T3 is a hormone secreted by the thyroid gland that helps regulate body temperature, heart rate and growth, according to MedicineNet. The technical name of T3 is triiodothyronine or liothyronine, and the hormone has three iodine molecules attached to its main chemical structure.
A blood test assesses how much T3 is secreted by the thyroid. This test is performed when a doctor thinks there is a possibility someone's thyroid is malfunctioning. Higher-than-normal levels of T3 may indicate overactive thyroid, goiter, liver disease or someone taking thyroid medication. Lower-than-normal T3 levels may be a sign of illness, starvation, thyroiditis or an underactive thyroid gland, notes MedlinePlus.
Normal ranges of T3 go from 100 to 200 nanograms per deciliter of blood. Normal values may change between laboratories conducting T3 tests, according to MedlinePlus. Certain drugs and medications may alter someone's T3 levels and create a false reading. Doctors may tell patients to stop taking certain supplements or medications before a T3 test. Birth control pills, steroids, lithium, estrogens and methadone can alter triiodothyronine levels secreted by the thyroid.
Most of T3 binds itself to a protein in the blood, according to WebMD. A blood test detects bound T3 and unattached T3. Less than 1 percent of T3 in the blood is not attached to thyroxine-binding globulin, and T3 levels are normally much lower than T4. Both T3 and T4 combine to regulate the body's metabolism.