Thyroid nodules often are discovered by a physician during a routine physical examination. The physician may feel a lump in the neck or discover that the thyroid gland is enlarged. In other cases, nodules may be discovered when a patient is undergoing an imaging test (e.g., x-ray, ultrasound, CAT scan, MRI) related to another condition, such as spinal arthritis or carotid artery disease.

When a nodule is discovered, the physician is likely to order blood tests to determine whether the thyroid is functioning properly and if thyroiditis is present. However, most patients with thyroid nodules will have normal results from thyroid blood tests.

A physician may also take steps to determine whether a nodule is cancerous. A fine-needle aspiration biopsy is the primary test used to determine whether or not a nodule is cancerous. A very thin needle is inserted into the nodule to remove cells that are analyzed under a microscope for signs of cancer. A local anesthetic may be applied, and discomfort is minimal. Several tissue samples are typically obtained during the procedure, which takes about 20 minutes and usually is performed in a physician’s office.

In 75 percent of cases, a fine-needle aspiration biopsy provides enough information to formulate an accurate treatment plan, according to the American Association of Clinical Endocrinologists.

Between 60 and 80 percent of test results indicate that a nodule is benign, according to the American Cancer Society. About 10 percent of results indicate a suspicious nodule, which usually turns out to be either a noncancerous follicular adenoma or follicular cancer. Such suspicious nodules may require a thyroid scan before a more definitive diagnosis can be made.

In about 5 percent of cases, a nodule can be clearly identified as cancerous. Such nodules are often a form of papillary cancer.

In up to 20 percent of cases, a fine-needle aspiration biopsy fails to provide enough information to make a conclusive diagnosis. This is most likely when patients have fluid-filled nodules or nodules consisting of a follicular cell type. Repeat testing may reveal more clues as to the nature of the nodule. A surgical biopsy may also be performed. In other cases, physicians rely on other clues and their own expertise to make a judgment as to whether or not the nodule requires treatment.

Imaging tests that may be used to help diagnose whether or not a thyroid nodule is cancerous include:

• Thyroid scan. A test in which the patient is given a small dose of a radioactive isotope before the thyroid is scanned with a special camera. The image that is created reveals whether the thyroid nodule is functioning normally or if it shows signs of being cancerous. However, the thyroid scan is often inconclusive regarding whether or not a nodule is cancerous. For this reason, the technique is infrequently used for this type of diagnosis. Instead, the thyroid scan is used to provide additional information after a fine-needle aspiration biopsy indicates that a nodule is suspicious.
  
  
• Thyroid ultrasound. Procedure in which images are generated by bouncing high-frequency sound waves off the thyroid. Ultrasound can reveal the presence of thyroid nodules that are as small as 2 to 3 millimeters. However, as with a thyroid scan, ultrasound rarely provides enough information to confirm whether or not a nodule is cancerous. More often, ultrasound is used to help guide a needle into the nodule during a fine-needle biopsy. A thyroid ultrasound may also be used to monitor the potential growth of a nodule after a fine needle aspiration biopsy has determined that the nodule is benign.