Updated November 18, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
Blood tests for thyroid function are an important part of the process of both diagnosing thyroid disease, and treating thyroid conditions.
Here is a summary to help you have a better idea of what your thyroid tests mean.

GENERAL GUIDELINES

Always ask for an actual copy of your thyroid laboratory test results, including reference ranges. Don't accept "they're normal" as a result.

THYROID STIMULATING HORMONE TSH)

• Thyroid Stimulating Hormone (TSH) - Start with TSH. This measures a pituitary hormone that responds to changes in thyroid blood levels, and is a key test for many doctors. At most labs in the U.S., the reference range is from around 0.5 to 5.5.
• TSH Below Normal: If the TSH level is below the normal cutoff, your doctor may determine that you are hyperthyroid (you have an overactive thyroid.)
• TSH Above Normal: If the TSH level is above the normal cutoff, your doctor may determine that you are hypothyroid (you have an underactive thyroid.)
• Important Note About the TSH Range: Some doctors believe that the TSH reference range is far too broad, and instead, consider 3.0 to be the top-end cutoff for hypothyroidism. Find out where your doctor stands on this controversy.

FREE T4/FREE THYROXINE

• Free T4/Free Thyroxine -- Free T4 measures the free, unbound levels of the hormone thyroxine -- the storage hormone -- in your bloodstream. 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.

• Free T4 in the Normal Range - Many doctors consider any level within the normal reference range to be fine.
• Elevated Free T4 - Elevated Free T4 may be indicative of hyperthyroidism.
• Low Free T4 - Low Free T4 levels below the reference range may be indicative of hypothyroidism.
• Optimal Free T4? - Some integrative physicians and hormone experts state that if free T4 is not in the top half of the reference range, it is not optimal. Find out where your practitioner stands on this issue.

FREE T3/FREE TRIIODOTHYRONINE

• Free T3/Free Triiodothyronine - Free T3 measures the free, unbound levels of the hormone triiodothyronine - the active thyroid hormone at the cellular level. Because the free levels of T3 represent immediately available hormone, free T3 is thought to better reflect the patient's hormonal status than total T3.
• Elevated Free T3 - Elevated Free T3 may be indicative of hyperthyroidism.
• Low Free T3 - Low Free T3 levels below the reference range may be indicative of hypothyroidism.
• Optimal Free T3? - Some integrative physicians and hormone experts state that if free T3 is not in the top half -- or even the top quarter - of the reference range, it is not optimal. Find out where your practitioner stands on this issue.

RT3/REVERSE T3

Reverse T3 (RT3) -- also known as Reverse Triiodothyronine -- is a controversial test. Conventional endocrinology for the most part dismisses the value of RT3 measurement in diagnosing, treating and managing hypothyroidism. Integrative physicians and those who focus on optimal hormone balance, however, consider elevated RT3 to be an key sign of an underactive or dysfuntional thyroid. Learn more about the Reverse T3 test now.

THYROID ANTIBODIES TESTS

• 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). TPO antibodies can be evidence of tissue destruction.
• Normal TPO Antibodies - If TPO antibodies fall within the reference range, they are considered normal. This does not entirely rule out Hashimoto's disease, but makes it less likely.
• Elevated TPO Antibodies - Elevated TPO antibodies can be indicative of Hashimoto's disease, or other forms of thyroiditis. TPO antibodies are also detectable as many as 85 percent of Graves' disease patients.

TgAb/THYROGLOBULIN ANTIBODIES

If you have already been diagnosed with Graves' disease, having elevated levels of Thyroglobulin Antibodies (TgAb) 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.

TSI/THYROID STIMULATING IMMUNOGLOBULINS

Thyroid-stimulating Immunoglobulins (TSI) are elevated 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.) Note: some people with Hashimoto's disease also have these antibodies, and this can cause periodic short term episodes of hyperthyroidism.

NEED MORE INFORMATION?

For more information, read What Are Your Thyroid Tests Telling You?