyroid tissue, and the ATA lists precautions to protect the people around you afterward. MedlinePlus says the uptake test has had no documented side effects.

Can I have a thyroid uptake scan while breastfeeding? The sources say no. MedlinePlus states that women who are pregnant or breastfeeding should not have this test. RadiologyInfo says the tests aren't recommended for breastfeeding women, and NIDDK says not to have them while breastfeeding. NIDDK says that for Graves' disease, a doctor may order a Doppler ultrasound instead during pregnancy or breastfeeding. Tell your doctor and the imaging team before the day if either applies to you.

Do I need an uptake scan if I have Hashimoto's? NIDDK and the ATA don't list one for diagnosing Hashimoto's. They describe blood tests, including thyroid antibodies, and sometimes an ultrasound. The test often comes up when blood work points to an overactive thyroid. If it's on your order, ask your doctor what question it's meant to answer.

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Educational, not medical advice.