Thyroid Stimulating Hormone (TSH)

The thyroid is a small, butterfly-shaped gland in the front of your neck. Your thyroid makes hormones that control how your body uses energy. Thyroid hormones affect nearly every organ and control many of your body's most important functions. For example, they affect your breathing, heart rate, weight, digestion, and even your mood. If you don't have enough thyroid hormones in your blood, many of your body functions will slow down. But if you have too much, many body functions will speed up.

Your thyroid is controlled by a gland in your brain called the pituitary gland. The pituitary gland makes thyroid-stimulating hormone (TSH). TSH tells your thyroid how much thyroid hormone it needs to make.

If the thyroid hormone level in your blood is too low, your pituitary gland makes larger amounts of TSH to tell your thyroid to work harder. If your thyroid hormone level is too high, the pituitary gland makes little or no TSH. By measuring the TSH level in your blood, you can find out if your thyroid is making the right level of hormones.

Your TSH test results can tell you if your thyroid is making too much or too little hormone. But the test alone can't explain why your TSH levels may be too high or too low.

A high TSH usually means the thyroid is underactive (hypothyroidism), because the pituitary is pushing the thyroid harder to make more hormone. A low TSH usually means the thyroid is overactive (hyperthyroidism), because the pituitary is dialing back its signal.

When TSH is just slightly off but free T4 is normal, it's called subclinical hypothyroidism (slightly high TSH) or subclinical hyperthyroidism (slightly low TSH). Your provider may decide to repeat the test or order more tests rather than start treatment right away.

If your test results aren't normal, your provider will probably order other thyroid blood tests to find out what's causing your thyroid problem.

In certain cases, an abnormal TSH result may be a sign of a pituitary gland problem, but this doesn't happen often. Severe illness and some medicines can also temporarily move TSH out of the normal range, so the result is best understood together with your symptoms and other thyroid tests.