Triiodothyronine (T3) Test

This test is used to evaluate free and bound T3 in order to help determine abnormal thyroid function. Triiodothyronine (T3) is a thyroid hormone that affects nearly every physiological process in the body. T3 is the active form of thyroxine (T4), and plays a key role in growth, development, regulating heart rate, metabolism and the body's temperature.

Thyrotoxicosis occurs when there is too much thyroid hormone in the blood. It can be caused by hyperthyroidism, Graves' disease, inflammation of the thyroid gland itself or from a benign tumor. Too little T3 can be a result of hypothyroidism which causes symptoms like fatigue, intolerance to cold temperatures, weight gain, low heart rate, decreased appetite, poor memory, depression, infertility and sore muscles.