Blood and lab tests
What Does TSH Mean on a Blood Test? Thyroid Test Explained
TSH (Thyroid-Stimulating Hormone) · say it: tee-ess-AYCHAlso written: thyroid-stimulating hormone, thyrotropin
Short answer
TSH is a hormone from the pituitary gland that tells the thyroid how much thyroid hormone to make; it is the usual first test of thyroid function.
What it means
The thyroid is a gland in the neck that makes hormones controlling how the body uses energy. The pituitary gland, at the base of the brain, makes TSH to tell the thyroid to work harder or ease off.
The relationship works like a thermostat. When thyroid hormone levels are low, the pituitary usually raises TSH; when they are high, TSH usually falls. So a high TSH often points to an underactive thyroid, and a low TSH to an overactive one, though there are exceptions.
TSH is often checked together with free T4 (and sometimes T3), which measure thyroid hormone directly. Illness, pregnancy, some medicines and supplements such as biotin can affect results.
Where you may see it
- Thyroid function tests
- Routine health check panels
- Fertility and pregnancy blood tests
📄 How it looks on a report
TSH 4.8 mIU/L (range printed by lab)
🙅 What it does not mean
- A high TSH does not mean the thyroid is overactive; it usually suggests the opposite, which surprises many people.
- One slightly abnormal TSH does not confirm a thyroid condition; doctors often repeat it or add other thyroid tests.
- TSH is not a measure of thyroid hormone itself; it measures the signal sent to the thyroid.
🗒️ Questions to ask your doctor
🧾 Reports where you will see “TSH (Thyroid-Stimulating Hormone)”
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💊 Medicines whose labels mention “TSH (Thyroid-Stimulating Hormone)”
Plain-English explanations of each medicine, from its official FDA label.
- Levothyroxine Sodium
a synthetic thyroid hormone (L-thyroxine, also called T4), described on the label as chemically identical to the hormone made by the human thyroid gland
🔗 Read more from trusted sources
- “TSH (Thyroid-Stimulating Hormone)” on MedlinePlus ↗
US National Library of Medicine
- NCI Dictionary definition ↗
US National Cancer Institute: “thyroid stimulating hormone”
- MedlinePlus lab test guides ↗
US National Library of Medicine
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