A thyroid nuclear scan is a two-visit imaging test that uses a small amount of radioactive tracer to create a picture of your thyroid gland and measure how actively it absorbs iodine. The entire process, from swallowing the tracer to completing the scan, typically spans about 24 hours across two separate appointments. It’s one of the primary tools for figuring out why your thyroid is overactive and whether nodules in the gland are producing too much hormone.
Why the Scan Is Ordered
The main reason for a thyroid nuclear scan is to determine the cause of hyperthyroidism. Blood tests can tell your doctor that your thyroid hormone levels are high, but they can’t explain why. The scan reveals a distinct pattern for each condition: Graves’ disease lights up the entire gland uniformly, a toxic multinodular goiter shows scattered bright patches, and a single overactive nodule appears as one hot spot with the rest of the gland suppressed. Thyroiditis, an inflammation that temporarily dumps stored hormone into the bloodstream, looks different from all three because the gland barely absorbs any tracer at all.
The scan also helps evaluate thyroid nodules found on ultrasound. A nodule that absorbs more tracer than surrounding tissue (a “hot” nodule) is almost always benign. A nodule that doesn’t absorb tracer (a “cold” nodule) isn’t necessarily cancerous, but it does warrant further evaluation, usually with a biopsy.
Preparing in the Days Before
Your preparation starts well before you arrive. The tracer works by mimicking iodine, so anything that floods your body with regular iodine will interfere with the results. You’ll need to follow a low-iodine diet for at least one week beforehand, keeping your daily iodine intake below 50 micrograms. That means avoiding iodized salt, dairy products, seafood, seaweed, egg yolks, and many processed foods that contain iodized salt as an ingredient. Use non-iodized salt for cooking during this period.
Certain medications also need to be paused. Thyroid hormone replacement like levothyroxine requires a 4 to 6 week washout, while the faster-acting liothyronine needs 10 to 14 days. Any supplements or medications containing iodine should be stopped at least a week ahead. Your doctor will give you a specific timeline based on what you’re taking. Don’t stop any medication without their guidance, since going weeks without thyroid hormone replacement requires medical oversight.
Which Tracer Is Used
Two tracers are commonly used, and the choice depends on what your doctor needs to see. Iodine-123 is the more common option for a full uptake and scan. It behaves almost identically to the natural iodine your thyroid absorbs, which makes it especially accurate for measuring gland function. You swallow it as a capsule.
The other option, technetium-99m, is injected into a vein rather than swallowed. It produces images faster, sometimes within 20 to 30 minutes, but it only shows whether tissue is trapping the tracer, not whether the thyroid is fully processing it the way it would with real iodine. In a study comparing both tracers in 122 patients, image quality was similar about 42% of the time, but there were meaningful discrepancies in a third of cases, particularly nodules that appeared active on technetium but inactive on iodine-123. Neither tracer is universally better, so the choice depends on the clinical question being asked.
What Happens at Each Visit
First Visit: Taking the Tracer
If your scan uses iodine-123, the first visit is brief. You’ll swallow a small capsule containing the radiotracer, and then you’re free to leave. There’s nothing to feel from the capsule itself. You’ll be asked to return 4 to 6 hours later for the first round of imaging, and again at 24 hours for a second measurement. Some facilities combine these into fewer visits depending on the protocol.
Second Visit: The Imaging
Right before imaging begins, you’ll be asked to drink some water to clear any residual tracer from your mouth and throat. Then you’ll lie on your back on an exam table with your neck extended slightly backward, sometimes with a small support under your shoulders. A technologist may use gentle head restraints to help you stay still.
A gamma camera, which looks like a large flat panel or cone-shaped detector, is positioned close to your neck. It doesn’t emit radiation. Instead, it detects the tiny amount of radiation coming from the tracer your thyroid has absorbed and converts it into an image. The camera captures an anterior (front) view of your neck, and additional images may be taken from different angles. The whole imaging session takes about 30 minutes or less. You won’t feel anything during the scan, though holding your neck in an extended position can get mildly uncomfortable.
How Uptake Is Measured
Beyond the visual image, the scan produces a number called the uptake percentage. This tells your doctor how much of the tracer your thyroid absorbed compared to the total dose you were given. Normal uptake at 6 hours falls between 7% and 15%. At 24 hours, the normal range is 10% to 30%.
A number above that range typically points to an overactive gland, as in Graves’ disease or a toxic nodule. A number below the range, sometimes near zero, suggests thyroiditis or excess iodine intake suppressing the gland. This quantitative measurement is what makes the nuclear scan different from an ultrasound, which shows structure but not function.
Understanding the Images
The scan produces a color-coded or grayscale map of tracer distribution across your thyroid. Areas that absorb more tracer appear brighter (“hot”), while areas absorbing less appear darker (“cold”). A healthy thyroid shows fairly even uptake across both lobes.
Hot nodules are producing thyroid hormone independently of the body’s normal signals. They’re almost always benign. Cold nodules aren’t functioning normally, and while most are also benign (cysts, colloid nodules, or areas of inflammation), they carry a higher statistical chance of being cancerous and typically need a fine-needle biopsy for further evaluation. Occasionally, a patient will have both hot and cold nodules at the same time, which complicates interpretation.
After the Scan
For diagnostic scans using iodine-123 or technetium-99m, the radiation dose is very small and clears your body quickly. Most people can resume normal activities immediately. Drinking plenty of fluids helps flush the remaining tracer through your kidneys faster.
The precautions are more significant for therapeutic doses of iodine-131, which is a different procedure used to treat thyroid cancer or severe hyperthyroidism. In that case, the radiation dose is much larger, and you’d need to limit close contact with young children and pregnant women for a period of time afterward. Children’s thyroid glands are smaller, so the same amount of radioactive iodine delivers a proportionally larger dose to their tissue. For a standard diagnostic scan, though, these precautions are minimal or unnecessary. Your facility will give you specific instructions based on the tracer and dose used.
Results are typically available within a few days. Your doctor will interpret the images alongside your blood work and any ultrasound findings to determine the next step, whether that’s medication, radioactive iodine therapy, or biopsy of a suspicious nodule.