Thyroid function tests are blood tests which are ordered by doctors to determine how well the thyroid gland is functioning. The thyroid function tests examine the levels of hormones in the bloodstream to determine how well the thyroid can respond to signals from the pituitary gland, and also how well the thyroid gland can produce the thyroid hormones. These tests are commonly used to monitor patients with thyroid disease, and also have a role in diagnosing thyroid disease.
Thyroid Gland Physiology
In order to make sense of thyroid function tests, it is important to first understand how the thyroid gland works. The thyroid gland is crucial in the control of body metabolism; and regulates processes such as growth, energy use and sensitivity to other hormones. The function of the thyroid gland depends on two glands within the brain, the hypothalamus and the pituitary gland.
When the body needs more thyroid hormone in the circulation, the hypothalamus releases thyrotropin releasing hormone (TRH), which travels to the anterior pituitary gland. TRH acts on the anterior pituitary to stimulate the release of thyroid stimulating hormone (TSH). TSH is released into the bloodstream, where it travels to the thyroid gland. When acting on the thyroid gland, TSH causes the thyroid to produce and release more of the thyroid hormones, known as T3 and T4. T3 and T4 enter the bloodstream and carry out their actions to increase the rate of metabolism in the body. A higher level of T3 and T4 in the bloodstream is sensed in the hypothalamus and the pituitary, and inhibits the release of TRH and TSH.
Thyroid Function Tests
The hormones produced by the pituitary and the thyroid are commonly measured in the blood to assess the function of the thyroid gland.
Thyroid Stimulating Hormone (TSH): TSH is the hormone produced by the pituitary gland that causes release of thyroid hormones. TSH is commonly ordered and is a sensitive test of overall thyroid function. The normal range for TSH is 0.5-5.0 mIU/L. Patients with hyperthyroidism have low levels of TSH, below 0.5mIU/L. This is because their thyroid gland is over-active and producing a large quantity of T3 and T4, which suppress the release of TSH from the pituitary gland. Conversely, patients with hypothyroidism have high levels of TSH, commonly elevated above 5.0mIU/L. This is because the thyroid gland is under-active and levels of T3 and T4 in the blood are low. This stimulates the release of large amounts of TSH from the pituitary in an attempt to increase production of T3 and T4 from the thyroid gland.
Free Thyroxine (T4): Thyroxine is an alternative name for T4. T3 and T4 are transported in the blood bound to proteins, such as albumin and thyroxine-binding globulin. Free thyroxine measures the amount of T4 in the blood that is circulating not bound to such proteins. The accepted normal range for free thyroxine is 12-22pmol/L. In patients with hyperthyroidism, free T4 is commonly elevated above 22pmol/L. Conversely, those with hypothyroidism will have low levels of free T4, often below 12pmol/L.
Free T3: Free T3 is similar to testing free T4; the test looks at the amount of T3 in the circulation that is not bound to proteins in the blood. T3 is the thyroid hormone that is more active than T4; however, it is produced and released in much smaller quantities than T4. Free T3 is much less commonly ordered than free T4, but is an important test in certain circumstances, such as suspected T3 thyrotoxicosis. The accepted normal range for free T3 is 3.1-6.8 pmol/L. Free T3 is often raised in patients with hyperthyroidism and lowered in patients with hypothyroidism.
Total T4 and total T3 can also be measured, but these tests are rarely ordered in modern practice. Both are raised in hyperthyroidism and lowered in hypothyroidism. These tests measure both the amounts of T3 and T4 bound to blood proteins, as well as the amount free in the circulation.
Anti-Thyroid Antibody: Anti-thyroid antibody is a test which may be ordered if an auto-immune thyroid condition is suspected, such as Hashimoto’s thyroiditis and Graves’ disease. An important class of anti-thyroid antibodies are known as anti-thyroid peroxidase antibodies (anti-TPO). This is an antibody against an enzyme which is involved in the production of T3 and T4. Anti-TPO antibodies are found in the majority of patients with Graves’ disease and Hashimoto’s thyroiditis; however, these can be present in normal individuals, and absent in some people who have either disease.