When your doctor is running blood tests to diagnose or manage a thyroid condition, the list of tests and acronyms can be confusing. Let’s take a look at the key tests used to diagnose and manage hypothyroidism (an underactive thyroid), and hyperthyroidism (an overactive thyroid), and how to understand your results.
Thyroid Stimulating Hormone (TSH) Test
TSH is a messenger hormone produced and released by your pituitary gland. Your pituitary releases more TSH when it detects that your thyroid is not producing enough thyroid hormone. And it cuts back on the release of TSH when your thyroid is producing more thyroid hormone than you need.
The reference range varies but generally runs from around .4 to 4.0 milliunits per liter (mU/L). Since TSH goes up when your thyroid is not making enough, levels above the top cutoff of the reference range — i.e., above 4.0 mU/L — are considered evidence of hypothyroidism. Because TSH drops when your thyroid is producing too much thyroid hormone, levels below the bottom number in the reference range — i.e., below .4 mU/L — are considered evidence of hyperthyroidism.
(Please note that reference ranges differ slightly based on geography. You should find out your laboratory’s specific reference range for any thyroid tests where your testing is performed.)
Thyroxine (T4) and Free Thyroxine (Free T4)
Thyroxine, also known as T4, is one of the key hormones produced by your thyroid gland. In order for the body to use the thyroid hormone, T4 is converted into the active thyroid hormone triiodothyronine (T3). The process is sometimes called T4-to-T3 conversion.
With thyroid hormones, you have a total amount, which includes the available hormone, as well as the hormone that is bound to proteins, and therefore not usable. The total T4 test measures all the available thyroxine circulating in your bloodstream. A free T4 test measures the amount of unbound and available thyroxine.
Again, while reference ranges vary, a common adult reference range for total T4 is from 4.5 to 12.5 µg/dL, and for free T4, 0.8 to 1.8 ng/dL.
Since these tests measure actual thyroid hormones, levels above the reference range — i.e., total T4 above 12. 5 µg/dL, or free T4 above 1.8 ng/dL — indicate that you have too much thyroid hormone (hyperthyroidism.) Levels below the reference range — i.e., total T4 below µg/dL, and free T4 less than 0.8ng/dL — indicate that you do not have enough thyroid hormone (hypothyroidism.)
Triiodothyronine (T3) and Free Triiodothyronine (Free T3)
Your thyroid gland produces some triiodothyronine (T3). The rest is produced when T4 is converted into T3. T3 is considered the active thyroid hormone, as it actively helps facilitate delivery of oxygen and energy to your organs, cells, tissues, and glands.
Total T3 measures all the T3, while free T3 measures the available levels of triiodothyronine that are unbound and usable by your body.
A typical adult range for total T3 is from 80 to 200 ng/dL, and for free T3, 2.3 to 4.2 pg/mL.
Levels above the reference range — i.e., total T3 of 200 ng/dL, or free T3 above 4.2 pg/mL —indicate that you have too much thyroid hormone (hyperthyroidism.) Levels below the reference range — total T3 below 80 ng/dL, and free T3 below 2.3 pg/mL — indicate that you do not have enough of the active thyroid hormone (hypothyroidism.)
Reverse Triiodothyronine/Reverse T3 (RT3)
When the body is under various types of stress, a mechanism can kick in that affects the normal T4-to-T3 conversion process. Some of the T4 that is supposed to convert to T3 gets hijacked and ends up being converted into an inactive form of T3 known as reverse T3. Reverse T3 is counted in both total T3 and Free T3 test levels, but because it is inactive and unusable by the body, it can make T3/Free T3 appear higher that it actually is.
Testing reverse T3 is considered controversial by the mainstream endocrinology community. Integrative and holistic physicians, however, often measure reverse T3 to determine if there is a deficiency of the active T3 hormone that can contribute to hypothyroidism.
The reference range for reverse T3 is typically 10 to 24 ng/dL. Ideally, your reverse T3 should fall in the lower half of the normal range. You physician may also want to calculate the ratio of your T3/Reverse T3, to help evaluate the situation.
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Mary Shomon is a thyroid disease, hormonal and autoimmune health writer, and patient advocate. For two decades, Mary has been a leading force advocating for more effective, patient-centered thyroid and hormonal health care. Mary is the New York Times bestselling author of “The Thyroid Diet Revolution,” “Your Healthy Pregnancy with Thyroid Disease,” “Living Well With Hypothyroidism,” and 10 other books on thyroid disease and integrative health. She co-stars in two PBS health specials, “Healthy Hormones,” and “Vibrant for Life.” Follow her on Twitter at @thyroidmary or at her Facebook communities: ThyroidSupport and ThyroidDiet.