Thyroid-Stimulating Hormone (TSH)

Test Overview

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.

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, have below-average intelligence. 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.

Why It Is Done

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).

How To Prepare

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.

How It Is Done

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 on the site and then put on a bandage.

How It Feels

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.

Risks

There is very little chance of a problem from having a blood sample taken from a vein.

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.

Results

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

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Results are usually available in 2 to 3 days.

A slightly high TSH value may not require treatment. The doctor will consider any symptoms you might have along with other test results to determine if treatment is needed.

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 tumour 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 non-cancerous (benign) tumour called a toxic nodule.
Damage to the pituitary gland that prevents it from making TSH (a condition called secondary hypothyroidism).
Taking too much thyroid medicine for treatment of an underactive thyroid gland.
Pregnancy during the first trimester.

What Affects the Test

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, or lithium.
Having had a recent X-ray with iodine dye or test using radioactive materials.
Having severe stress or a long-term (chronic) illness.

What To Think About

The thyroid-stimulating hormone (TSH) test is the best screening test for conditions that can affect the thyroid gland.
If the TSH results are abnormal, the results of a TSH test should be considered along with the results of thyroid hormone tests, especially free thyroxine (FT4) results. To learn more about FT3 and FT4 testing, see the topic Thyroid Hormone Tests.

Medical Tests: Questions to Ask the Doctor

References

Citations

Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.