What Is TSH (Thyroid-Stimulating Hormone)?

TSH tells your thyroid gland how to produce other hormones.

TSH, or thyroid-stimulating hormone, is produced by the pituitary gland (a pea-sized structure at the base of the brain).

It helps the thyroid gland — located at the base of the neck — produce the hormones thyroxine (T4) and triiodothyronine (T3), which affect your metabolism and other bodily functions.

Your body can't perform properly without the correct amounts of these hormones.

The Pituitary Gland

The pituitary gland is often referred to as the "master gland" because it controls several other hormone-producing glands, including the thyroid gland.

It sits behind the bridge of the nose at the base of the brain, close to the optic nerves.

This gland works closely with the hypothalamus, a region in the brain.

The hypothalamus produces thyrotropin-releasing hormone (TRH), which tells the pituitary gland to produce more TSH.

The pituitary gland works like a thermostat. If it senses that your thyroid hormone levels are low, it releases more TSH.

If your thyroid hormone levels are high, it slows or stops the release of TSH.

TSH Test

A TSH test can be used to check how well your thyroid gland is functioning. It measures the amount of TSH that your pituitary gland is secreting.

This test can diagnose both hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid).

Your doctor may order this test if you have signs of an enlarged or nodular thyroid, or a known or suspected thyroid disorder.

TSH tests are also used to screen newborn babies for underactive thyroid, and to diagnose fertility problems in women.

TSH levels can vary throughout the day, so it's best to have this test done early in the morning.

Your healthcare provider may order further testing if your TSH levels aren't normal.

TSH Levels

A normal TSH range is anywhere from 0.4 to 4.0 milli-international units per liter (mIU/L).

If you're being treated for a thyroid disorder, your doctor will probably try to keep your TSH level between 0.5 and 4.0 mIU/L.

Some labs use different measurements, so be sure to talk to your doctor about what your results mean.

Also, the normal range of TSH may be different for you if you:

• Are pregnant
• Have thyroid cancer
• Have a pituitary disorder

Graves' Disease

Graves' disease is an immune system disorder that causes hyperthyroidism (overactive thyroid).

If you have Graves' disease, your TSH levels will probably be very low, because your pituitary gland will try to compensate for high levels of thyroid hormones in the bloodstream.

Your doctor will probably order a TSH test if there's reason to believe you have this condition.

Hashimoto's Thyroiditis

Hashimoto's thyroiditis is a disease in which your immune system attacks your thyroid gland, often causing hypothyroidism (underactive thyroid).

It's the most common cause of hypothyroidism in the United States.

If you have Hashimoto's thyroiditis, your TSH levels will probably be very high.

Your doctor may order a TSH test while trying to find out if you have this condition.