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Finding the right tests…
Hormonal
T4, or thyroxine, is the hormone your thyroid gland releases in the largest amount, and your body converts it into the more active T3 as needed in tissues throughout the body. This test measures T4 in your blood, usually the free fraction that isn't bound to carrier proteins, and it's one of the standard tests your doctor orders alongside TSH to work out whether your thyroid is underactive or overactive. It's also used to monitor thyroid medicine, since a dose that's too low or too high shows up as a T4 level outside your target range before you necessarily feel it. Newborn screening programmes check T4 as well, to catch congenital hypothyroidism early.
This test reports T4 in your blood, most often as free T4 — the fraction not attached to carrier proteins, considered the more accurate measure of your thyroid's actual output. MedlinePlus lists a typical free T4 range of roughly 0.8 to 1.9 ng/dL in adults, though this shifts with age and pregnancy and your lab's range may differ. It's usually ordered with TSH as the standard first check for a suspected thyroid problem, and repeated periodically if you're on thyroid medicine to confirm your dose is right. A single blood sample is enough, with no special preparation.
T4, or thyroxine, is the hormone your thyroid gland makes in the largest quantity, and it acts mostly as a reservoir — your body converts T4 into the more active T3 in tissues throughout the body as needed, rather than using T4 directly.
TSH, made by the pituitary gland, rises or falls to tell the thyroid to make more or less T4, so the two work as a feedback pair. A low TSH with a high T4 points toward an overactive thyroid; a high TSH with a low T4 points toward an underactive one. Checking T4 alongside TSH, rather than either alone, is what lets your doctor tell a genuine thyroid problem apart from a pituitary one, since in pituitary disease both TSH and T4 can be low together.
If you're taking thyroid hormone replacement, periodic T4 testing shows whether your dose needs adjusting, since a dose that's too low or too high can show up on a blood test before you notice symptoms. T4 is also part of routine newborn screening, since catching congenital hypothyroidism in the first weeks of life prevents the developmental problems it would otherwise cause.
MedlinePlus reports a typical free T4 range of about 0.8 to 1.9 ng/dL in adults. This range shifts with age and pregnancy, and can vary slightly by lab, so check the figures on your own report.
Free T4 measures only the portion of thyroxine not attached to carrier proteins, which is considered the more accurate reflection of thyroid activity, while total T4 includes the bound portion too. Most labs now report free T4 as the standard test.
TSH and T4 work as a feedback pair — TSH tells the thyroid how much T4 to make — so reading them together lets your doctor tell whether an abnormal result comes from the thyroid itself or from the pituitary gland that controls it.
This combination usually points to an underactive thyroid (hypothyroidism), often from Hashimoto's disease, where the thyroid isn't making enough hormone despite the pituitary sending a stronger signal to try to compensate.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
This depends on how stable your dose has been and your individual situation, so it's best to ask your doctor how often you specifically need retesting.
Yes, T4 is part of standard newborn screening programmes, since catching an underactive thyroid in the first weeks of life allows treatment to start before it affects development.