T4 and T3 are hormones produced by the thyroid gland. They help control the rate at which the body uses energy, and are regulated by a feedback system. TSH from the pituitary gland stimulates the production and release of T4 (primarily) and T3 by the thyroid. Most of the T4 and T3 circulate in the blood bound to protein. A small percentage is free (not bound) and is the biologically active form of the hormones.
Laboratory tests can measure either total amount of hormone (bound plus unbound) or just the free portion. The free T4 test is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 test. However, some professional guidelines still recommend the total T3 test so either free T3 or total T3 may be ordered by a health practitioner.
Typically, the preferred initial test for thyroid disorders is a TSH test. If the TSH level is abnormal, it will usually be followed up with a test for free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically. This is known as reflex testing and it saves the health practitioner time from having to wait for the results of the initial test and then requesting additional testing to confirm or clarify a diagnosis. Reflex tests are typically performed on the original sample that was submitted when the initial test was requested.
As an alternative, a thyroid panel may be requested by the health practitioner. This means that all three tests will be performed at the same time to get a more complete initial picture of thyroid function.
If the feedback system involving the thyroid gland is not functioning properly due to one of a variety of disorders, then increased or decreased amounts of thyroid hormones may result. When TSH concentrations are increased, the thyroid will make and release inappropriate amounts of T4 and T3 and the person may experience symptoms associated with hyperthyroidism. If there is decreased production of thyroid hormones, the person may experience symptoms of hypothyroidism.
The following table summarizes some examples of typical test results and their potential meaning.
Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)
The above test results alone are not diagnostic but will prompt a health practitioner to perform additional testing to investigate the cause of the excess or deficiency and thyroid disorder. As examples, the most common cause of hyperthyroidism is Graves disease and the most common cause of hypothyroidism is Hashimoto thyroiditis. (See the condition article on Thyroid Diseases for more on these and other related diseases.)
In the past, panels of tests were more common. More recently, however, the practice has been to order, where possible, one initial or screening test and then follow up with additional testing, if needed, to reduce the number of unnecessary tests. With thyroid testing, one strategy is to screen with a TSH test and then order additional tests if the results are abnormal or if clinical suspicions warrant.
It is important to note that thyroid tests are a "snapshot" of what is occurring within a dynamic system. An individual person's total T4, free T4, total T3, free T3, and/or TSH results may vary and may be affected by:
Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3
This article was last reviewed on March 11, 2015. | This article was last modified on October 29, 2015.
The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.
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