Health Encyclopedia

T4 test

Definition

T4 (thyroxine) is a hormone produced by the thyroid gland. A laboratory test can be done to measures the amount of the T4 in your blood.

Alternative Names

Thyroxine test

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

Your health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test.

Drugs that can increase T4 measurements include:

Birth control pills
Clofibrate
Estrogens
Methadone

Drugs that can decrease T4 measurements include:

Anabolic steroids
Androgens
Antithyroid drugs (for example, propylthiouracil and methimazole)
Interferon alpha
Interleukin-2
Lithium
Phenytoin
Propranolol

This list may not be all inclusive.

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 is done as part of an evaluation of thyroid function. Your doctor may order this test if you have signs of a thyroid disorder. Thyroid function is complex and depends on the action of many different hormones, including thyroid-stimulating hormone (TSH) and T3 (triiodothyronine).

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

Most T4 is transported by proteins. If T4 is not attached to a protein, it is called "free" T4.

Conditions under which the test may be performed:

Normal Values

A typical normal range is 4.5 to 11.2 micrograms per deciliter (mcg/dL).

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

Greater-than-normal levels of T4 along with low levels of TSH may be due to conditions that involve an overactive thyroid, including:

Early Hashimoto's disease
Graves disease
Germ cell tumors
Iodine-induced hyperthyroidism
Subacute or chronic thyroiditis
Toxic multinodular goiter
Trophoblastic disease

Lower-than-normal levels of T4 may indicate:

Hypothyroidism (including Hashimoto's disease and several other disorders involving an underactive thyroid)
Illness
Malnutrition or fasting
Use of certain medications

What the risks are

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

Excessive bleeding
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)

References

AACE Thyroid Task Force. American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism. Endocr Pract. 2002;8(6) 459.

Review Date:10/24/2007
Reviewed By:Robert Hurd, MD, Professor of Endocrinology, Department of Biology, Xavier University, Cincinnati, OH, and physician in the Primary Care Clinic, Cincinnati Veterans Administration Medical Center, Cincinnati, Ohio. Review provided by VeriMed Healthcare Network.

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