TSH (Thyroid-Stimulating Hormone): Normal Range, High & Low Causes
What TSH measures
TSH is made by the pituitary gland. When thyroid hormone is low, TSH rises to push the thyroid harder, and when thyroid hormone is high, TSH falls. It is the usual first screen for thyroid problems.
TSH reference ranges
| Category | Conventional units | SI units |
|---|---|---|
| Typical adult range | 0.4 – 4.0 mIU/L | Same unit (µIU/mL) |
| Often high (underactive thyroid) | Above about 4.0 – 4.5 mIU/L | Same unit |
| Often low (overactive thyroid) | Below about 0.4 mIU/L | Same unit |
Ranges differ between labs and change with age and pregnancy. mIU/L and µIU/mL are the same number.
Common causes of a high result
- Hypothyroidism, most often Hashimoto thyroiditis
- Not enough thyroid medicine
- Iodine deficiency or excess
- Recovery from illness (temporary rises)
Common causes of a low result
- Hyperthyroidism, such as Graves disease
- Too much thyroid medicine
- First trimester of pregnancy
- Pituitary problems or severe illness
How to prepare for the test
No fasting is needed. Tell your clinician if you take biotin supplements, which can interfere with some thyroid tests.
How often to retest
About 6 to 8 weeks after a medicine change, then once or twice a year when stable, or as advised.
Track TSH over time
Upload your reports and Labgist plots every TSH result on one timeline, converts units and shows what changed.
Get the appFrequently asked questions
What is a normal TSH level?
About 0.4–4.0 mIU/L for most adults, although labs use slightly different limits.
Can I have symptoms with a normal TSH?
Yes. Many conditions share thyroid-like symptoms such as tiredness or weight change. Your clinician may check free T4, antibodies or other causes.
Does biotin affect TSH results?
High-dose biotin can distort some thyroid blood tests. Ask your clinician whether to pause it before testing.
Related tests
Sources
This page is general education and was last updated 2026-10-30. It is not a substitute for medical advice. See our editorial approach.