T4 test

Definition

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.

How the Test is Performed

A blood sample is needed. For information on how this is usually done, see: Venipuncture

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
• Raloxifene
• Tamoxifene

Drugs that can decrease T4 measurements include:

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

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. You may feel some throbbing after the test.

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
• Thyroid nodule

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 may be due to conditions that involve an overactive thyroid, including:

• 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 thyroiditis
• Toxic multinodular goiter
• Trophoblastic disease
• Over-treatment with thyroid hormone medicine

Lower than normal levels of T4 may be due to:

• 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)