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.
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.
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.