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 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.

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 to check your thyroid function. Thyroid function is complex and depends on the action of many different thyroid hormones, including thyroid-stimulating hormone (TSH) and T3 (triiodothyronine).

Your doctor may order this test if you have signs of a thyroid disorder, including:

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

Normal Results

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.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

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
• High levels of the protein that carries T4 in the blood (can occur with pregnancy, use of birth control pills or estrogen, liver disease, and as part of an inherited condition)
• 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

Risks

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)

Alternative Names

Thyroxine test

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):457-469.

Ladenson P, Kim M. Thyroid. In: Goldman L and Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders; 2007:chap 244.

Update Date: 4/20/2012

Reviewed by: Ari S. Eckman, MD, Division of Endocrinology and Metabolism, Johns Hopkins School of Medicine, Baltimore, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.