Thyroid blood tests are used to measure the health of the thyroid, a butterfly-shaped gland at the base of the neck. The thyroid secretes hormones involved in regulating a person’s metabolism, the physical and chemical processes necessary for the maintenance of life. Thyroid blood tests are important tools to screen for and monitor thyroid disease. There are two major forms of thyroid disease:

• Hypothyroidism, characterized by low thyroid levels
  
  
• Hyperthyroidism, characterized by high thyroid levels

The thyroid gland produces two major hormones: thyroxine (T4) and triiodothyronine (T3). Thyroxine makes up more than 90 percent of the hormone produced by the thyroid. Various thyroid disorders can cause the thyroid gland to release too much or too little T4 or T3, resulting in a range of symptoms

Another hormone called thyroid-stimulating hormone (TSH) causes the thyroid gland to manufacture thyroxine. Levels of TSH may also be measured to help physicians diagnose thyroid problems.

The major thyroid blood tests include:

• TSH test. If a physician suspects a thyroid problem, a TSH test is usually the first test to be performed. High levels of TSH typically are present when the patient’s thyroid is failing because of a disorder that directly affects the thyroid (primary hypothyroidism). When TSH levels are low, it usually indicates that the thyroid gland is overactive (hyperthyroidism), although it also can signal a problem with the pituitary gland (secondary hypothyroidism).
  
  
• T4 test. T4 circulates in the body either bound to proteins or freely circulating on its own. Although freely circulating T4 makes up just 0.1 percent of total T4, free T4 is the part of the hormone that penetrates target tissues and delivers its effects. T4 testing can be used to measure both bound and free T4 (total T4 test). However, a newer test allows a narrower measurement of just the circulating levels of T4 (free T4 test). This free T4 (FT4)  test generally is viewed as more accurate than total T4 testing.
  
  
• T3 test. As with T4, T3 circulates in the body either attached to proteins (about 99.7 percent of T3 in the body) or freely circulating on its own. A T3 test may be performed to diagnose hyperthyroidism, with elevated levels of T3 indicating the condition. Typically, this test is performed after abnormal results are reported in TSH and T4 testing. T3 testing is usually not helpful in diagnosing hypothyroidism. T3 testing can be used to measure either all T3 in the body (total T3 test) or just circulating levels of T3 (free T3 test).
  
  
• Serum TBG level. This test measures the level of a protein called thyroxine binding globulin (TBG) in the blood. TBG transports thyroid hormone throughout the body. The test can be used to identify abnormalities in a patient’s TGB or to help indicate conditions including thyroid disorders, kidney disease and acromegaly.
  
  
• Autoantibody test. Normally, the immune system produces antibodies to protect the body against diseases. However, patients with autoimmune disorders have immune systems that mistakenly produce autoantibodies (antibodies that attack healthy tissue). Some patients with hypothyroidism or hyperthyroidism have antibodies that attack the thyroid gland, causing it to either produce too much or too little thyroid hormone.

Other thyroid blood tests may also be used to help physicians diagnose thyroid problems. For example, a TRH test, in which a physician injects TRH (TSH-releasing hormone) into the patient, can help reveal if hypothyroidism is due to a problem with the pituitary gland or hypothalamus. Improved TSH testing has largely replaced the TRH test.

Additionally, a procedure known as a TSI test measures for thyroid-stimulating immunoglobulin (TSI), a substance often found in the blood of patients with Graves’ disease. However, TSI tests are rarely performed because the information they provide usually is not necessary to formulate a treatment plan.

If thyroid cancer is suspected, a combination of thyroid blood tests may be conducted. These may include a TSH test to assess the condition of the gland and a calcitonin test to help confirm or rule out a type of cancer called medullary thyroid carcinoma. Scientists have found that a newer blood test for a substance called TSH receptor message ribonucleic acid (TSHR mRNA) can also indicate thyroid cancer.

Thyroid blood tests may be recommended by physicians for a number of reasons. They are most often performed when a physician suspects that a patient’s symptoms are related to a thyroid disorder. They also may be performed to monitor the health of patients previously diagnosed with thyroid conditions.

Additionally, in the United States, TSH and T4 testing is routinely performed on newborns to screen for congenital hypothyroidism, a condition that can cause mental retardation if left untreated. A combination of TSH and T4 testing may also be used to diagnose and monitor female infertility problems and to diagnose pituitary problems.

Some experts have suggested that adults over the age of 35 should undergo routine thyroid blood testing. Others organizations recommend screening for adults over age 60 or for women who are 50 and older. To date, there is no consensus on the need for routine screening.

Depending on thyroid blood test results, a physician may recommend additional assessments, including thyroid imaging tests such as a thyroid scan or an ultrasound.