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TSH normal range: what does a high or low TSH mean?
TSH is the main blood test for thyroid function. Here is the usual normal range, how to read TSH together with free T4 and when an abnormal result needs treatment.
This guide is general information and does not replace medical advice. Reference ranges differ between laboratories – always compare your result with the range printed on your own report and discuss abnormal results with a doctor.
TSH (thyroid-stimulating hormone) is made by the pituitary gland and tells the thyroid how much hormone to produce. When the thyroid is underactive, TSH rises to push it harder; when the thyroid is overactive, TSH falls. Because TSH reacts strongly to small changes, it is the best first test of thyroid function.
Normal ranges
| Test | Typical adult range |
|---|---|
| TSH | about 0.4–4.0 mU/L (laboratories vary, e.g. 0.27–4.2) |
| Free T4 (FT4) | about 12–22 pmol/L (≈ 0.9–1.7 ng/dL) |
| Free T3 (FT3) | about 3.1–6.8 pmol/L |
mU/L and µIU/mL are the same unit, so a TSH of 2.5 mU/L equals 2.5 µIU/mL.
How to read TSH with free T4
| TSH | Free T4 | Usual meaning |
|---|---|---|
| High | Low | Underactive thyroid (hypothyroidism), most often Hashimoto's thyroiditis |
| High | Normal | Subclinical hypothyroidism – often monitored rather than treated |
| Low | High | Overactive thyroid (hyperthyroidism), e.g. Graves' disease or thyroid nodules |
| Low | Normal | Subclinical hyperthyroidism, or too high a dose of thyroid medicine |
| Low or normal | Low | Rare pituitary problem or recovery from serious illness – needs specialist assessment |
High TSH
Symptoms of an underactive thyroid include tiredness, feeling cold, weight gain, constipation, dry skin, hair loss, low mood and heavy periods. Treatment with levothyroxine is usually recommended if TSH is above 10 mU/L, or if free T4 is low. With a TSH between the upper limit and 10 mU/L and a normal free T4, doctors often repeat the test after 6–12 weeks and check thyroid antibodies (anti-TPO) first, because TSH can rise temporarily after an illness.
Low TSH
An overactive thyroid can cause weight loss, a fast or irregular heartbeat, tremor, anxiety, sweating, heat intolerance and loose stools. A low TSH always needs free T4 and free T3 checked, and usually a specialist referral. In people taking thyroid tablets, a low TSH often simply means the dose is too high.
Things that can change your result
- Pregnancy – TSH is lower, especially in the first trimester, and trimester-specific ranges are used.
- Age – TSH rises slightly with age; values up to about 6–7 mU/L can be normal in people over 70–80.
- Biotin supplements, recent illness, amiodarone, lithium, steroids and iodine-containing contrast agents.
When should you see a doctor?
See a doctor if your TSH is outside the range, especially with symptoms, if you are pregnant or planning pregnancy, or if you take thyroid medicine and your result has changed. Seek urgent care for a very fast or irregular heartbeat, chest pain, high fever with agitation or confusion, or marked drowsiness and low body temperature.
Sources
- NICE NG145 – Thyroid disease: assessment and management (2019): nice.org.uk
- British Thyroid Foundation – Thyroid function tests: btf-thyroid.org
- Pearce SH et al. 2013 ETA Guideline: Management of subclinical hypothyroidism. Eur Thyroid J 2013;2:215–228.
- Alexander EK et al. 2017 ATA Guidelines for thyroid disease during pregnancy and the postpartum. Thyroid 2017;27:315–389.
Frequently asked questions
What is a normal TSH level?
In most laboratories the adult reference range is roughly 0.4–4.0 mU/L (some use 0.27–4.2 mU/L). Older adults can have slightly higher values without disease, and pregnancy has its own, lower ranges.
Should I take my thyroid tablet before the blood test?
For TSH monitoring it usually does not matter much, but many doctors prefer you to have the blood test before your daily levothyroxine dose and at a similar time each time, so results are comparable.
Can supplements affect thyroid tests?
Yes. High-dose biotin (vitamin B7), found in many hair and nail supplements, can interfere with laboratory methods and give falsely high or low results. Stop biotin for at least 2 days before the test unless your doctor says otherwise.