This test measures the amount of triiodothyronine, or T3, in the blood. T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4).
Measurement of T3 helps to diagnose hyperthyroidism, an excess production of T3, with symptoms such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes. If the thyroid gland produces insufficient amounts of thyroid hormones, then the patient may have symptoms associated with hypothyroidism and a slowed metabolism, such as weight gain, dry skin, fatigue, and constipation. About 99.7% of the T3 found in the blood is attached to a protein (primarily thyroxine-binding globulin - TBG - but also several other proteins) and the rest is free (unattached). Separate blood tests can be performed to measure either the total (both bound and unattached) or free (unattached) T3 hormone in the blood. A total or free T3 test may be ordered when a patient has an abnormal TSH and/or T4 test result. It may be ordered as part of the investigative workup when a patient has symptoms suggesting hyperthyroidism. One of the T3 tests may sometimes be ordered at intervals to monitor a known thyroid condition and to help monitor the effectiveness of treatment for hyperthyroidism.