T3, Free

This test measures free T3 (triiodothyronine), the thyroid hormone available for your body to use. T3 helps regulate energy, metabolism, body temperature, and heart rate. Testing can help evaluate an overactive thyroid (hyperthyroidism), particularly when TSH is low but free T4 is normal. It may help investigate symptoms such as a racing heartbeat, unexplained weight loss, shakiness, or sensitivity to heat. Free T3 is generally less useful for identifying an underactive thyroid and is not routinely needed to monitor most thyroid hormone treatment.