Free thyroxine (free T4) tests are used to help evaluate thyroid function and diagnose thyroid diseases, including hyperthyroidism and hypothyroidism, usually after discovering that the thyroid-stimulating hormone (TSH) level is abnormal.
To help evaluate thyroid gland function; to help diagnose thyroid disease; to screen for hypothyroidism in newborns; to monitor the effectiveness of treatment.
Your healthcare provider will collect your blood into a tube or vial and send it to a lab for testing. Typical results for the total T4 test in adults generally range from 4.5 to 11.2 micrograms per deciliter (μg/dL). Results for children vary based on age. So talk to your doctor about the normal ranges expected for your child. There may also be some variation between labs.
In general, high free T4 results may indicate an overactive thyroid gland, and low free T4 results may indicate an underactive thyroid gland. The test results alone are not diagnostic but will prompt the health practitioner to perform additional testing to investigate the cause of the excess or deficiency.