T4, Free

This test measures free T4 (thyroxine), the form of the main thyroid hormone that is not attached to proteins in your blood and is available for your body to use. Your body converts T4 into T3, a more active hormone that helps regulate energy, metabolism, and body temperature. Free T4 is commonly measured alongside TSH to help identify an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism), especially when TSH results are abnormal. It may also help monitor thyroid treatment. Testing can help investigate fatigue, unexplained weight changes, temperature sensitivity, or changes in heart rate.