What it measures
- TSH (thyroid-stimulating hormone)
- Free T4 (FT4)
- Free T3 (FT3)
Why test your thyroid?
Your thyroid sets the pace of your metabolism. When it’s underactive, everything slows down: you may feel tired, cold, low or gain weight. When it’s overactive, things speed up: anxiety, weight loss, a racing heart and feeling hot. Thyroid problems are common, especially in women, and very treatable.
What this test measures
- TSH is made by the pituitary gland to control the thyroid. It’s the most sensitive first test: high TSH usually means an underactive thyroid, low TSH an overactive one.
- Free T4 and free T3 are the thyroid hormones themselves, which show how severe any imbalance is.
Add TPO antibodies to check for an autoimmune cause such as Hashimoto’s.
Who it’s for
- Tiredness, weight gain or feeling cold (underactive thyroid)
- Anxiety, weight loss, a racing heart or feeling hot (overactive thyroid)
- Hair thinning, dry skin or irregular periods
- Monitoring thyroid medication
How to prepare
- Drink plenty of water beforehand; it makes the sample quicker and easier to take.
- You don’t need to fast.
- Stop biotin (vitamin B7) supplements 48 hours before, as biotin can affect hormone and thyroid results.
- If you take levothyroxine, have your test before your daily dose, unless your doctor has told you otherwise.
Thyroid Function Profile Blood Test: your questions
What are the symptoms of an underactive thyroid?
Common symptoms include tiredness, weight gain, feeling cold, constipation, dry skin, thinning hair, low mood and heavy or irregular periods.
Should I take my thyroid medication before the test?
If you take levothyroxine, it’s usually best to have your blood test before your daily dose. Follow your doctor’s advice if it differs.
Information on this page is general and doesn’t replace advice from your GP, midwife or consultant. If you feel unwell or are worried about symptoms, contact your GP, call NHS 111 or, in an emergency, 999.