The first test a health practitioner will usually order to detect thyroid dysfunction is a test for thyroid stimulating hormone (TSH). If the TSH level is abnormal, the health practitioner will usually order a test for free thyroxine (free T4) to confirm the diagnosis.
T4 is not very soluble in blood so it is carried by serum proteins. A very small percentage of the total T4 is not bound to protein and it is this fraction that is biologically active. In the past, laboratories estimated the concentration of free T4 by measuring the total T4 and also determining the level of binding proteins. Today, however, it is much more common for laboratories to estimate the free T4 directly using immunoassay.
- TSH – to test for hypothyroidism, hyperthyroidism, screen newborns for hypothyroidism, and monitor treatment for thyroid disorders
- Free T4 – to test for hypothyroidism, hyperthyroidism, screen newborns for hypothyroidism, and to monitor treatment of thyroid disease
- Free T3 – primarily to test for hyperthyroidism, especially when the free T4 is not elevated; when people are iodine-deficient, the thyroid makes much more T3 than T4.
Additional tests that may be performed include:
- Total T4 and total T3 may still be needed in some rare circumstances.
- Thyroid antibodies – to help differentiate different types of thyroiditis and identify autoimmune thyroid conditions
- Thyroid peroxidase (TPO) antibody—a marker for autoimmune thyroid disease; it can be detected in Graves disease or Hashimoto thyroiditis. It may be especially helpful in early Hashimoto thyroiditis when the TSH is elevated but the remaining thyroid is still able to maintain a normal free T4 level.
- Thyroglobulin (TG) antibody—also a marker for both Graves disease and Hashimoto thyroiditis; this antibody targets thyroglobulin, the storage form of thyroid hormones.
- Thyroid stimulating hormone receptor (TSHR) antibodies—a marker for Graves disease; these may be measured in two different ways:
- Thyroid stimulating immunoglobulin (TSI) assay measures the stimulation of thyroid cells in a culture dish.
- Thyroid binding inhibitory immunoglobulin (TBII) assay measures the ability of a person's serum to block TSH from binding to receptors.
Some patients with Graves disease have antibodies that can only be detected in one of these two approaches. Rarely, autoimmune antibodies to the TSH receptor can cause hypothyroidism (rather than hyperthyroidism).
- Calcitonin – to help detect the presence of excessive calcitonin production, which can occur with C-cell hyperplasia and medullary thyroid cancer
- Thyroglobulin – to monitor the treatment of thyroid cancer and to detect recurrence
- Biopsies – often a fine-needle biopsy, a procedure that involves inserting a needle into the thyroid and removing a small amount of tissue and/or fluid from a nodule or other area that the health practitioner wants to examine; an ultrasound is used to guide the needle into the correct position. (See the article on Anatomic Pathology for more general information on biopsies.) If the diagnosis (benign vs. malignant) is unclear, examining the biopsy material with genetic tests for mutations in certain genes (e.g., BRAF, RAS, RET/PTC, Pax8-PPARG, or galectin-3) may help.
Screening for thyroid disease in asymptomatic adults is controversial, and there is no consensus in the medical community as to who would benefit from screening and at what age to begin begin (except for screening in newborn infants, which all recognize is necessary and beneficial). In 2004, the U.S. Preventive Services Task Force found insufficient evidence to recommend for or against routine screening for thyroid disease in asymptomatic adults. However, the American Thyroid Association and the American Association of Clinical Endocrinologists released clinical practice guidelines in 2012 that recommend that screening for hypothyroidism should be considered in people over the age of 60. Because the signs and symptoms of both hypothyroidism and hyperthyroidism are so similar to those seen in many common disorders, health practitioners often need to rule out thyroid disease even though the patient has another problem.
- Thyroid scans – tests that use radioactive iodine or technetium to look for thyroid gland abnormalities and to evaluate thyroid function (for iodine) in different areas of the thyroid
- Ultrasound – an imaging scan that allows health practitioners to determine whether a nodule is solid or fluid-filled and can help measure the size of the thyroid gland
For more on imaging studies, see the web site RadiologyInfo.org.