Thyroid Stimulating Hormone (TSH) - Serum

Why Is This Test Done?

Your doctor may order a TSH test to check for thyroid problems, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It’s often the first test used to diagnose thyroid issues because it’s sensitive and easy to measure. TSH is also used to monitor thyroid treatment if you’re on medication.

What to Expect Before the Test

No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the test if your blood is drawn early in the morning. If you’re taking thyroid medication, ask your doctor whether to stop temporarily or adjust your timing. Avoid caffeine or smoking right before the draw, as they might affect results.

What Happens During the Test?

A healthcare provider will insert a small needle into a vein in your arm or hand to draw a blood sample. You may feel a slight pinch, but it usually doesn’t cause pain. The blood is then sent to a lab for testing.

What Do the Results Mean?

Results are reported in units per liter (U/L) or milli-international units per liter (mIU/L). High TSH levels often suggest hypothyroidism, while low levels may indicate hyperthyroidism. Your doctor will interpret the results in the context of your symptoms and other tests.