Thyroid (TSH) blood test: what it measures and what can change it
TSH is the signal the pituitary sends to the thyroid. It moves before the thyroid hormones themselves do.
General information about the test, from Hea's guide and the NHS · Last reviewed 5 November 2026
What it measures, and why GPs check it
TSH is the signal the pituitary sends to the thyroid. It moves before the thyroid hormones themselves do.
It is the first-line thyroid test; free T4 is added when TSH is outside the laboratory range.
Levels in UK guidance
Ranges differ between labs — your report shows your lab's range.
- NICE asks for free T4 when TSH is above the laboratory range, and free T4 with free T3 when it is below. NICE NG145 1.2.8
- When TSH stays at 10 mIU/L or above on two tests three months apart, NICE says the GP considers levothyroxine; below 10 without symptoms most people are watched rather than treated. NICE NG145 1.5.3–1.5.4, 1.5.7
What can shift the number
Now or in the last 3 months
- Pregnant. In pregnancy the usual ranges for many tests are different: show these results to your midwife or GP. NICE NG201 Antenatal care (2021); NICE NG3 Diabetes in pregnancy (2015, updated 2020); RCOG Green-top Guideline 76: thyroid disorders in pregnancy (2025; PMID 40240075); NICE NG239 (2024) rec 1.3.2; BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Lipid profile; C-reactive protein (CRP); ESR
- Had a baby or a pregnancy loss in the last 12 months. In the 12 months after a pregnancy thyroid tests can run high, then low, then recover. RCOG Green-top Guideline 76: thyroid disorders in pregnancy (2025; PMID 40240075); NHS.uk: Underactive thyroid
- An infection or illness in the 6 weeks before. NICE advises against testing the thyroid during an acute illness, because the illness itself can change the results. NICE NG145 Thyroid disease: assessment and management (2019, updated 2023), rec 1.2.6
Some medicines and supplements can change this result too. When you add your results, Hea asks which ones you take.
What a GP reads it with
Read together with: Free T4, Free T3. NICE NG145 Thyroid disease: assessment and management (2019), rec 1.2.8
A GP usually repeats TSH with free T4 after about three months, and may check thyroid antibodies once.
Questions to ask your GP
- Should this be repeated, and when?
- Do my symptoms fit with this result?
- Are thyroid antibodies worth checking once?
Safety first
If your lab or GP has flagged your result as urgent, follow their advice first.
Not for emergencies: call 999 or NHS 111.
Questions people ask
What is a thyroid function test?
It is a blood test that checks how well your thyroid gland is working. It measures TSH, a hormone from the pituitary gland that controls the thyroid, and often T4 and T3, the main thyroid hormones, also called free T4 and free T3. The usual range depends on your age and on the laboratory's method.
NHS: Overactive thyroid (hyperthyroidism) – Diagnosis, Page last reviewed 11 April 2023Is TSH the only thyroid test I need?
Often it is the first one. NICE suggests TSH alone as the first test for most adults, unless a pituitary problem is suspected. If TSH is above the laboratory's range, free T4 is measured in the same sample. If it is below the range, free T4 and free T3 are measured.
NICE NG145: Thyroid disease: assessment and management, Last updated 12 October 2023What can affect a thyroid blood test?
An acute illness can change the results, so NICE advises against testing during one unless the thyroid may be the cause. A high intake of biotin from supplements can make results read falsely high or low, so your GP may ask about supplements you take. If symptoms change, NICE suggests waiting at least 6 weeks before repeating the test.
NICE NG145: Thyroid disease: assessment and management, Last updated 12 October 2023Why would my GP repeat a thyroid test?
NICE bases decisions about a raised TSH on more than one test. If TSH is 10 mIU/L or higher on 2 tests 3 months apart, NICE says the GP considers treatment. Below that, symptoms and age are taken into account. People who are not treated have TSH and free T4 checked again over time.
NICE NG145: Thyroid disease: assessment and management, Last updated 12 October 2023What are the symptoms of an underactive thyroid?
Common symptoms include feeling extremely tired, feeling cold more than usual, putting on weight, constipation, low mood, difficulty concentrating, and dry skin or hair. They usually develop slowly and may be mild. Many of them are common and have other causes, so the NHS advises getting them checked by a GP.
NHS: Underactive thyroid (hypothyroidism), Page last reviewed 28 April 2025Got your result? Start with this question in your AI.
Join Hea, add it to Claude or ChatGPT, and your chat opens with this question ready. Hea shows what the lab printed, your past results and what can shift a number; your GP decides what it means for you.
Hea, my thyroid (TSH) result came back — help me understand it with my history
Closed beta for adults (18+) living in the UK. Free in the beta.
Sources
Each line about the test above names the guideline it comes from, as Hea's guide cites it. The answers to the questions come from:
- NHS: Overactive thyroid (hyperthyroidism) – Diagnosis, Page last reviewed 11 April 2023
- NICE NG145: Thyroid disease: assessment and management, Last updated 12 October 2023
- NHS: Underactive thyroid (hypothyroidism), Page last reviewed 28 April 2025