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

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:

• Hypopituitarism
• Hypothyroidism - primary
• Hypothyroidism - secondary
• Thyrotoxic periodic paralysis

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.

Your health care provider will tell you, if necessary, to stop taking 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 include all medications.

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.