What does a TSH test measure?

TSH stands for thyroid stimulating hormone, and the thing most people find counterintuitive is that it is not made by the thyroid. It is made by the pituitary gland, and its job is to tell the thyroid how much thyroid hormone to produce.

That is why the number often moves in the direction opposite to expectation. If the thyroid is underactive, the pituitary pushes harder and TSH goes up. If the thyroid is overactive, the pituitary eases off and TSH goes down. A high TSH is generally a signal of an underactive thyroid, not an overactive one.

TSH is usually interpreted alongside other thyroid measurements rather than alone, and reference ranges differ between laboratories. A result slightly outside a range is a reason to discuss it, not a diagnosis.