T4 and another hormone called triiodothyronine (T3) are produced by the thyroid gland. They help control the rate at which the body uses energy and are regulated by a feedback system. TSH stimulates the production and release of T4 (primarily) and T3 from the thyroid gland.
Most of the T4 and T3 circulates in the blood bound to protein, while a small percentage is free (not bound). Blood tests can measure total T4, free T4, total T3, or free T3. The total T4 test has been used for many years, but it can be affected by the amount of protein available in the blood to bind to the hormone. Free T4 is not affected by protein levels and is the active form of thyroxine. 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.
Monitor the effectiveness of treatment in a person with known thyroid disorder
Monitor people with pituitary disease, to make sure that the thyroid is still working, and to monitor thyroid hormone treatment if it is not
Monitor individuals with thyroid cancer, in which the tumors respond to TSH. TSH and T4 levels will be regularly checked to make sure that enough thyroid hormone is being given to keep TSH low without making T4 too high.
In the United States, newborns are commonly screened for T4 levels as well as TSH concentrations to check for congenital hypothyroidism, which can cause mental retardation if left untreated.
Free T4 along with TSH may sometimes be used to screen for thyroid disorders, but expert opinions vary on who can benefit from screening and at what age to begin.
If a health practitioner suspects that someone may have an autoimmune-related thyroid condition, then thyroid antibodies may be ordered along with a free T4 test.
The eyes may be affected: puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.
Symptoms of hypothyroidism may include:
Menstrual irregularity in women
Enlarged thyroid gland (goiter)
Free T4 testing may be ordered along with other thyroid tests on a regular basis when a person is undergoing treatment for a thyroid disorder.
In pregnant women with thyroid disorders, the health practitioner is likely to order thyroid testing early and late in the pregnancy and for a time period following delivery to monitor the mother and baby.
In general, high free T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free T4 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the health practitioner to perform additional testing to investigate the cause of the excess or deficiency.
Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)
When used for monitoring treatment for thyroid or pituitary disorders, results of thyroid tests will inform the health practitioner whether treatment is effective and/or whether an adjustment to dose is necessary. For example, in people with hyperthyroidism, free T4, free T3, and TSH are regularly checked while they are on anti-thyroid drugs to assure that the drugs are working and to decrease doses if thyroid hormone levels get too low. In people with hypothyroidism, TSH and free T4 are regularly checked to assure that the right dose of thyroid hormone is being given to make TSH normal.
It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness, as thyroid hormone levels may be affected the stress of an illness.
It is important to note that thyroid tests are a "snapshot" of what is occurring within a dynamic system. An individual person's results may vary and may be affected by:
Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3
Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T4 test results and their use should be discussed with the health practitioner prior to testing. In general, free T4 levels are not affected by these medications.
This article was last reviewed on October 16, 2014. | This article was last modified on June 6, 2016.
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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