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Hormonal
A free T4 test measures the thyroxine (T4) circulating unattached to carrier proteins in your blood, the active portion your tissues can use. T4 is the main hormone your thyroid gland makes, and your body converts it into T3 as needed. Unlike T3 testing, where total T3 is often preferred, free T4 is considered the more accurate way to check T4 levels, which is why it's used more often than total T4. Doctors order it, usually alongside TSH, to help diagnose an underactive or overactive thyroid, or to monitor treatment for a known thyroid condition.
The test measures free T4, the small unbound fraction of thyroxine in your blood that's available for your tissues to use. A low free T4 usually points to hypothyroidism, while a high result points to hyperthyroidism, though your doctor reads it together with TSH to work out whether the problem starts in the thyroid gland or in the pituitary gland that controls it.
Thyroxine (T4) is the main hormone your thyroid gland releases into your bloodstream, and your body converts it into the more active T3 as needed. Most T4 travels bound to carrier proteins; the free T4 measured in this test is the small unattached portion available for your tissues to use.
- Symptoms of an underactive thyroid, such as fatigue, weight gain, or feeling cold - Symptoms of an overactive thyroid, such as weight loss, rapid heartbeat, or heat intolerance - Following up an abnormal TSH result - Monitoring thyroid hormone replacement therapy - Investigating a suspected pituitary gland problem
A low free T4 can point to hypothyroidism, Hashimoto's disease, or a pituitary problem, while a high result can suggest hyperthyroidism, Graves' disease, or a toxic nodule in the thyroid. Because free T4 is considered more accurate than total T4, it's the version most labs use for routine thyroid checks. Even so, your doctor typically orders it alongside TSH, and sometimes T3, rather than reading FT4 on its own, since the combination shows whether the issue lies in the thyroid gland or the pituitary gland that signals it.
A low free T4 often points to an underactive thyroid, called hypothyroidism, which can also stem from conditions like Hashimoto's disease or a pituitary problem. Your doctor will confirm the cause using your TSH result and symptoms together.
A high free T4 can indicate hyperthyroidism, an overactive thyroid gland, which may be caused by conditions like Graves' disease or a toxic nodule. Your doctor will look at your TSH and symptoms alongside this result.
Fasting isn't usually required for this test. Confirm with your doctor or the centre when you book, since instructions can vary by lab.
TSH tells your doctor how hard your pituitary gland is working to control your thyroid, while FT4 shows the actual hormone level. Reading them together shows whether a problem starts in the thyroid or in the pituitary gland.
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.
A commonly cited range is roughly 0.8 to 1.9 ng/dL, but exact ranges vary by lab and testing method, so check the reference range printed on your own report. Ask your doctor to explain what your specific number means.
Yes, thyroid medicines, biotin supplements, and some other drugs can change your FT4 reading. Tell your doctor about everything you're taking before the test.
Most doctors and labs prefer free T4 over total T4 because it isn't affected by changes in the proteins that carry the hormone, such as during pregnancy or with certain medicines. That's why free T4 is used more often for routine thyroid checks.