Cysts, which are fluid-filled, are usually benign, whereas solid nodules may be malignant. However, it is important to understand that an ultrasound showing a solid nodule is not sufficient to establish a diagnosis of thyroid cancer. This is because most solid nodules are benign. Nodules may be partially cystic or partially solid, and in either circumstance, may be benign or malignant.
In this test, your doctor will have you drink a solution containing the radioactive isotope iodine-131. Because thyroid cells use iodine to make thyroid hormone, the iodine-131 will be taken up by and concentrated in the thyroid gland. The radioiodine emits gamma rays. These gamma rays are counted and turned into an image by a “gamma camera” in a process known as scintigraphy. The gamma camera is placed at the neck to determine how much radioactivity is being emitted by the thyroid gland, and areas in which the radioactivity may be particularly high or low.
If a thyroid nodule takes up less radioiodine than surrounding thyroid tissue, it is called a cold nodule.
If a thyroid nodule takes up more radioiodine than surrounding thyroid tissue, it is then a hot nodule.
Hot nodules are not likely to be thyroid cancer. However, a cold nodule may be either benign or malignant.
Whole-body radioiodine scans may be useful in detecting metastases of thyroid cancer, because any thyroid-derived tissue, anywhere in the body, will show up on the scan. These scans are also frequently used to monitor a patient’s response to therapy. This technique is only useful in well-differentiated forms of thyroid cancer, such as papillary or follicular thyroid cancer, as they readily take up radioiodine. Poorly-differentiated forms of these cancers, as well as medullary or anaplastic thyroid cancers, do not take up radioiodine, and the scan therefore has no diagnostic value.
Positron Emission Tomography Scans
Posititron emission tomography (PET) scans use a modified sugar compound called fluorodeoxyglucose (FDG). The scan picks up areas of cells that preferentially absorb the FDG. PET scans may be useful with tumors that do not take up radioiodine, such as poorly-differentiated papillary and follicular thyroid cancers, in order to monitor for possible recurrence. In medullary thyroid cancer, a PET scan is not considered as sensitive as an octreotide scan, and is thus not the preferred test.
The gold standard in diagnosing thyroid cancer is fine-needle aspiration
(FNAB). In this test, a topical anesthetic, such as ethyl chloride, is first applied to the neck area. Sometimes, your doctor may inject a local anesthetic.
The material collected from the FNAB is then sent to a pathology laboratory. The pathologist examines the thyroid cells under a microscope for certain defining characteristics of malignancy.
This test measures thyroglobulin, a protein produced by the thyroid. It may be elevated in thyroid cancer. Testing for thyroglobulin is not used to establish a diagnosis of thyroid cancer, as many conditions may cause elevated thyroglobulin levels.
The test is used most often to check for recurrence or metastasis after surgical removal of the thyroid and radioiodine ablation (destruction of residual thyroid tissue). This is because after these treatments, there should be hardly any thyroid cells left to produce thyroglobulin, so if elevated levels are seen, it suggests that they have not been sufficiently eradicated, and the disease may have recurred and/or metastasized.
Thyroid stimulating hormone
This test measures blood levels of thyroid stimulating hormone (TSH) produced by the pituitary gland. TSH levels may be higher in patients with thyroid cancer. Just as with thyroglobulin, this is not diagnostic of thyroid cancer.
The importance of testing for TSH generally comes after the patient is treated for thyroid cancer with a thyroidectomy and radioiodine ablation. These patients are generally treated with T4 (thyroxine) in order to suppress TSH levels. High levels of TSH may stimulate thyroid tumor growth. Therefore, TSH suppression to a very low level should help reduce tumor growth and lessen the chance of recurrence or metastasis. Hence, your physician will regularly monitor your TSH blood levels to make sure the suppression therapy is working.
This test is used in patients with suspected medullary thyroid cancer. Because medullary thyroid cancer occurs in the C-cells of the thyroid, which produce the hormone calcitonin, increased calcitonin levels may be indicative of the disease. Very high levels of calcitonin are suggestive of advanced disease.
Carcinoembryonic antigen test
This is a blood test for carcinoembryonic antigen (CEA). CEA is produced by medullary thyroid cancer cells, so determining CEA levels may indicate the extent of the disease. This test is not diagnostic with other thyroid cancers.