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

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How It Is Done

A health professional uses a needle to take a blood sample, usually from the arm.

How long the test takes

The test will take a few minutes.

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

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), and 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.