Thyroid . Patient guide
Thyroid-Stimulating Hormone (TSH) Blood Test
Also known as: Thyrotropin
What is TSH
TSH (thyroid-stimulating hormone) is a pituitary hormone that signals the thyroid gland to produce more thyroid hormone, and it is the most sensitive first-line test of thyroid function. According to NICE CKS Hypothyroidism, the typical adult reference range is approximately 0.27 to 4.2 mIU/L on the Roche Elecsys assay used widely in UK labs; a TSH above the reference range usually points to an underactive thyroid (hypothyroidism) and a TSH below it to an overactive thyroid (hyperthyroidism). Around 2 in 100 UK adults have hypothyroidism, with prevalence rising with age and in women.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in mIU/L. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Typical adult range | 0.27 to 4.20 | mIU/L (varies slightly between UK labs) |
| Suggesting hypothyroidism | over 4.20 | |
| Suggesting hyperthyroidism | under 0.27 | |
| Treatment target on levothyroxine (NICE) | 0.4 to 4.0 |
What it is
The hypothalamus releases TRH, which stimulates the pituitary to release TSH, which in turn stimulates the thyroid gland to release T4 and T3. The system is a feedback loop: low thyroid hormones drive TSH up, and high thyroid hormones suppress it.
Why a clinician would order it
TSH is the cornerstone of thyroid testing. It is included in any work-up for fatigue, weight change, hair shedding, cold or heat intolerance, anxiety, palpitations, low mood, irregular periods, fertility review, and in patients taking levothyroxine.
If your level is outside the range
Symptoms of low TSH
- Anxiety, palpitations
- Tremor
- Weight loss
- Heat intolerance, sweating
- Loose stools
- Sleep disruption
What low can indicate. Hyperthyroidism (Graves disease, toxic nodule, thyroiditis), over-replacement with levothyroxine, recent illness (transient suppression), or rarely pituitary disease.
Symptoms of high TSH
- Fatigue
- Weight gain
- Cold intolerance
- Hair shedding
- Dry skin
- Constipation
- Low mood
What high can indicate. Primary hypothyroidism (Hashimoto thyroiditis is the commonest UK cause), under-replacement with levothyroxine, recovery from illness, or assay interference.
Testing tips
No fasting required. A morning sample is preferable as TSH follows a mild diurnal rhythm, so testing at a similar time of day makes repeat results easier to compare. If you take levothyroxine, take it AFTER the blood draw rather than before. Stop biotin, including hair, skin and nail supplements, for 3 days beforehand, because it interferes with thyroid immunoassays.
If you book Thyroid-Stimulating Hormone as part of Hair Loss Essentials, Advanced Hair & Hormone Check, Pre-Transplant Screening, Pre-Transplant + BBV Screen or General Wellness, the panel needs a fast of at least 12 hours (water is fine), so follow the panel instructions rather than the single-marker advice above.
Related reading
- Thyroid Blood Tests: Symptoms of an Underactive or Overactive Thyroid, and Exactly What to Check An under- or overactive thyroid causes fatigue, weight change and hair loss. The thyroid blood tests to check (TSH, Free T4, Free T3, antibodies) a...
- Why Am I Always Cold? Could It Be Your Thyroid? Always cold, with icy hands and feet and low energy? An underactive thyroid is one cause, but iron, weight and circulation matter too. What to check.
Where you can get this tested
Thyroid-Stimulating Hormone is included in the following WMG Health panels. Same-day appointments are usually available at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Looking to book a test?
Comprehensive private thyroid blood test
TSH, Free T4, Free T3 + TPO antibodies. NICE-aligned interpretation by GMC-registered doctors.
Symptoms often investigated with Thyroid-Stimulating Hormone
Thyroid-Stimulating Hormone is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Read this marker alongside another
Thyroid-Stimulating Hormone is most useful when interpreted together with the markers below. Each guide walks through the 4-quadrant matrix our clinicians use when both come back at once.
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.
Common questions about TSH
What is a normal TSH range?
Typical adult range: 0.27 to 4.20 (mIU/L (varies slightly between UK labs)). Suggesting hypothyroidism: over 4.20. Suggesting hyperthyroidism: under 0.27. Treatment target on levothyroxine (NICE): 0.4 to 4.0. Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low TSH result mean?
Hyperthyroidism (Graves disease, toxic nodule, thyroiditis), over-replacement with levothyroxine, recent illness (transient suppression), or rarely pituitary disease.
What does a high TSH result mean?
Primary hypothyroidism (Hashimoto thyroiditis is the commonest UK cause), under-replacement with levothyroxine, recovery from illness, or assay interference.
Do I need to fast or prepare for the TSH blood test?
No fasting required. A morning sample is preferable as TSH follows a mild diurnal rhythm, so testing at a similar time of day makes repeat results easier to compare. If you take levothyroxine, take it AFTER the blood draw rather than before. Stop biotin, including hair, skin and nail supplements, for 3 days beforehand, because it interferes with thyroid immunoassays. If you book Thyroid-Stimulating Hormone as part of Hair Loss Essentials, Advanced Hair & Hormone Check, Pre-Transplant Screening, Pre-Transplant + BBV Screen or General Wellness, the panel needs a fast of at least 12 hours (water is fine), so follow the panel instructions rather than the single-marker advice above.
Can I order a TSH blood test privately in London?
Yes. WMG Health offers TSH in several standard panels and as part of a custom panel at our 134 Harley Street clinic. Results are released as each marker is reviewed, and the completion date is set by the slowest marker in your panel. We confirm the expected timeframe at booking. Book a standard panel online, or ask for a custom panel via /contact/ or 020 3239 3378.