Thyroid blood tests indicate how well a person’s thyroid gland is functioning. The thyroid produces hormones that are essential to regulating the body’s metabolism, the physical and chemical processes necessary for the maintenance of life. These blood tests may help physicians diagnose conditions that relate to the thyroid gland, such as forms of hyperthyroidism (e.g., Graves’ disease) and hypothyroidism.

The major thyroid blood tests include:

• TSH test. Normally, this the first test of thyroid function to be performed. It measures levels of thyroid-stimulating hormone (TSH), a substance that regulates the thyroid gland.
  
  
• T4 test. Measures levels of the hormone thyroxine (T4), which circulates in the body either bound to proteins or freely on its own.
  
  
• T3 test. Measures levels of the hormone triiodothyronine (T3). As with T4, T3 circulates in the body either attached to proteins or freely circulating on its own.
  
  
• Autoantibody test. Some patients with thyroid disorders have antibodies that affect the thyroid gland, causing it to produce either too much or too little thyroid hormone. This test can reveal the presence of those autoantibodies.
  
  
• Serum TBG. Measures the level of a protein called thyroxine binding globulin (TBG) in the blood.

The use of various medications, including aspirin, birth control pills and thyroid medication, can affect the outcome of the tests. Certain medical conditions may also affect results. Patients should inform their physician of any medications they may be taking as well as whether they are ill or may be pregnant.

Patients should discuss the results of their thyroid blood tests with a physician. In some cases other diagnostic tests may be required, such as imaging tests or biopsies.