--- title: "Thyroid function test: What to Expect | HealthAnswers" description: "A thyroid function test checks TSH and often free T4 to diagnose underactive or overactive thyroid — what results mean and how treatment is monitored." url: https://healthanswers.co.uk/tests/thyroid-function-test/ robots: noindex --- # Thyroid function test: What to Expect | HealthAnswers Tests & investigations A thyroid function test checks TSH and often free T4 to diagnose underactive or overactive thyroid — what results mean and how treatment is monitored. Written by HealthAnswers editorial team Medically reviewed by [HealthAnswers UK doctor review panel](https://healthanswers.co.uk/our-doctors/) Reviewed 3 July 2026 ## Quick answer What is a thyroid function test? A thyroid function test is a blood test that usually measures TSH and often free T4 to check for underactive or overactive thyroid. No fasting is needed. It is commonly requested for tiredness, weight change, palpitations or cold/heat intolerance. Results guide medicines such as levothyroxine. ## Who is this page for? This page is for people in the UK who want a plain-English overview of Thyroid function test, including when to seek help and what NHS care usually involves. It explains general information, not what is wrong with you personally. ## Who should skip this page? Skip this page if you need a diagnosis, a prescription, or emergency care. It cannot replace a GP, pharmacist, NHS 111 or 999. If you are in immediate danger, call 999; for urgent advice that is not life-threatening, call 111 in England, Scotland or Wales, or use your GP out-of-hours service in Northern Ireland. On this page 5 sections ## Key facts about the thyroid function test - TSH is the main screening test for thyroid problems. - High TSH usually suggests an underactive thyroid; low TSH an overactive one. - Free T4 (and sometimes T3) helps confirm the pattern. - Autoimmune thyroid disease is a common cause in the UK. - Levothyroxine dose is adjusted using repeat blood tests. ## How thyroid hormones work The pituitary gland releases **TSH** (thyroid-stimulating hormone), which tells the thyroid how much hormone to make. The thyroid produces mainly **T4**, which is converted to active **T3** in the body. These hormones regulate metabolism — energy, temperature, heart rate and weight. A thyroid function test uses a blood sample to see whether this feedback loop is working normally. ## What the test measures Most GP requests start with TSH. If TSH is abnormal, or symptoms are strong, **free T4** is measured too. Free T3 is sometimes added when overactive thyroid is suspected. Antibody tests (such as TPO antibodies) may be used when autoimmune thyroid disease is likely. ## Underactive thyroid (hypothyroidism) Typical blood pattern: **high TSH** with **low free T4**. Common symptoms include tiredness, weight gain, constipation, dry skin and feeling cold. Treatment is usually lifelong **levothyroxine**, with the dose adjusted until TSH is in the target range. ## Overactive thyroid (hyperthyroidism) Typical pattern: **low TSH** with **high free T4 or T3**. Symptoms can include weight loss, tremor, palpitations, heat intolerance and anxiety. Causes include Graves’ disease and overactive nodules. Treatment may involve antithyroid tablets (such as carbimazole), radioiodine, or surgery, depending on the cause. ## Monitoring over time After starting or changing levothyroxine, blood tests are usually repeated every 6 to 8 weeks until stable, then about yearly. Take levothyroxine consistently — often in the morning on an empty stomach — and separate it from calcium or iron supplements by several hours unless advised otherwise. Pregnancy, starting the combined pill, or major weight change can alter dose needs — ask a GP for advice rather than adjusting tablets yourself. ## ! When to speak to a doctor about the thyroid function test See a GP if you have symptoms of a thyroid problem or an abnormal thyroid result that has not been explained. Seek urgent advice for very rapid heartbeat, severe agitation, or extreme weakness with known thyroid disease. Call **111** if you are unsure what to do. Call **999** for chest pain, severe breathlessness, or collapse. ## Common questions about the thyroid function test What symptoms suggest I might need a thyroid test? Underactive thyroid can cause tiredness, weight gain, cold intolerance, dry skin and constipation. Overactive thyroid can cause weight loss, tremor, heat intolerance, palpitations and anxiety. These symptoms overlap with many other conditions, so a blood test helps clarify. Do I need to fast before a thyroid function test? No fasting is needed. If you already take levothyroxine, many clinicians prefer the blood test before that day's dose — follow the instructions from your GP or lab. What does a high TSH mean? A high TSH often means the pituitary is pushing a underactive thyroid to work harder. Free T4 helps confirm hypothyroidism. Mild TSH rises are sometimes monitored before treatment, depending on symptoms and levels. How is thyroid function monitored in pregnancy? Thyroid hormone needs change in pregnancy, so monitoring is more frequent and targets are stricter. Tell your GP or midwife promptly if you are pregnant and take thyroid medicine — dose adjustments are common. Can biotin supplements affect thyroid tests? High-dose biotin (vitamin B7) in some supplements can interfere with certain lab assays and give misleading results. Tell the person requesting the test about supplements you take. ## Sources - [NHS — Blood tests](https://www.nhs.uk/conditions/blood-tests/) - [NHS — Underactive thyroid](https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/)