TSH
The pituitary hormone that tells the thyroid how much hormone to make; the main thyroid screening test.
| Typical reference range | 0.4–4.5 mIU/L |
| Commonly cited optimal range | 0.5–2.5 mIU/L |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
Thyroid-stimulating hormone is released by the pituitary gland in response to TRH from the hypothalamus. It stimulates the thyroid to produce T4 and T3, which in turn suppress TSH in a feedback loop.
Why it matters
Because TSH changes exponentially with small changes in thyroid hormone, it is the most sensitive screening test for thyroid dysfunction. High TSH suggests an underactive thyroid; low TSH suggests an overactive one.
What a high result can mean
High TSH usually indicates primary hypothyroidism, most often from Hashimoto's thyroiditis, or insufficient thyroid hormone replacement. Mild elevations can occur during recovery from illness, with iodine deficiency or excess, lithium, amiodarone and with aging.
What a low result can mean
Low TSH suggests hyperthyroidism (Graves' disease, toxic nodules, thyroiditis) or excess thyroid hormone medication. It can also be low with pituitary disease, high-dose corticosteroids, severe illness and early pregnancy.
What affects the result
TSH has a daily rhythm, highest at night and early morning, and can vary 20-40% between days; morning testing is most consistent. Biotin supplements can interfere with some assays (stop 2-3 days before). Interpret with free T4, free T3 and thyroid antibodies.
Units and conversion
Usually reported in mIU/L.
- From uIU/mL: multiply by 1 to get mIU/L
- From mU/L: multiply by 1 to get mIU/L
- From uU/mL: multiply by 1 to get mIU/L
- From mIU/mL: multiply by 1,000 to get mIU/L
Also called on lab reports
Thyroid Stimulating Hormone, Thyrotropin, TSH, 3rd Generation, TSH, Ultrasensitive, Thyroid-Stimulating Hormone, TSH, High Sensitivity, TSH, Serum
Related tests: Thyroid
This page is educational and does not replace advice from a healthcare professional.