T3 test

Definition

Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body's control of metabolism.

A laboratory test can be done to measure the amount of T3 in your blood.

Alternative Names

Triiodothyronine; T3 radioimmunoassay

How the test is performed

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

How to prepare for the test

Your health care provider will instruct you, if needed, to stop taking drugs that may affect the test results.

Drugs that can increase T3 measurements include:

• Birth control pills
• Clofibrate
• Estrogens
• Methadone

Drugs that can decrease T3 measurements include:

• Amiodarone
• Anabolic steroids
• Androgens
• Antithyroid drugs (for example, propylthiouracil and methimazole)
• Lithium
• Phenytoin
• Propranolol

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 depends on the action of many different hormones, including thyroid-stimulating hormone (TSH) and T4.

Sometimes it can be useful to measure both T3 and T4 when looking at thyroid function. For example, in some cases of hyperthyroidism, T3 may be increased but T4 may be normal.

The T3 test measures both the T3 that is attached to proteins and floating free in the blood.

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

• Disorder in which the pituitary gland does not produce norml amounts of some or all of its hormone
• Overactive thyroid gland
• Underactive thyroid gland

Normal Values

The range for normal values is 100 - 200 ng/dL (nanograms per deciliter).

The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different specimens.Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

High levels of T3 in the blood may occur in pregnancy, with the use of birth control pills or estrogen, from liver disease, or as part of an inerited condition.

Higher-than-normal levels of T3 may be a sign of:

• Overactive thyroid gland (for example, Graves disease)
• T3 thyrotoxicosis (rare)
• Toxic nodular goiter 

Lower-than-normal levels may be due to:

• Short- or long-term illness
• Thyroiditis (swelling or inflammation of the thyroid gland -- Hashimoto's disease is the most common type)
• Starvation
• Underactive thyroid gland

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)

Special considerations

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References

Kim M, Ladenson P. Thyroid. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 233.

Salvatore D, Davies TF, Schlumberger MJ, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 11.