Thyroid lab tests

TSH, or thyroid-stimulating hormone, comes from a small gland at the base of your brain and tells your thyroid how hard to work. A high level may point to an underactive thyroid, and a low level an overactive one.

Your thyroid, the gland in your neck, mainly makes a hormone called T4. Free T4 is the part ready to act, and it’s usually read with TSH, the signal that tells your thyroid how hard to work.

Free T3 is the part of T3, your most active thyroid hormone, that isn’t attached to proteins and is ready to act. It helps set your energy, heart rate and body temperature.

Thyroid peroxidase, or TPO, is a protein your thyroid uses to make its hormones. TPO antibodies can be a sign that your immune system is targeting your thyroid.

Reverse T3 is an inactive form of thyroid hormone. Your body can make more of it during illness, physical stress or strict dieting, as a way to save energy.

Thyroglobulin is a protein your thyroid uses to make its hormones. This test looks for antibodies, immune proteins, against it, which can be a sign that your immune system is targeting your thyroid.

Calcitonin is a hormone made by certain cells in your thyroid, the gland in your neck. It doesn’t show how well your thyroid works, but it can help look for a rare type of thyroid cancer.

T3 is your body’s most active thyroid hormone. This test counts all of it in your blood, both the part carried by proteins and the small free part.

Despite its name, this test doesn’t measure T3 itself. It’s an older test that shows how much open space is left on the proteins that carry thyroid hormone, and it’s used with total T4 to estimate free thyroid hormone.

This is one of the most accurate ways to measure free T4, the active part of your thyroid’s main hormone. The lab first separates the free hormone from the hormone attached to proteins, then measures it.

TSH, or thyroid-stimulating hormone, tells your thyroid how hard to work. If your TSH is outside the normal range, the lab automatically adds free T4, the thyroid’s main hormone, to help explain why.

It starts with TSH, the main thyroid screening test. If your TSH is outside the normal range, the lab automatically adds follow-up thyroid tests, so extra tests are run only when they’re needed.