Thyroid
TSH blood test
A TSH (thyroid-stimulating hormone) blood test measures the hormone your pituitary gland releases to tell your thyroid how much thyroid hormone to make. It is the most sensitive first-line check of thyroid function: a high TSH usually points to an underactive thyroid, and a low TSH to an overactive one.
- What it measures
- Thyroid-stimulating hormone (the pituitary's signal to the thyroid)
- Sample
- Venous blood sample
- Fasting
- Not required (test in the morning if monitoring)
- Also known as
- thyroid-stimulating hormone, thyrotropin
- Included in 57 Health tests
- 8 panels, from £60
TSH reference range
Guide only| Category | Guide range |
|---|---|
| Adults (non-pregnant) | 0.4–4.0 mIU/L |
TSH follows a daily rhythm, higher in the early morning and lower in the afternoon, and has different targets in pregnancy and older age. Exact cut-offs vary by laboratory and assay, and TSH is usually interpreted alongside free T4 (FT4).
These are general UK adult guide ranges from public health sources, shown for orientation only. Reference ranges vary by laboratory, assay, age and sex, so your result is always interpreted against the range quoted by the analysing laboratory on your report.
What does a high TSH level mean?
A high TSH usually means an underactive thyroid (hypothyroidism): the pituitary is working harder to stimulate a thyroid that isn't producing enough hormone.
A mildly raised TSH with a still-normal free T4 is called subclinical hypothyroidism. Thyroid antibodies (anti-TPO) help show whether an autoimmune cause such as Hashimoto's disease is behind it.
What does a low TSH level mean?
A low or suppressed TSH usually means an overactive thyroid (hyperthyroidism), where excess thyroid hormone switches off the pituitary's signal.
Causes include Graves' disease, thyroid nodules, and taking too much thyroid medication. It is confirmed by measuring free T4 and free T3, which are typically raised.
When to consider a TSH test
- Persistent tiredness or low mood
- Unexplained weight gain or loss
- Feeling unusually cold or unusually hot
- Palpitations, anxiety or tremor
- Hair, skin or bowel changes
- Irregular periods or fertility questions
- A family history of thyroid or autoimmune disease
- Monitoring an existing thyroid condition or its treatment
Frequently asked questions
This page is general information about a blood marker and is not medical advice or a diagnosis. A TSH test screens and gives context; results are interpreted by a clinician alongside your symptoms, history and other markers. Always follow the guidance on your own report.