Explainer · updated 2026-09-09
TSH, explained in plain English
Key facts
- Typical adult reference interval: 0.4-4 mIU/L (ATA)
- High TSH with a normal free T4 is called: subclinical hypothyroidism, usually watched rather than treated (GARBER 2012)
- Low or undetectable TSH with normal thyroid hormones is called: subclinical hyperthyroidism, which is followed up (GARBER 2012)
- What TSH measures: the pituitary's instruction to the thyroid; a high TSH points to an underactive gland (GARBER 2012)
What it is
TSH stands for thyroid stimulating hormone. It is the pituitary gland's instruction to the thyroid, and the most sensitive thyroid test there is. TSH moves the opposite way to the hormone it is asking for, so a high TSH points to an underactive thyroid gland while a low or undetectable TSH points to an overactive one.
What the number is telling you
TSH is a signal, so it is read together with the thyroid hormone beside it, usually free T4. One reading says little; time and trend matter. Changes are usually judged across weeks and months, because TSH responds slowly to treatment.
What high or low can mean
High: A TSH above the reference limit with a normal free T4 is called subclinical hypothyroidism, and it is usually watched rather than treated. Where the free T4 beside it has also fallen, the underactivity is clearer, and treatment is usually the next step.
Low: A low or undetectable TSH with normal thyroid hormones is called subclinical hyperthyroidism, and it is followed up. It tells your doctor the pituitary has turned the signal down, which fits an overactive gland.
What a typical result looks like
Typical adult reference intervals from the American Thyroid Association run from 0.4 to 4 mIU/L. The range on your own report is the one that counts.
Questions worth asking your doctor
- Where does my result sit in relation to the reference interval?
- What does the free T4 result beside it add to the picture?
- If my TSH is high, what would make you treat it rather than watch it?
- How does this compare with my previous TSH results?
If a result worries you, ask your doctor.
Sources
- Guidelines for the Treatment of Hypothyroidism (2014)
- Clinical Practice Guidelines for Hypothyroidism in Adults (2012)
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