Glossary term
TSH
Also known as: Thyroid Stimulating Hormone
Part of: Thyroid Function Tests
- Reading time
- 3 min read
- Last reviewed
- Last reviewed January 1970
- Reading level
- Written in plain English
- Reviewed by
- Reviewed by Medipher Editorial Team
A hormone made by the pituitary gland that tells the thyroid how much thyroid hormone to produce.
Medipher explains medical wording in plain English — it does not diagnose or replace a clinician. If this is a medical emergency, call your local emergency number immediately.
Simple visual explanation
What is this?
TSH (Thyroid Stimulating Hormone) is made by a gland at the base of the brain called the pituitary. It tells the thyroid gland in your neck how much thyroid hormone to make.
TSH works on a feedback loop — if thyroid hormone is low, TSH rises to push the thyroid to produce more; if thyroid hormone is high, TSH falls.
Because of this feedback loop, TSH is often the most sensitive early sign of a thyroid problem.
Why is it important?
TSH is the first-line screening test for thyroid disease. It picks up both underactive and overactive thyroid, often before symptoms are troublesome.
It's used during pregnancy, when starting or adjusting thyroid medication, and to investigate symptoms such as tiredness, weight change, mood change or palpitations.
Where you'll see it: Thyroid function blood tests.
How doctors use this result
Clinicians may use this result to:
- Screen for underactive or overactive thyroid.
- Investigate tiredness, weight change, mood change or palpitations.
- Adjust thyroid medication doses safely.
- Monitor thyroid health during pregnancy and after treatment.
What's the normal range?
| Adults (typical) | 0.4–4.0 mIU/L |
Ranges are stricter in pregnancy. Always use the range on your own report.
TSH varies through the day and after starting or stopping medication, so a small shift often just needs a repeat test after a few weeks.
Normal ranges are guides, not verdicts. Interpretation depends on your age, sex, symptoms, medical history, medications and other investigations — which is why your clinician looks at the whole picture rather than a single value.
Common reasons it may be high
Underactive thyroid
The pituitary produces more TSH to try and push the thyroid to work harder.
Medication adjustment
TSH takes weeks to stabilise after a dose change, so temporary rises are common.
Recovery after thyroid inflammation
TSH often overshoots as the thyroid returns to normal function.
Common reasons it may be low
Overactive thyroid
High thyroid hormone suppresses the pituitary's release of TSH.
Over-replacement of thyroid medication
Too much levothyroxine can shut down TSH; a dose review is usually all that's needed.
Pituitary conditions (uncommon)
Rarely, the pituitary itself doesn't produce enough TSH; specialist review is needed.
Does an abnormal result always mean something is wrong?
Not necessarily. A number that sits outside the printed range is a prompt for interpretation — not a diagnosis.
- A single result rarely gives the full picture — clinicians interpret it alongside your symptoms.
- Your medical history, medications and other blood tests all shape the meaning of one value.
- Small movements just outside the reference range are common and often not concerning.
- Repeat testing over time is frequently more useful than any single number.
What might happen next?
- Repeat TSH after a few weeks to confirm.
- Additional tests such as free T4, free T3 or thyroid antibodies.
- Medication adjustment if you are already on thyroid treatment.
- Specialist referral if the picture is unclear.
One result rarely tells the whole story. Clinicians look at your symptoms, medical history, medications and other tests together — and a single value that sits slightly outside the reference range is often normal for you. Repeat testing over time is frequently more useful than any one number. Medipher helps you understand what a result means in plain English, but it cannot diagnose medical conditions or replace advice from a doctor, nurse or pharmacist.
When should I speak to a healthcare professional?
Speak with your GP or another healthcare professional if:
- You've been asked to repeat the test.
- You're experiencing new or worsening symptoms.
- Your clinician has flagged the result as needing follow-up.
- You don't understand your results or feel unsure what to do next.
If you have severe symptoms — chest pain, severe breathlessness, fainting, heavy bleeding or feel very unwell — seek urgent help. If this is a medical emergency, call your local emergency number immediately. Medipher is designed to help explain medical information and is not an emergency service.
Frequently asked questions
Related guides
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Sources
General patient information referenced when writing this page.
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This glossary entry is general health information, not medical advice. Medipher explains medical wording in plain English and does not diagnose, provide treatment advice, or replace a GP, doctor, nurse or pharmacist. Always discuss your results with a qualified clinician.