Two Different Tests, Two Different Questions
If your doctor has found a thyroid abnormality or your blood tests show thyroid dysfunction, you may need imaging to understand what is happening. The two most common thyroid imaging studies are thyroid ultrasound and thyroid nuclear scan. They are not interchangeable. Each one answers a fundamentally different question about your thyroid gland.
Thyroid ultrasound asks: What does the thyroid look like? What is the structure?
Thyroid nuclear scan asks: How is the thyroid functioning? What is it doing?
Understanding this distinction helps you make sense of why your doctor ordered a particular test, and why sometimes both are needed.
What a Thyroid Ultrasound Shows
A thyroid ultrasound uses sound waves to create images of the thyroid gland. It is painless, involves no radiation, and is performed by placing a probe on the front of your neck. The test takes about 15 to 20 minutes.
Ultrasound excels at showing structural detail:
- Size: Is the thyroid enlarged (goitre)?
- Nodules: Are there nodules? How many? How large? What do they look like?
- Characteristics of nodules: Are they solid or fluid-filled (cystic)? Do they have calcifications? Are the margins regular or irregular?
- Lymph nodes: Are the nearby lymph nodes in the neck normal in appearance?
Radiologists use standardised scoring systems (such as TI-RADS) to assess whether thyroid nodules look suspicious and whether a biopsy (fine needle aspiration) is recommended. Ultrasound is the primary tool for evaluating thyroid nodules and guiding biopsy decisions.
However, ultrasound cannot tell you how a nodule is functioning. It shows anatomy, not physiology.
What a Thyroid Nuclear Scan Shows
A thyroid nuclear scan (also called a thyroid scintigraphy) uses a small amount of radioactive tracer, either Tc-99m pertechnetate or radioactive iodine (I-123 or I-131), which is taken up by thyroid tissue. After the tracer is administered (usually injected for Tc-99m or swallowed as a capsule for radioactive iodine), images of the thyroid are acquired with a gamma camera.
The scan shows how the thyroid tissue is functioning:
- Overall uptake: Is the thyroid taking up more tracer than normal (overactive) or less (underactive)?
- Distribution: Is the uptake uniform across the gland, or are there focal areas of increased or decreased activity?
- Nodule function: Critically, the scan shows whether a thyroid nodule is “hot,” “warm,” or “cold.”
Hot, Warm, and Cold Nodules
This is where the thyroid scan provides information that no other test can:
- Hot nodule: Takes up more tracer than the surrounding thyroid tissue. This means the nodule is overactive, producing thyroid hormone independently. Hot nodules are almost always benign. They are a common cause of hyperthyroidism.
- Warm nodule: Takes up tracer at the same rate as surrounding tissue. These are also usually benign.
- Cold nodule: Takes up less tracer than surrounding tissue. Cold nodules are not necessarily cancerous, but a higher proportion of cold nodules are malignant compared to hot or warm ones. A cold nodule often warrants further evaluation, typically with ultrasound-guided biopsy.
When Your Doctor Orders a Thyroid Ultrasound
Ultrasound is usually the first imaging test ordered when:
- A nodule is felt during a physical examination
- A nodule is incidentally found on another imaging study (CT, MRI, or PET scan)
- There is a need to assess the size and characteristics of known nodules
- A biopsy is being planned and ultrasound guidance is needed
- Follow-up monitoring of previously identified nodules
- Evaluation of an enlarged thyroid (goitre)
When Your Doctor Orders a Thyroid Nuclear Scan
A thyroid scan is typically ordered in more specific clinical situations:
- Hyperthyroidism: When blood tests show an overactive thyroid (low TSH, high T3/T4), the scan helps determine the cause. Is it Graves’ disease (diffuse increased uptake), a toxic nodule (focal hot spot), or thyroiditis (low uptake)?
- Nodule with low TSH: If you have a thyroid nodule and your TSH is low or suppressed, the scan determines whether the nodule is hot (autonomously functioning). If it is, biopsy is generally not needed because hot nodules are almost always benign.
- Post-thyroid cancer surgery: Radioactive iodine whole-body scans are used to detect residual or recurrent thyroid cancer tissue after thyroidectomy.
- Evaluating thyroid tissue in unusual locations: Such as substernal goitre or lingual thyroid.
Can You Need Both?
Yes. The tests complement each other. A common scenario: a patient has a thyroid nodule and mildly suppressed TSH. The ultrasound characterises the nodule’s structure and size. The thyroid scan determines whether it is hot or cold. Together, these results guide the next step. A hot nodule on scan, even if it looks mildly suspicious on ultrasound, is almost certainly benign and typically does not need a biopsy. A cold nodule with suspicious ultrasound features likely warrants biopsy.
Your endocrinologist or thyroid surgeon will decide which tests you need based on your clinical picture, blood work, and examination findings.
Discussing Your Results
Neither test alone gives the complete picture. Thyroid imaging results are always interpreted in the context of your symptoms, blood tests (TSH, T3, T4, thyroid antibodies), and clinical examination. Your doctor will explain what the findings mean for your specific situation and what, if any, next steps are needed.
For more information about how thyroid nuclear scans are performed, visit our thyroid scan page.