The most common thyroid test is the blood test that measures the amount of thyroid-stimulating hormone (TSH) in your bloodstream. The test is sometimes called the thyrotropin-stimulating hormone test.
TSH that is elevated, or above normal, is considered indicative of hypothyroidism. TSH that is “suppressed” or below normal, is considered evidence of hyperthyroidism.
As of 2003, the American Association of Clinical Endocrinologists is recommending that the normal range run from 0.3 to 3.0, versus the older range of 0.5 to 5.5. So, according to the new standards, levels above 3.0 are evidence of possible hypothyroidism, and levels below 0.3 are evidence of possible hyperthyroidism. Keep in mind that there is disagreement among practitioners, and some follow the older range, others use the newer range.
Free T4 / Free Thyroxine
Free T4 measures the free, unbound thyroxine levels in your bloodstream. Free T4 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.
Free or unbound T4 levels represent the level of hormone available for uptake and use by cells. Bound levels represent a circulating hormone that may not all be immediately available, because it is affected by other drugs, illness, and physical changes such as pregnancy. Because the free levels of T4 represent immediately available hormone, free T4 is thought to better reflect the patient's hormonal status than total T4 (below).
Total T4/Total Thyroxine/Serum Thyroxine
This test measures the total amount of circulating thyroxine in your blood. Thyroxine, a hormone produced by the thyroid, is also known as T4. A high value can indicate hyperthyroidism, a low value can indicate hypothyroidism. Total T4 levels can be elevated due to pregnancy, and other high estrogen states, including use of estrogen replacement or birth control pills.
Total T3/Total Triiodothyronine
Triiodothyronine is the active thyroid hormone, and is also known as T3. Total T3 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.
Free T3 / Free Triiodothyronine
Free T3 measures the free, unbound levels of triiodothyronine in your bloodstream. Free T3 is considered more accurate than Total T3. Free T3 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.
T3 Resin Uptake (T3RU)
When done with a T3 and T4, the T3 resin uptake (T3RU) test is sometimes referred to as the T7 test. This test measures the amount of unsaturated binding sites on the transport (binding) hormones. Elevated T3RU is more commonly seen with hyperthyroidism.
Thyroglobulin (Tg) levels are low or undetectable with normal thyroid function but can by elevated in thyroiditis, Graves’ disease, or thyroid cancer. Monitoring of Tg levels is frequently used to evaluate the effectiveness of treatment for thyroid cancer and to monitor for thyroid cancer recurrence.
When the body is under stress, instead of converting T4 into T3 - the active form of thyroid hormone - the body conserves energy by making what is known as Reverse T3 (RT3), an inactive form of the T3 hormone. The value of RT3 tests in diagnosis is controversial, as some practitioners believe that the body continues to manufacture RT3 instead of active T3, causing various symptoms that are identified as the so-called “Wilson’s syndrome.”
Thyroid Peroxidase (TPO) Antibodies (TPOAb) / Antithyroid Peroxidase Antibodies
Thyroid Peroxidase (TPO) antibodies, are also known as Antithyroid Peroxidase Antibodies. (In the past, these antibodies were referred to as Antithyroid Microsomal Antibodies or Antimicrosomal Antibodies). These antibodies work against thyroid peroxidase, an enzyme that plays a part in the T4-to-T3 conversion and synthesis process. TPO antibodies can be evidence of tissue destruction, such as Hashimoto's disease, less commonly, in other forms of thyroiditis such as post-partum thyroiditis.
It’s estimated that TPO antibodies are detectable in approximately 95 percent of patients with Hashimoto's thyroiditis, and 50 to 85 percent of Graves’ disease patients. The concentrations of antibodies found in patients with Graves' disease are usually lower than in patients with Hashimoto's disease.
Thyroglobulin Antibodies / Antithyroglobulin Antibodies
Testing for thyroglobulin antibodies (also called antithyroglobulin antibodies) is common. If you have already been diagnosed with Graves' disease, having high levels of thyroglobulin antibodies means that you are more likely to eventually become hypothyroid. Thyroglobulin antibodies are positive in about 60 percent of Hashimoto's patients and 30 percent of Graves' patients.
Thyroid-Stimulating Immunoglobulins (TSI) / TSH Stimulating Antibodies (TSAb)
TSH receptor antibodies (TRAb) are seen in most patients with a history of, or who currently have, Graves' disease. Testing is usually done for a specific type of stimulating TRAb that goes by several different names, including:
- Thyroid-Stimulating Immunoglobulins (TSI)
- TSH stimulating antibodies (TSAb)
Thyroid-stimulating immunoglobulins (TSI) can be detected in the majority - some estimates say as many as 75 to 90 percent - of Graves' disease patients. The higher the levels, the more active the Graves' disease is thought to be. (The absence of these antibodies does not, however, rule out Graves' disease.) Less commonly, some people with Hashimoto's disease also have these antibodies, and this can cause periodic short term episodes of hyperthyroidism.
When monitoring TSI, elevated levels may help predict relapse of Graves' disease, and lowered TSI levels may indicate that Graves' disease treatment is working.