TSH test

Definition

A TSH test measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It prompts the thyroid gland to make and release thyroid hormones into the blood.

Alternative Names

Thyrotropin; Thyroid stimulating hormone; Hypothyroidism - TSH; Hyperthyroidism - TSH; Goiter - TSH

How the Test is Performed

A blood sample is needed. Other thyroid tests that may be done at the same time include:

• T3 test (free or total)
• T4 test (free or total)

How to Prepare for the Test

There is no preparation needed for this test. Ask your health care provider about any medicines you are taking that may affect the test results. DO NOT stop taking any medicines without first asking your provider.

Medicines you may need to stop for a short time include:

• Amiodarone
• Dopamine
• Lithium
• Potassium iodide
• Prednisone or other glucocorticoid medicines

The vitamin biotin (B7) can affect the results of the TSH test. If you take biotin, talk to your provider before you have any thyroid function tests.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Why the Test is Performed

Your provider will order this test if you have symptoms or signs of an overactive or underactive thyroid gland. It is also used to monitor treatment of these conditions.

Your provider may also check your TSH level if you are planning to get pregnant.

Normal Results

Normal values range from 0.5 to 5 microunits per milliliter (µU/mL).

TSH values can vary during the day. It is best to have the test early in the morning. Experts do not fully agree on what the upper number should be when diagnosing thyroid disorders.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

If you are being treated for a thyroid disorder, your TSH level will likely be kept between 0.5 and 4.0 µU/mL, except when:

• A pituitary disorder is the cause of the thyroid problem. A low TSH may be expected.
• You have a history of certain types of thyroid cancer. A TSH value below the normal range may be best to prevent the thyroid cancer from coming back.
• A woman is pregnant. The normal range for TSH is different for women who are pregnant. Your provider may suggest that you take thyroid hormone, even if your TSH is in the normal range.

What Abnormal Results Mean

A higher-than-normal TSH level is most often due to an underactive thyroid gland (hypothyroidism). There are many causes of this problem.

A lower-than-normal level may be due to an overactive thyroid gland, which can be caused by:

• Graves disease
• Toxic nodular goiter or multinodular goiter
• Too much iodine in the body (due to receiving iodine contrast used during imaging tests, such as CT scan)
• Taking too much thyroid hormone medicine or prescribed natural or over-the-counter supplements that contain thyroid hormone

Use of certain medicines may also cause a lower-than-normal TSH level. These include glucocorticoids/steroids, dopamine, certain chemotherapy drugs, and opioid painkillers such as morphine.

Risks

There is little risk involved with having your blood taken.Veins and arteries vary in size from one person 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 lightheaded
• Multiple punctures to locate veins
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

References

Guber HA, Farag AF. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 24.

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. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 11.

Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.