Would a thyroid blood screen detect thyroid cancer? How is a nodule detected?

M.B.

The thyroid function tests that are ordered routinely measure the level of circulating thyroid hormone and the level of TSH (thyroid-stimulating hormone, a pituitary hormone that stimulates the thyroid gland to produce more thyroid hormone). These tests say nothing about whether a person has thyroid cancer. They are mostly useful for determining whether the thyroid gland is overactive or underactive.

But can any blood test detect thyroid cancer? A blood test may suggest the presence of cancer, but ultimately the diagnosis must be made using clinical and pathological evidence.

Here are some examples of clinical evidence: How does the thyroid gland feel? Is there a growing nodule on the gland? (This is NOT always a sign of cancer. In fact, most nodules are not cancerous.) Are there any signs of local invasion by a tumor? (For example, a thyroid tumor can erode into the recurrent laryngeal nerve, which supplies the vocal cords; destruction of this nerve can cause the voice to become breathy.)

Pathological evidence is TISSUE. One of the most common ways of obtaining tissue is with a fine needle aspiration biopsy. At times, removal of a portion of the thyroid gland is necessary, since a needle biopsy does not always provide sufficient information.

But back to the blood tests: There are two well-established blood tests that relate to thyroid cancer. One blood test measures the level of the protein thyroglobulin. Some thyroid cancers (papillary carcinoma and follicular carcinoma) produce large quantities of thyroglobulin. If this is true before cancer surgery, then by checking the thyroglobulin level after the surgery, the doctor can determine whether all of the cancer has been eliminated, or whether the cancer has recurred.

Can a doctor do this blood test to detect cancer? Unfortunately, many noncancerous thyroid problems lead to an elevated thyroglobulin level, so if the blood test detects a high level of thyroglobulin, this does NOT mean the patient has cancer.

Another useful blood test measures the level of the hormone calcitonin. This hormone is elevated in many patients with medullary carcinoma of the thyroid. Medullary carcinoma is an uncommon tumor that often has a familial (inherited) pattern. In such families, the blood test for calcitonin can be performed on unaffected family members as a screening procedure. As with thyroglobulin, calcitonin levels can be measured postoperatively to detect recurrence in patients who have had a thyroid removal.

Your second question is much easier. How is a nodule detected? Usually, a nodule is detected by palpation -- either you feel a lump or your doctor feels a lump. Unfortunately, the thyroid tends to be a lumpy gland in some folks. As I mentioned above (but this is worth repeating), most lumps are NOT cancers.

Ultrasound is another useful tool for studying nodules. Using ultrasound, a radiologist can measure the diameter of the nodule, providing hard, objective evidence for future reference. Doctors and patients are not very good at figuring out whether a lump is growing or shrinking based solely on the evidence of palpation. If growth (or shrinkage) is marked, the answer is obvious; but if the change is subtle, ultrasound provides a distinct advantage over palpation. Ultrasound may also reveal the presence of many other small nodules -- this is actually good news, since it suggests that the patient merely has a multinodular goiter, which is generally a benign condition.

CT and MRI are occasionally useful, but palpation and ultrasound are the primary tools of detection.