What is a radionuclide scan?
The thyroid gland takes up iodine from our bodies and synthesizes thyroid hormones. The distribution of iodine in the thyroid gland reflects the function and shape of the thyroid gland and helps your doctor determine lesions.
A radionuclide scan, also called a “thyroid scan,” is one of the most important methods for diagnosing thyroid disease. It takes advantage of the “iodine uptake” of the thyroid gland. After injecting a radionuclide (called a “developer”, usually iodine or technetium), the thyroid is scanned and the doctor can determine the nature of the nodule by comparing the density of the developer absorbed by the nodule with that of a normal gland.
Radioactive iodine (I-131) is the most commonly used imaging agent. It differs from normal iodine in that it spontaneously emits radiation during decay, which is then detected by the instrument and forms the image of the thyroid scan. Another commonly used imaging agent is technetium (TcO4). It is a congener of iodine and can also be taken up by the thyroid gland, but the specificity of the image is not as high as that of I-131.
Nuclear imaging is usually used to evaluate thyroid nodules larger than 1 cm in diameter.
When do I need a radionuclide scan?
Currently, there is very limited use of radionuclide scans for the diagnosis of thyroid nodules. It is usually not needed if the thyroid nodule is small (<1 cm), or if the ultrasound clearly indicates benign or strongly suspects malignancy.
In general, doctors consider this test only if a thyroid nodule is accompanied by a lower-than-normal serum thyroid stimulating hormone (TSH) level.
If the test suggests that the nodule is capable of picking up contrast, also known as a “hot nodule,” it is very likely to be benign, and a fine needle aspiration biopsy is not usually needed.
In addition, because radionuclide scans can specifically show thyroid tissue, they can be used to locate metastases throughout the body when distant metastases from thyroid cancer occur.
How do the radionuclide imaging results look?
There are a number of terms that are used professionally to describe thyroid nodules as hot, warm, cool, and cold, depending on their ability to “take up” iodine.
“Hot nodules”
Iodine uptake is strongest and manifests as a concentrated image of the developer.
This nodule is very unlikely to be malignant and is usually seen in cases of functionally autonomous thyroid adenomas, congenital absence of one lobe of the gland and compensated function of the opposite side.
“Warm nodules”
The iodine uptake capacity of the nodule is about the same as normal thyroid tissue. These nodules are also generally not malignant and are commonly seen in conditions such as functioning thyroid adenomas, nodular goiters, and thyroiditis.
“Cool nodules” and “cold nodules”
Nodules have lower iodine uptake than normal thyroid tissue, with “cold nodules” being essentially non iodine uptake.
These two types of nodules can be thyroid cancer, but they can also be thyroid cysts, cystic thyroid adenomas, or chronic lymphocytic thyroiditis.
In fact, most thyroid nodules present as “cool” or “cold” nodules, which need to be further determined with ultrasound findings and, if necessary, a fine needle aspiration to confirm the diagnosis.
Will this test expose me to dangerous radiation?
Usually, this test is safe for most people.
But pregnant and breastfeeding women should not have this test because small amounts of I-131 can pass through the placenta into the bloodstream of the fetus and can be secreted through breast milk, potentially affecting the growth and development of the fetus or baby.
Co-written by Dr. Naisi Huang, Cancer Hospital of Fudan University