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hormoneSynthetic thyroid hormone (T4), the standard prescription treatment for an underactive thyroid.
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Lab test
What is a TSH test, and what are normal TSH levels?
A TSH test measures thyroid stimulating hormone, the pituitary signal that tells the thyroid how much hormone to make. It is usually the first test for an underactive or overactive thyroid. ARUP Laboratories lists 0.27 to 4.20 mU/L for adults aged 20 and older; pregnancy and other labs use different ranges.
What does the TSH test measure?
TSH is made by the pituitary gland and tells the thyroid how much thyroid hormone to make. When thyroid hormone is low, TSH rises; when thyroid hormone is high, TSH falls. MedlinePlus
Measuring TSH shows whether the thyroid is making the right level of hormones, but it cannot show what is causing a thyroid problem. MedlinePlus
Why is the TSH test ordered?
- To find out whether symptoms may come from hypothyroidism (fatigue, weight gain, feeling cold, dry skin) or hyperthyroidism (weight loss, rapid heartbeat, heat sensitivity, trouble sleeping). MedlinePlus
- To monitor people who take thyroid hormone medicine, or after treatment for hyperthyroidism. MedlinePlus
- Alongside other tests when a lump or nodule is found on the thyroid. MedlinePlus
What are typical TSH ranges?
ARUP Laboratories publishes TSH intervals by age, with separate intervals in pregnancy. ARUP Laboratories
Reference ranges differ between laboratories and testing methods, and many also differ by age, sex, pregnancy or the phase of the menstrual cycle. The range printed on your own report is the one that applies to your result, and a clinician interprets it alongside your history.
| Group | Reference interval |
|---|---|
| Adults, 20 years and older | 0.27 to 4.20 mU/L |
| 12 to 19 years | 0.53 to 3.59 mU/L |
| Pregnancy, 1st trimester (10 to 13 weeks) | 0.03 to 3.40 mU/L |
| Pregnancy, 2nd trimester (14 to 20 weeks) | 0.19 to 4.06 mU/L |
What can affect TSH results?
- Some medicines can affect the result; tell the person ordering the test about everything you take, and do not stop a medicine unless they tell you to. MedlinePlus
- Biotin can interfere with TSH and other thyroid immunoassays and cause incorrect results. Tell the person ordering the test if you take it. FDA
- Pregnancy changes TSH, which is why ARUP publishes separate trimester intervals. ARUP Laboratories
When should you talk to a clinician about a TSH result?
- A TSH outside the range is usually followed by a free T4 test, and sometimes T3 or thyroid antibodies, before anything is concluded. MedlinePlus
- New symptoms such as a rapid or irregular heartbeat, unexplained weight change or a swelling in the neck are worth discussing with a clinician. MedlinePlus
DiscoverMedica explains what a test measures and what named sources publish. It does not interpret individual results; the clinician who ordered your test, or one you choose, does.
Where can you get a TSH test?
A clinician can order it as part of routine or follow-up care. Some services also let you order lab tests yourself; check that a service’s panel includes this marker, which laboratory runs it, and who reviews the result with you.
- Function HealthAnnual membership for broad lab panels, drawn at Quest sites or at home.
- Quest HealthOrder individual lab tests from Quest Diagnostics online, without a doctor’s visit.
TSH Test: questions, answered
What is a normal TSH level?
ARUP Laboratories lists 0.27 to 4.20 mU/L for adults aged 20 and older, with different intervals for teenagers and in pregnancy. Other laboratories and assays print slightly different ranges, so read a result against the range on your own report and discuss it with the clinician who ordered it.
What does it mean when TSH is high or low?
According to MedlinePlus, TSH rises when thyroid hormone is low and falls when thyroid hormone is high, so TSH outside the range can point toward an underactive or overactive thyroid. A TSH result cannot show the cause, which is why clinicians usually add a free T4 test before drawing conclusions.
Can biotin affect a TSH test?
Yes. The FDA warns that biotin, common in hair, skin and nail supplements, can interfere with some lab tests and cause incorrect results that may go undetected. Tell whoever orders your test if you take biotin, and ask whether to pause it beforehand.
Is TSH different in pregnancy?
Yes. ARUP Laboratories publishes separate pregnancy intervals: 0.03 to 3.40 mU/L in the first trimester and 0.19 to 4.06 mU/L in the second. Pregnancy results are interpreted by the clinician caring for the pregnancy. Other laboratories publish their own pregnancy ranges, so the range on the report is the one to use.
Sources
- MedlinePlus: TSH (Thyroid Stimulating Hormone) Test
- ARUP Laboratories: Thyroid Stimulating Hormone 3rd Generation (test 0070225)
- MedlinePlus: Thyroxine (T4) Test
- FDA: Biotin Interference with Lab Tests
Last reviewed October 7, 2026. Every source above was checked on or before that date. Educational information only, not medical advice; it does not replace a conversation with a licensed clinician. DiscoverMedica does not employ clinicians.
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