Thyroid Diagnostic Testing
Even though cancerous nodules are rare, it is extremely important to have available the best diagnostic tests in order to detect those nodules that are malignant. The different diagnostic tests for the thyroid are listed below.
A blood test is usually the first test conducted to monitor thyroid function and the production of thyroid hormones. TSH is the most sensitive index of thyroid function and stimulated the thyroid gland to produce thyroid hormone. Low TSH indicates that the thyroid is producing too much hormone while an elevated TSH means too little thyroid hormone is being produced. Therefore, high levels of thyroid hormones and a low TSH indicate hyperthyroidism while low levels of thyroid hormones and a high TSH are indicative of hypothyroidism. If an autoimmune thyroid condition is suspected (Hashimoto's thyroiditis), then anti-thyroid peroxidase and antithyroglobulin antibody measurements in the blood may be made to confirm the diagnosis. Other thyroid hormones that can be measured in the blood include: T3, T4, thyroid peroxidase (TPO), thyroid antibody (Tgab), TSH receptor antibody, and thyroid binding immunoglobulin.
In addition, measurements of thyroglobulin may be performed. Such a measurement is very useful in monitoring individuals who have had their thyroid removed for thyroid cancer since it is an excellent tumor marker for papillary and follicular cancers of the thyroid gland.
For the rare medullary carcinoma case, calcitonin can be measured. It is a protein made by special cells of the thyroid gland called C-cells. When these cells become malignant, they can produce large amounts of calcitonin. This blood test is used diagnostically and as a tumor marker for patients with medullary thyroid carcinoma.
Your endocrinologist will work closely with your particular case to determine which blood tests are right for you.
Computed Tomography (CT) Scan
In computed tomography (CT), digital geometry processing creates a 3-D image of the interior of the body from a large number of 2-D X-ray images. CT data is then manipulated through a process called "windowing" that illuminates different angles of an organ or structure by blocking X-ray beams, much the same way a camera creates an image by blocking out light. Scanners then enable the data to be reformatted as a 3-D scan.
CT scans are especially valuable in detecting cancers, infection, heart disease, brain disorders, abnormal blood flow, or organ, blood cell or bone disorders.
Fine Needle Aspiration
For nodules that are larger than one centimeter or nodules that have suspicious characteristics based on an ultrasound, a fine needle aspiration of the nodule is recommended.
A fine needle aspiration (FNA) biopsy is best carried out under ultrasound guidance. A small needle, generally smaller than those used for drawing blood, is placed directly into the nodule and multiple cells or fluid is withdrawn from the nodule. Specially trained pathologists interpret the tissue and determine whether the cells are benign, malignant or atypical. Ultrasound-guided FNA usually yields adequate specimens in 85 percent of individuals. For those 15 percent of individuals where the specimens are deemed inadequate, repeat aspiration is required. Newer molecular diagnostic tests can provide further information to clarify the diagnosis. Your endocrinologist will work closely with you and refer you to molecular diagnostic testing if needed.
Certain tumors, for example medullary carcinoma of the thyroid, are associated with well-defined genetic alterations. It is important to determine if an individual with medullary carcinoma has a genetic alteration. There is a strong possibility it may be transmitted to their children who should also be tested. Cedars-Sinai experts then perform analysis of the RET oncogene. In addition, we have the capability of performing genetic analysis on thyroid cancer specimens that are removed in order to profile them, which may be helpful for determining the degree of aggressiveness of the tumor.
Learn more on genetic testing available at Cedars-Sinai Medical Genetics Institute.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) is a medical imaging technique that produces detailed images of the body in any plane, illuminating structures and function with a high degree of clarity. It is able to create greater contrast between soft tissues than computed tomography (CT). MRI also uses a strong magnetic field in conjunction with radiofrequency to create a rotating magnetic field, detected by a scanner, which produces an image of the body.
Nuclear Medicine Scan
Nuclear medicine uses small amounts of radiopharmaceuticals to view the structure and functions of organs. Combining chemistry, physics, mathematics, computer technology, and medicine, nuclear medicine technology can often detect abnormal growths on tissues very early. Nuclear medicine scanning creates contrast through the amount of radioactive pharmaceuticals absorbed by an organ.
Positron Emission Tomography (PET) & Computerized Tomography Scan
PET and CT are standard imaging tools that help physicians accurately determine the location of any malignancy. Of the two, a PET scan is more sensitive to the metabolic signals emanating from cancer cells. A CT scan can then create an accurate picture of its size, shape and location. Together, these two scanning technologies provide a complete picture of a tumor's location and metabolism.
Radioactive Iodine Scan
A radioactive iodine scan is a three-step diagnostic process for evaluating thyroid nodules, hyperthyroidism or goiters. It helps your physician identify the size, shape, location and physiology of these nodules.
- Phase 1: Patient swallows a radioactive iodine capsule (I-123).
- Phase 2: Four hours later, a thyroid uptake measurement is taken, as well as images of the thyroid gland and a nuclear medicine physician examines the thyroid.
- Phase 3: Twenty-four hours after taking the capsule, a second uptake measurement of the thyroid gland is taken.
A diagnosis can be made based on both the iodine uptake at the two time periods and the thyroid scan.
The initial diagnostic test to evaluate a thyroid nodule is a high resolution ultrasound of the thyroid. The ultrasound will show the characteristics of the nodule including smoothness of the edges of the nodule, whether the nodule has calcifications, whether there are multiple nodules or a single nodule, and whether the nodule(s) has benign characteristics suspicious for malignancy.
An ultrasound uses cyclic sound pressure, higher than the upper limit of human hearing (approximately 20 kilohertz or 20,000 hertz) to illuminate details about the inner structure of organs or tissues. For a thyroid ultrasound, a transducer (like a small microphone) is placed at the neck, from which sound waves pass into the skin. As the transducer converts the echoes into electronic signals, images of the neck appear on a television monitor for physician review. An ultrasound of the neck is a painless procedure that takes about 10 minutes.
Urinalysis effectively determines if the body is excreting an excess of calcium or iodine. Iodine is a key ingredient in the production of two thyroid hormones: T3 and T4. A deficiency in iodine can cause hypothyroidism, as does excessive iodine intake.
Thyrogen Stimulation Testing
The Food and Drug Administration (FDA) has approved recombinant human TSH (rhTSH) for both diagnostic testing and for therapy. Recombinant TSH is used for diagnostic testing when evaluating for recurrence of thyroid cancer. It is also used therapeutically to destroy the residual normal thyroid tissue left after thyroid surgery as well as to help destroy any residual thyroid cancer following such surgery. Diagnostic testing with use of rhTSH is performed in an outpatient setting over five days:
- Day 1 - Blood draw for thyroglobulin followed by thyrogen injection.
- Day 2 - Second thyrogen injection.
- Day 3 - The patient may undergo a PET/CT scan (without iodine contrast material) and/or receive a tracer dose of radioactive iodine for a nuclear medicine whole body scan.
- Day 5 - More blood drawn for thyroglobulin to see if there is a change after stimulation by rhTSH. If radioactive iodine was given, a total body scan is performed to see what tissues take up the radioactive iodine. This test is used diagnostically to see if there is any residual thyroid tissue, and to localize the tissue.