What are a thyroid uptake test and scan?
Thyroid cells use iodine to make thyroid hormone. A thyroid uptake test and scan use a small amount of a radioactive tracer so a nuclear medicine team can measure this activity and create an image of the gland.
- Thyroid uptake test: Measures how much tracer the thyroid collects over a defined period.
- Thyroid scan: Shows where the tracer is distributed within the thyroid.
The tests are often performed together, but they answer different questions. Blood tests, symptoms, medical history, and other imaging remain important parts of the diagnosis.
Iodine-123, or I-123, is commonly used for thyroid imaging. Radioactive iodine treatment generally uses iodine-131, or I-131, at a therapeutic activity intended to damage thyroid cells. The purpose, activity, preparation, and safety instructions are different.
Why might these tests be ordered?
A clinician may order an uptake test or scan to help determine why thyroid hormone levels are high. The tests can provide functional information that an ultrasound cannot.
They may help distinguish Graves' disease, one or more autonomously functioning thyroid nodules, thyroid inflammation, or iodine-related causes of hyperthyroidism. They are not required for every patient with abnormal thyroid tests or every thyroid nodule.
What should patients discuss before the test?
Tell the ordering clinician and nuclear medicine team about:
- Pregnancy, possible pregnancy, or breastfeeding
- Recent CT contrast or other iodine exposure
- Thyroid medicines and other prescription medicines
- Vitamins, supplements, kelp, or products containing iodine
- Previous thyroid procedures or radioactive iodine treatment
Some medicines, supplements, foods, and recent contrast studies can affect the result. Do not stop a prescribed medicine unless the treating team gives specific instructions.
What happens during testing?
The tracer may be swallowed as a capsule or liquid. The facility then measures how much tracer the thyroid has collected. Measurements are often made several hours after the tracer and again the following day, although protocols vary.
For the scan, a gamma camera or probe detects the tracer from outside the body. The patient may need to sit or lie still with the neck positioned for imaging. The detector does not produce the radiation and the imaging itself should not be painful.
The nuclear medicine facility should provide the exact schedule, preparation instructions, and any instructions to follow after the test.
How are the results interpreted?
The amount and pattern of uptake are interpreted together with thyroid blood tests and the clinical history.
- Diffuse increased uptake may support Graves' disease.
- Focal or uneven uptake may identify one or more autonomously functioning nodules.
- Low uptake may occur with thyroiditis, recent iodine exposure, or other causes in which the thyroid is not actively producing excess hormone.
A scan pattern is not interpreted by itself and does not replace biopsy when a nodule meets appropriate criteria for tissue sampling.
Radiation and safety
Diagnostic thyroid testing uses a small amount of radioactive material, but it should not be described as completely harmless. The ordering clinician and nuclear medicine team should consider whether the expected diagnostic benefit justifies the exposure.
Radioactive iodine uptake testing is generally not performed during pregnancy or breastfeeding. The instructions following a diagnostic test depend on the tracer and activity used, so patients should follow the facility's directions rather than assume that no precautions are needed.
A previous reaction to seafood or iodinated CT contrast is not the same as an allergy to radioactive iodine. Patients should still tell the clinical team about all previous reactions and allergies.
Questions to ask the clinical team
- Why is this test useful in my situation?
- Am I having an uptake measurement, a scan, or both?
- Which tracer will be used?
- Should I change any medicines, supplements, or foods beforehand?
- Could recent contrast imaging affect the result?
- When will measurements and images be taken?
- Do I need to follow any precautions afterward?
- How will the result affect the next step in my care?
Sources and medical review
Medically reviewed by Richard B. Guttler, MD, FACP, FACE, ECNU · September 2026
Dr. Guttler's 2010 video documents historical laboratory procedures, including checking a shipment for leakage, verifying an administered activity, checking for contamination, preparing a capsule, and positioning a patient. It is not being treated as current operating or safety guidance.