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T4 test

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Definition:

A T4 test measures the amount of the T4 hormone in the blood. T4 is produced by the thyroid gland.

Alternative Names:

Thyroxine test

How the test is performed:

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test:

The health care provider may advise you to stop taking drugs that may affect the test (see "special considerations").

For infants and children:

The preparation you can provide for this test depends on your child's age and experience. For specific information regarding how you can prepare your child, see the following topics:

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

How the test will feel:

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

This test may be performed as part of an evaluation of thyroid function. Thyroid function is complex and depends on the action of many different hormones:

1. Thyroid-stimulating hormone (TSH) is a secreted by the pituitary gland.
2. TSH causes the thyroid gland to produce two more hormones, T4 (thyroxine) and T3 (triiodothyronine).
3. Finally, TSH itself is stimulated by another hormone, thyroid-releasing hormone (TRH), which is made by the hypothalamus .

In people with normal thyroid function, having enough T3 and T4 inhibits both TSH and TRH, which prevents the body from making too much T3 and T4.

T4 levels are important, because T4 increases numerous enzymes that produce energy for the body.

Most T3 and T4 is transported by a protein called TBG (thyroxine binding globulin), but smaller amounts are found on prealbumin and albumin . When not bound to proteins, they are called "free" T3 or T4.

• Review Date: 1/18/2006
• Reviewed By: Nikheel S. Kolatkar, MD, Clinical and Research Fellow, Division of Endocrinology, Diabetes, and Hypertension, Brigham and Women's Hospital, Harvard Medical School, Boston, MA. Review provided by VeriMed Healthcare Network.

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