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To help evaluate thyroid gland function; to diagnose thyroid disease, including hyperthyroidism, and determine the cause; to monitor effectiveness of treatment of a thyroid disorder
A blood sample taken from a vein in your arm
None needed; however, certain medications, multivitamins and supplements can interfere with the free T3 and total T3 tests, so tell your healthcare practitioner about any prescribed or over-the-counter drugs and/or supplements that you are taking. Acute illness may affect thyroid testing test results. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness.
Triiodothyronine (T3) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called thyroxine (T4) and together they help control the rate at which the body uses energy. Almost all of the T3 (and T4) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. Tests can measure the amount of free T3 or the total T3 (bound plus unbound) in the blood.
Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.
T3 levels are regulated by the feedback system that the body uses to maintain stable amounts of thyroid hormones in the blood. T4 and TSH, along with its regulatory hormone thyrotropin releasing hormone (TRH), which comes from the hypothalamus, are also part of the feedback system.
When all three organs (hypothalamus, pituitary and thyroid) are functioning normally, thyroid production is regulated to maintain relatively stable levels of thyroid hormones in the blood.
If the thyroid gland produces excessive amounts of T4 and T3, then the person affected may have symptoms associated with overactive thyroid (hyperthyroidism), such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes and in some cases, bulging eyes. Graves disease is the most common cause of hyperthyroidism. (See the article on Graves Disease.)
If the thyroid gland produces insufficient amounts of T4 and T3, then the person may have signs and symptoms associated with underactive thyroid (hypothyroidism) and a slowed metabolism, such as weight gain, dry skin, fatigue, and constipation. Hashimoto thyroiditis is the most common cause of hypothyroidism in the U.S. (See the article on Hashimoto Thyroiditis for more details.)
Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH. The effect of these conditions on thyroid hormone production can be detected and monitored by measuring the free T3 or sometimes total T3.
A free or total triiodothyronine (free T3 or total T3) test is used to assess thyroid function. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor treatment of a person with a known thyroid disorder.
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 (unbound plus bound), free T4, total T3 (bound plus unbound), or free T3.
Since most T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not. However, some professional guidelines recommend the total T3, so either test may be used to assess thyroid function. For example, free T3 or sometimes total T3 may be ordered along with thyroid antibodies to help diagnose Graves disease, an autoimmune disorder that is the most common cause of hyperthyroidism.
The free or total T3 test is usually ordered following an abnormal TSH, particularly if the free T4 test is not elevated.
A free T3 or total T3 test may be ordered when someone has an abnormal TSH test result. It may be ordered as part of the investigative workup when a person has symptoms suggesting hyperthyroidism, especially if the free T4 level is not elevated.
Signs and symptoms may include:
Free or total T3 may sometimes be ordered at intervals to monitor a known thyroid condition and to help monitor the effectiveness of treatment for hyperthyroidism.
In general, high total or free T3 results may indicate an overactive thyroid gland (hyperthyroidism) and low total or free T3 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the healthcare practitioner to perform additional testing to investigate the cause of the excess or deficiency.
When used for monitoring treatment for thyroid or pituitary disorders, results of thyroid tests will inform the healthcare 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.
The following table summarizes some examples of typical test results and their potential meaning.
|Note: Laboratory results must always be correlated with the clinical findings of the patient.|
|TSH||Free T4||Total or Free T3||Most likely diagnosis|
|Normal||Normal||Normal||Normal thyroid function (e.g., "euthyroid")|
|Normal or decreased||Normal or decreased||Decreased||Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome)|
|Increased||Normal||Normal||Subclinical hypothyroidism1; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given|
|Increased||Decreased||Normal of decreased||Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism)|
|Normal or increased||Increased||Increased||Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance)|
|Decreased||Normal||Normal||Subclinical hyperthyroidism2; in a person with hypothyroidism, too much thyroid hormone is being given|
|Decreased||Normal||Increased||Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis)|
|Decreased||Increased||Increased||Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism)|
|Decreased||Decreased||Decreased||Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism)|
1In affected adults, the diagnosis of subclinical hypothyroidism is applied when the TSH level is elevated and the free T4 level is normal on repeat testing over a number of weeks or months. Adults with subclinical hypothyroidism may have few or no overt symptoms of hypothyroidism. However, subclinical hypothyroidism places affected adults at somewhat increased risk for an elevated LDL cholesterol level, increased risk for cardiovascular disease, and reduced mental acuity.
2In affected adults, the diagnosis of subclinical hyperthyroidism is applied when the TSH level is decreased and the free T4 level and T3 levels are normal on repeat testing over a number of weeks or months. Adults with subclinical hyperthyroidism may have few or no overt symptoms of hyperthyroidism. However, subclinical hyperthyroidism places affected persons at somewhat increased risk for atrial fibrillation and osteoporosis.
It is important to note that thyroid tests are a "snapshot" of what is occurring within a dynamic system. An individual person's total T3, free T3, total T4, free T4, and/or TSH results may vary and may be affected by:
Many multivitamins, supplements (especially hair, skin, and nail) and over-the-counter and prescription medications may affect thyroid test results, and their use should be discussed with the healthcare practitioner prior to testing. For example, biotin (vitamin B7) can interfere with some lab tests, so your healthcare practitioner may advise you to refrain from taking biotin or supplements that contain biotin for a few days before having blood drawn for thyroid hormone testing. If you have a procedure done in which fluorescein dyes are injected into your blood (e.g., angiography), you may need to wait a few days before having thyroid testing done.
This test was once used to help calculate the Free Thyroxine Index (FTI), an estimation of the free T4 concentration. It is determined from the total T4 test and some estimation of the level of thyroid hormone binding proteins. The T3 uptake test was the original test for estimating the level of binding proteins, and later versions were called T-uptake methods. These are rarely used now that there are methods available to measure free T4 and free T3 directly.
Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when the liver converts T4 to T3, it also produces a certain percentage of RT3. When the body is under stress, such as during a serious illness, it tries to prevent many tissues that depend on T3 from being metabolically active by producing more RT3 than T3. This is believed to be a way of conserving energy until the stress is relieved and it causes a syndrome called non-thyroidal illness (NTI). RT3 may also be elevated in hyperthyroidism. Use of the RT3 test remains controversial and it is not widely requested.
Pregnancy causes normal changes in the function of many endocrine glands, including the thyroid gland. Healthcare practitioners do not generally test asymptomatic women, but those with symptoms and/or a known thyroid disorder will usually be tested at intervals to detect and monitor hyperthyroidism or hypothyroidism during pregnancy and after delivery of the baby.
Some experts have advocated screening pregnant women during the first trimester (or preconception) even if they do not have a history of thyroid disease. However, most guidelines do not support this course of action. For more information, see the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) webpage: Thyroid Disease and Pregnancy.
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