Thyroid-Stimulating Hormone (TSH)

A thyroid-stimulating hormone (TSH) blood test is used to check for thyroid gland problems. TSH is produced when the hypothalamus releases a substance called thyrotropin-releasing hormone (TRH). TRH then triggers the pituitary gland to release TSH. See illustrations of the thyroid gland and the pituitary gland.

TSH causes the thyroid gland to make two hormones: triiodothyronine (T3) and thyroxine (T4). T3 and T4 help control your body's metabolism.

Triiodothyronine (T3) and thyroxine (T4) are needed for normal growth of the brain, especially during the first 3 years of life. A baby whose thyroid gland does not make enough thyroid hormone (congenital hypothyroidism) may, in severe cases, be mentally retarded. Older children also need thyroid hormones to grow and develop normally.

This test may be done at the same time as tests to measure T3 and T4.

A test for thyroid-stimulating hormone (TSH) is done to:

• Find out whether the thyroid gland is working properly.
  • An underactive thyroid gland (hypothyroidism) can cause symptoms such as weight gain, tiredness, dry skin, constipation, a feeling of being too cold, or frequent menstrual periods.
  • An overactive thyroid (hyperthyroidism) can cause symptoms such as weight loss, rapid heart rate, nervousness, diarrhea, a feeling of being too hot, or irregular menstrual periods.
• Find the cause of an underactive thyroid gland (hypothyroidism). TSH levels can help determine whether hypothyroidism is due to a damaged thyroid gland or some other cause (such as a problem with the pituitary gland or the hypothalamus).
• Keep track of treatment with thyroid replacement medicine for people who have hypothyroidism.
• Keep track of thyroid gland function in people who are being treated for hyperthyroidism. This treatment may include antithyroid medicine, surgery, or radiation therapy.
• Double-check the diagnosis of an underactive thyroid gland in a newborn (congenital hypothyroidism).

Tell your doctor if you have had any tests in which you were given radioactive materials or had X-rays that used iodine dye within the last 4 to 6 weeks. Your test results may not be correct if you have had iodine contrast material before having a thyroid-stimulating hormone (TSH) test.

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then put on a bandage.

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

A thyroid-stimulating hormone (TSH) blood test is used to check for thyroid gland problems.

The normal ranges for the results of this test may vary from laboratory to laboratory. Results are usually available within 2 to 3 days.

High values

High TSH levels may be caused by:

• An underactive thyroid (hypothyroidism). Hashimoto's thyroiditis is the most common cause of primary hypothyroidism.
• A pituitary gland tumor that is making too much TSH. This is uncommon.
• Not taking enough thyroid hormone medicine for treatment of an underactive thyroid gland.

Low values

Low TSH levels may be caused by:

• An overactive thyroid gland (hyperthyroidism). Causes of hyperthyroidism include Graves' disease, a type of goiter (toxic multinodular goiter), or a noncancerous (benign) tumor called a toxic nodule.
• Damage to the pituitary gland that prevents it from making TSH (secondary hypothyroidism).
• Taking too much thyroid medicine for treatment of an underactive thyroid gland.

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking medicine, such as corticosteroids, levodopa, heparin, dopamine, lithium (such as Carbolith, Duralith, or Lithane), methimazole (Tapazole), and propylthiouracil.
• Having had a recent X-ray with iodine dye or test using radioactive materials.
• Having severe stress or a long-term (chronic) illness.
• Pregnancy during the first trimester.

• The thyroid-stimulating hormone (TSH) test is the best screening test for conditions that can affect the thyroid gland.
• The results of a TSH test should be considered along with the results of thyroid hormone tests, especially thyroxine (T4) results. For more information about T3 and T4 testing, see the medical test Thyroid Hormone Tests.
• Another test that measures TSH levels in the blood is called the thyrotropin-releasing hormone (TRH) challenge test. TSH blood levels are measured before and after an injection of TRH. Normally the injection causes the pituitary gland to release TSH. TSH levels that do not rise after the injection can indicate the presence of conditions such as a damaged pituitary gland (secondary hypothyroidism), Graves' disease, or any condition that causes an overactive thyroid gland (hyperthyroidism).
• The upper value of the normal TSH range for adults is 4.5 mIU/L or 4.5 mU/L (SI units).

Other Works Consulted

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.