A thyroid scan is a nuclear medicine imaging test that uses a small amount of radioactive material to create a picture of your thyroid gland and show how it’s functioning. Unlike an ultrasound, which only reveals the shape and size of the gland, a thyroid scan shows which parts of your thyroid are actively absorbing iodine and producing hormones. It’s most commonly ordered when blood tests suggest your thyroid is overactive or when a nodule needs to be evaluated for its hormone-producing behavior.
How a Thyroid Scan Works
Your thyroid gland naturally absorbs iodine from your bloodstream and uses it to make thyroid hormones. A thyroid scan takes advantage of this by giving you a small dose of radioactive iodine in pill form. The tracer travels through your bloodstream, and your thyroid absorbs it just like regular iodine. A special camera (called a gamma camera) then detects the radiation coming from your thyroid and produces an image showing exactly where and how much of the tracer your gland has taken up.
This is often paired with a radioactive iodine uptake test (RAIU), which measures the percentage of the tracer your thyroid absorbs at specific time points. Normal uptake is 5% to 15% at 6 hours and 10% to 30% at 24 hours. Higher-than-normal uptake points toward conditions where the thyroid is overproducing hormones. Lower-than-normal uptake suggests inflammation or damage to the gland.
Why Your Doctor Ordered One
Thyroid scans serve two main purposes: figuring out why your thyroid is overactive, and evaluating what a thyroid nodule is doing. If your blood work shows a low TSH level (the hormone that tells your thyroid to work), that’s a signal your thyroid may be producing too much on its own. A scan helps pin down the cause.
For hyperthyroidism, the scan can distinguish between Graves’ disease (where the entire gland lights up because it’s uniformly overactive) and a toxic nodule (where one spot is doing all the overproducing). It can also identify thyroiditis, an inflammation that temporarily dumps stored hormones into your blood but actually shows low uptake on the scan because the gland isn’t actively making new hormone. This distinction matters because each condition is treated very differently. A radionuclide uptake study is considered the preferred test for this purpose because it directly measures thyroid activity rather than inferring it from indirect signs like blood flow.
For nodules, the scan is specifically useful when your TSH is already low. The American Thyroid Association guidelines recommend scintigraphy primarily for cases with suppressed TSH, because finding that a nodule is autonomously producing hormones can spare you from needing a needle biopsy. If your thyroid function is normal, an ultrasound is typically the better first-line test for evaluating nodules.
Hot Nodules vs. Cold Nodules
The scan categorizes nodules based on how much tracer they absorb compared to the surrounding thyroid tissue. A “hot” nodule takes up more radioactive iodine than the tissue around it, meaning it’s actively churning out thyroid hormones on its own. Hot nodules are rarely cancerous. They’re typically benign growths that have essentially gone rogue, overproducing hormones independent of your body’s normal feedback signals.
A “cold” nodule takes up less iodine than the surrounding tissue, appearing as a gap or dim spot on the image. Cold nodules carry a higher concern for cancer, but the scan alone can’t tell you whether a cold nodule is malignant or not. Most cold nodules are still benign. If your scan reveals a cold nodule, your doctor will likely recommend a fine-needle aspiration biopsy (a quick procedure where a thin needle draws out a small sample of cells) to get a definitive answer.
How to Prepare
Preparation for a thyroid scan is more involved than most imaging tests because anything containing iodine can interfere with your thyroid’s ability to absorb the tracer, which skews the results.
- Low-iodine diet: For at least one week before the test, you’ll need to avoid high-iodine foods like table salt (iodized), seafood, and saltwater fish.
- Thyroid medications: Levothyroxine needs to be stopped 4 to 6 weeks before the test. Liothyronine requires a shorter pause of 10 to 14 days. Your doctor will give you specific guidance on this.
- Contrast dye: If you’ve had a CT scan or other imaging test that used iodine-based contrast dye, you’ll need to wait at least 6 weeks before a thyroid scan can be done accurately.
- Iodine-containing medications: Certain supplements and medications (including some cough syrups and heart medications) contain iodine and should be stopped at least a week beforehand.
Don’t stop any medication without your doctor’s guidance. Pausing thyroid medication for weeks can cause symptoms, so your care team will weigh the timing and may monitor you during that period.
What the Procedure Feels Like
The test itself is painless. You’ll swallow a pill containing the radioactive iodine tracer, then go home and return for imaging. The uptake measurement is typically done at 6 hours and again at 24 hours after swallowing the pill. During each visit, you’ll sit or lie still while a gamma camera is positioned close to your neck. The camera doesn’t touch you and doesn’t emit any radiation; it simply detects the tracer already in your thyroid. Each imaging session takes roughly 20 to 30 minutes.
The amount of radioactive iodine used is very small, and it’s cleared from your body relatively quickly. You won’t feel any effects from the tracer. There are no dietary restrictions after the scan, and you can resume normal activities immediately.
Pregnancy and Breastfeeding
Radioactive iodine crosses the placenta and can be absorbed by a developing baby’s thyroid, so thyroid scans are not performed during pregnancy. If there’s any chance you could be pregnant, a pregnancy test is typically done before the scan is scheduled.
For breastfeeding mothers, the situation requires careful timing. Radioactive iodine used in diagnostic scans (as opposed to iodinated contrast used in CT scans) can be excreted in breast milk, and guidelines generally call for temporarily stopping breastfeeding after the procedure. The exact duration depends on the specific tracer and dose used, so your nuclear medicine team will give you a specific timeline. This is a conversation worth having with your doctor before the test is scheduled so you can plan accordingly, including pumping and storing milk in advance if needed.
What Happens After the Results
Your results will show a combination of the uptake percentage and the scan image. High overall uptake with a uniformly bright gland points toward Graves’ disease. A single bright spot with the rest of the gland suppressed suggests a toxic adenoma, a benign nodule that’s overproducing hormones. Low overall uptake in someone with hyperthyroid symptoms typically indicates thyroiditis, where the gland is leaking stored hormone rather than actively overproducing it.
If the scan was done to evaluate a nodule, the key result is whether it’s hot or cold. A hot nodule generally means no biopsy is needed, since the risk of cancer is extremely low. A cold nodule usually triggers the next step: a biopsy guided by ultrasound. Results from a thyroid scan are typically available within a few days, and your doctor will use them alongside your blood work and ultrasound findings to map out a treatment plan.