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TSH (thyroid-stimulating hormone)

The pituitary's messenger hormone that tells the thyroid how much hormone to produce - the cornerstone thyroid blood test.

TSH - thyroid-stimulating hormone - is produced by the pituitary gland, a small gland at the base of the brain. Its job is to tell the thyroid how much T4 and T3 to make. When thyroid hormone levels drop, the pituitary releases more TSH to stimulate the thyroid; when levels are high, TSH falls.

Because of this feedback loop, TSH is often the most sensitive early indicator of thyroid function. A TSH above the reference range suggests the thyroid may not be producing enough hormone (hypothyroidism); a TSH below the range suggests it may be overactive (hyperthyroidism). In Graves' disease, TRAb antibodies mimic TSH and keep the thyroid permanently switched on - so TSH becomes very suppressed.

The standard UK reference range for TSH in adults is approximately 0.4 - 4.0 mIU/L, though ranges vary between laboratories. Levels also vary naturally with age, during pregnancy, and at different times of day.

TSH is measured in a routine blood test. It is typically the first test ordered when a thyroid problem is suspected and the main test used to monitor thyroid conditions and adjust treatment doses.

Common questions

What does a high TSH mean?
A high TSH generally means the pituitary is working harder to stimulate a thyroid not producing enough hormone - pointing towards hypothyroidism. The free T4 result and symptoms help build the full picture. Always discuss results with your doctor.
What does a low TSH mean?
A low or suppressed TSH generally means the pituitary has detected high thyroid hormone levels - pointing towards hyperthyroidism, or that someone is taking too much levothyroxine. Context and the free T4 value matter.
Can TSH vary day to day?
Yes. TSH has a natural daily rhythm (highest in the early hours, lowest mid-afternoon) and varies with illness and other factors. For consistency, many people are advised to have blood drawn before taking their levothyroxine that morning.

Read more

TSH, T4 and T3 explained Why lab ranges differ

Related terms

Free T4 (free thyroxine) Free T3 (free triiodothyronine) Hypothyroidism Hyperthyroidism Graves' disease Euthyroid

ThyroidTools is an independent, patient-led project - not a charity, not a medical service, and not affiliated with any healthcare organisation or clinical body. We're not your doctor. We're the friend who helps you get more from seeing one. Nothing here is medical advice; always speak to a qualified clinician about your own situation.