How is Thyroid cancer diagnosed?

Detecting and diagnosing thyroid cancer

A cancer on the rise

Thyroid cancer is a cancerous tumor or growth (nodule) located within the thyroid gland.  Thyroid cancer is uncommon, accounting for only about 1 out of every 100 cancers in the United States.  For reasons that are not quite clear, the incidence of thyroid cancer in women is rising faster than any other cancer in the United States. Some think that the rising incidence is due to the accidental detection and early diagnosis of very small thyroid cancers with widespread use of radiology studies of the head and neck. Other researchers are worried that there is as yet some unknown cause for the rise in thyroid cancer cases.

In 2016, at least 64,000 new cases of thyroid cancer were diagnosed and treated. Because of the excellent overall survival after treatment for thyroid cancer, there are at least 600,000 thyroid cancer survivors living in the United States.

Of these thyroid cancers, 65% to 80% are diagnosed as papillary thyroid cancer, 10% to 15% are follicular, 5% to 10% are medullary, and 3% to 5% are anaplastic.  If you or someone you know has been diagnosed with thyroid cancer, you will be glad to know that the outlook with treatment is usually excellent – most thyroid cancers can be totally removed with surgery.

Thyroid cancer is diagnosed about three times more often in women than men. The reason for this higher rate of thyroid cancer in women is unclear.

How is thyroid cancer diagnosed?

In the past thyroid cancer was often discovered by patients themselves. You may see or feel a lump or nodule on the front of your neck, or your doctor may notice a nodule during a routine physical examination.  But today, it is more common for a thyroid cancer to be incidentally identified on a CT/MRI or neck ultrasound done for some reason unrelated to the thyroid.

The most common initial finding is the appearance of a painless lump in the lower anterior neck in the region of the thyroid gland. In most cases the thyroid function is normal when measured by blood tests.

Usually the diagnosis of thyroid cancer is suspected because a nodule or mass is detected in the front of the neck. In most cases, a needle biopsy of the nodule is needed to obtain cells for careful evaluation under a microscope. In most cases, microscopic analysis of the cells obtained from a needle biopsy can readily determine if a nodule is benign (not cancer) or malignant (cancer). While thyroid blood tests are usually done to evaluate the function of the thyroid, and a thyroid ultrasound is often done to evaluate the structure of the thyroid gland, neither of these types of tests are sufficient to confidently determine if a thyroid nodule is benign or malignant.

What is a thyroid nodule?

A thyroid nodule is a collection of cells within the thyroid that grow and produce a lump. Sometimes these lumps can be felt by physical examination of the thyroid gland, but oftentimes they are detected as an incidental finding on radiology studies (such as ultrasound, MRI, or CT scan) done for an unrelated reason. Fortunately, about 90-95% of thyroid nodules are benign (not cancer).

Are thyroid nodules common?

Yes, thyroid nodules can be detected with ultrasonography in nearly 40- 50% of otherwise completely healthy adults. Most nodules are so small that they are never noticed by the patient or doctor.  However, the wide spread use of CT, MRI, and  neck ultrasonography has led to the detection of many very small, asymptomatic thyroid nodules that may or may not require further evaluation. 

What is a fine needle aspiration (FNA) biopsy?

If the nodule is large enough to warrant additional evaluation, a fine needle aspiration biopsy (FNAB) is often used to learn whether a thyroid nodule is benign or cancerous. With this test, a very small needle is inserted through the skin into the thyroid nodule in order to remove samples of tissue or fluid, which are then analyzed in a lab. The test is fast, safe, and usually causes little discomfort with some patients reporting a feeling of pressure to the area during the procedure.

Do all thyroid nodules require fine needle aspiration biopsy?

No. As a general rule, thyroid nodules less than 1 cm (approximately ½ inch) can be followed with observation without the need for fine needle aspiration. These small nodules are often found incidentally on CT, MRI or neck ultrasound done for some other reason. They are very common and are rarely thyroid cancer. Therefore, in the absence of other high risk features, these small nodules are usually observed with a repeat thyroid ultrasound in 6-12 months reserving biopsy for those few nodules that increase in size over time

The 2016 ATA guidelines also note that a biopsy may not be required for nodules as large as 2 cm if the ultrasonographic features suggest that the nodule is not likely to be thyroid cancer.

What is the role of molecular testing in the evaluation of thyroid nodules?

The last 10 years has seen an explosion in our understanding of the molecular basis of thyroid cancer.  This improved understanding has led to the development of several molecular tests that can provide clinically useful information with regard to whether a thyroid nodule is likely to be benign or cancerous.  These tests are always used within the context of an understanding of the risk that a nodule is likely to be cancer based on ultrasonographic findings, clinical findings, and the results of the fine needle aspiration biopsy.

When the FNA classifies a nodules as benign, malignant or suspicious for malignancy, the molecular testing probably plays very little role.  However, when the FNA biopsy is read as inconclusive (indeterminate, follicular neoplasm, suspicious for follicular neoplasm, atypia of undetermined significance, follicular lesion of undetermined significance), molecular testing has been shown to be of help in determining if that nodule is more likely to be benign or malignant.

Can thyroid cancer be detected with a blood test?

No. Despite extensive research, there is no single blood test that can accurately detect or diagnose thyroid cancer. The usual thyroid function tests are almost always normal in patients with thyroid cancer. Therefore, normal thyroid blood tests do not rule out a thyroid cancer.

Are radioactive iodine thyroid scans used to diagnose thyroid cancer?

If the thyroid blood tests are normal, radioactive iodine scans are seldom used in the United States in the evaluation of thyroid nodules. Radioactive iodine scans of the neck will document the location and general size of the isotope-concentrating thyroid but not as precisely as will an ultrasound. The portion of the gland which does not concentrate the radioisotope will not be visualized. It does provide a measure of the gland’s ability to “pick-up” or concentrate the radioactive isotope, a gross measure of thyroid function.

Most thyroid tumors, benign and malignant, will not concentrate the isotope but, on the contrary, a small portion of tumors that do so may be malignant. Thus, the radioactive isotope scan provides little help in distinguishing between benign and malignant tumors.