Biomarker 101: Thyroid-stimulating hormone (TSH)

TSH is usually the first thyroid test ordered, and it is also the one people most often read backwards.

What TSH is

Thyroid-stimulating hormone is not made by the thyroid. It is made by the pituitary gland in the brain, and its job is to tell the thyroid how much hormone to produce.

That instruction runs on a feedback loop. When circulating thyroid hormone is low, the pituitary raises TSH to ask for more. When thyroid hormone is plentiful, it lowers TSH to ease off.

This is why the number runs counter to intuition. A high TSH generally points to an underactive thyroid, not an overactive one — it is the sound of the pituitary asking louder because it is not getting what it wants.

Why it's measured

TSH is sensitive. It tends to move before the thyroid hormones themselves drift outside their ranges, which makes it a useful early indicator and the reason it is used as a first-line test.

It is measured when thyroid dysfunction is suspected, and to monitor a known thyroid condition over time. Because thyroid symptoms are vague and overlap with a great many other things — fatigue, weight change, mood, temperature sensitivity — a blood test is what separates thyroid from the alternatives.

What the numbers mean

A common adult reference range runs roughly 0.4 to 4.0 mIU/L, though this varies between laboratories and assays.

A result slightly outside that range is not automatically a problem. TSH shifts with age, with pregnancy, with the time of day the sample was taken, and with several medications. Mildly raised readings in older adults can be unremarkable.

There is no single figure that defines a dangerous result, because interpretation depends on context — your symptoms, your history, and what T4 and T3 are doing alongside it.

What moves TSH

Raised TSH most often reflects an underactive thyroid, where the gland is not producing enough hormone and the pituitary compensates. Iodine deficiency can do it, since iodine is required to make thyroid hormone. Pituitary conditions, some medications and thyroid inflammation after pregnancy are other causes.

Low TSH most often reflects an overactive thyroid producing hormone in excess, prompting the pituitary to reduce its signal. Pregnancy lowers TSH in the first trimester because hCG stimulates the thyroid directly. Pituitary dysfunction and some medications also lower it.

What TSH won't tell you

• It isn't a complete thyroid assessment. Free T4 and free T3 measure the hormones themselves; TSH measures the instruction to produce them.
• It doesn't explain why. Distinguishing between causes usually requires antibody testing, and sometimes imaging.
• It isn't a diagnosis. A number outside a reference range is a reason to look further, not a conclusion.
• One reading isn't a pattern. TSH fluctuates. Borderline results are commonly repeated before anything is concluded.

The bottom line

TSH is the pituitary's instruction to the thyroid, which is why it moves in the opposite direction to the hormones it controls. High usually means underactive; low usually means overactive.

It is a sensitive first-line test and a poor last word. Read alongside T4, T3 and your symptoms, it is genuinely informative.