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The film in words
A thyroid blood test can look at TSH, free T4 and free T3, and at antibodies such as TPO and Tg.
TSH is a hormone from a small gland at the base of the brain. It tells the thyroid how much hormone to make. When thyroid hormone is low, TSH goes up. When it’s high, TSH goes down.
Guidance for doctors is to test TSH first. If TSH is above range, free T4 is tested from the same blood sample. If it’s below range, free T4 and free T3 are.
T4 is the main hormone the thyroid makes. The test measures the “free” part, which the body can use.
The body turns T4 into T3, the form it actually uses. Free T3 is usually only tested to check for an overactive thyroid, or how overactive it is.
TSH and the hormones show how the thyroid is working. Antibodies can show why.
Positive TPO antibodies mean the cause of an underactive thyroid is autoimmune, such as Hashimoto’s.
TPO antibodies come from the immune system. They’re high in Hashimoto’s and sometimes in Graves’ disease. More than 9 in 10 people with Hashimoto’s have them, and so do about 1 in 10 people with no thyroid problem.
When TSH is above range, guidance for doctors is to consider one TPO antibody test, but not to repeat it.
Tg antibodies are sometimes high in Hashimoto’s. They’re also checked after thyroid cancer, because they can make another test used then less accurate.
A usual thyroid blood test looks at TSH and free T4, and sometimes free T3. A fuller test can add the antibodies.
Whichever test, your doctor reads the results together with your symptoms and how you feel.
Sources: British Thyroid Foundation: thyroid function tests; NICE NG145; British Thyroid Foundation: thyroid antibodies
More films
How your thyroid worksThe gland, its two hormones, and the signal from the brain that runs it.1 min 15
What your blood test showsWhat a usual test looks at, TSH and free T4 side by side, and lab ranges.1 min 31
Hashimoto’s: why the thyroid slows downThe immune system, TPO antibodies, and why the test isn’t repeated.1 min 04
Levothyroxine, and how to take itWhat it does, taking it so it’s absorbed, missed doses, and why changes take time.1 min 41
Still not feeling right?When results are in range but symptoms aren’t, and what to ask your GP.1 min 16
Lab ranges and your reportThe units on a report, typical ranges as a guide, age, and antibody cut-offs.1 min 17
TPO antibodies, in detailWhat a positive result can mean, with or without a thyroid problem, and why it stays.1 min 11
Tg antibodies, in detailThyroglobulin, why it’s watched after thyroid cancer, and what its antibodies change.58 sec
Iron, B12 and vitamin DOther results you may have, why they come up with the thyroid, and coeliac disease.1 min 25
Pregnancy and your thyroidPlanning a pregnancy, the dose and tests in pregnancy, and after the birth.1 min 33
General information, not a diagnosis. Your GP can tell you what your results mean for you.