TSH
Thyroid Stimulating Hormone
What this test measures
TSH is a hormone that tells your thyroid gland to make thyroid hormones. These hormones control your metabolism, energy levels, and many body functions. This test checks if your thyroid is working properly.
Understanding the numbers
Typical range: 0.4-4.0 mU/L
Typical UK range. Some labs use slightly different ranges (0.4-4.5 or 0.38-5.33). Always check the range on your own report.
- Low (overactive thyroid possible): 0–0.4 mU/L
- Normal: 0.4–4 mU/L
- Subclinical underactive: 4–10 mU/L
- Overt underactive: 10–20 mU/L
Key numbers
- 0.4 mU/L — Below = consider overactive thyroid
- 4.0 mU/L — Upper limit of normal
- 10 mU/L — Treatment usually offered
- 2.5 mU/L — Target in pregnancy / on levothyroxine
Normal range based on NICE NG145 thyroid guidance and typical UK laboratory ranges. Treatment is usually offered for TSH above 10 mU/L or above 4 with symptoms. Pregnancy has tighter targets. TSH is always interpreted alongside free T4.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Your thyroid may be underactive (hypothyroidism)
- Symptoms can include tiredness, weight gain, and feeling cold
- This is more common in women and older adults
- Treatment with thyroid hormone tablets usually helps
When not to wait
Severe overactive or underactive thyroid is a medical emergency. Don't wait for a GP appointment if you have any of these features with known or suspected thyroid disease.
- Very fast or irregular heartbeat
- Severe chest pain or breathlessness
- High fever with known thyroid disease
- Confusion, drowsiness, or fits
- Severe weakness or feeling about to collapse
- Very low body temperature, slow breathing, and extreme sleepiness
- Neck swelling that is painful or growing quickly
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Questions you could ask your GP
- Do I need to take thyroid medication, and is it lifelong?
- How often should my thyroid levels be monitored?
- What symptoms should I look out for that might mean my dose needs adjusting?
- Should I have my T4 or T3 levels checked as well?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Tired, weight gain, feeling cold
Anjali had been tired, putting on weight, and feeling cold even in summer. Her TSH was 14.2 mU/L and her free T4 was low at 8. Her thyroid antibodies were positive.
What the GP said: “Your thyroid is underactive (hypothyroidism), probably from an autoimmune cause. Treatment is a daily tablet called levothyroxine. You'll need a blood test in 6-8 weeks and a small dose adjustment if needed.”
What happened next: Anjali started levothyroxine 50 micrograms. At 8 weeks her TSH was 6.1 and her dose was increased to 75. Three months later her TSH was 2.2 and her symptoms had mostly settled.
A TSH above 10 with symptoms usually means treatment. Levothyroxine is a replacement — not a drug to push things — so the aim is to normalise the TSH (usually 0.4-2.5). Doses are adjusted slowly.
Mildly raised TSH, no symptoms
Rachael's TSH was 5.6 mU/L on a screening blood test. Her free T4 was normal and she felt well. Antibodies were negative.
What the GP said: “This is subclinical hypothyroidism — a mildly high TSH with a normal T4 and no symptoms. We don't always treat this. I'd like to repeat in three months to see if it settles on its own, which it often does.”
What happened next: At three months Rachael's TSH was 3.8 — back in range. No treatment was needed.
Subclinical hypothyroidism (TSH 4-10 with normal T4) often resolves without treatment. NICE guidance is to repeat the test before starting anything, and to treat mainly when TSH is above 10 or when symptoms are present.
Racing heart, weight loss
Deepak had lost 6 kg without trying, felt hot and anxious, and his resting heart rate was 110. His TSH was suppressed at 0.02 and his free T4 was raised at 32.
What the GP said: “Your thyroid is overactive (thyrotoxicosis). I'll start a medicine called carbimazole to bring the hormones down, a beta-blocker to ease the symptoms quickly, and I'm referring you to the endocrinology clinic for further assessment.”
What happened next: The endocrinology clinic diagnosed Graves' disease. Deepak took carbimazole for 18 months, came off it, and has been in remission for two years.
A very low TSH with raised T4 means an overactive thyroid. Treatment is urgent but very effective. The three options — tablets, radioiodine, or surgery — are discussed in the clinic based on your age, other health, and preferences.
Six weeks pregnant, on levothyroxine
Sarah has hypothyroidism and takes levothyroxine. She's 6 weeks pregnant. Her TSH was 3.8 — in the 'normal' range for non-pregnant adults but too high for pregnancy.
What the GP said: “In pregnancy the target is tighter, usually below 2.5. We need to increase your levothyroxine dose by about 25%, and repeat your TSH every 4-6 weeks to keep you in target through the pregnancy.”
What happened next: Sarah's dose was increased from 75 to 100 micrograms. Her TSH dropped to 1.8 at the next check. Her baby was born healthy at term.
Pregnancy changes thyroid targets. If you're on levothyroxine and planning pregnancy — or just found out you're pregnant — contact your GP the same week. Doses usually need to go up early.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.