T4, Free (Direct): Free T4 is the active form of thyroxine, one of the main thyroid hormones your body produces. Because it is “free,” it is available for your body to use to regulate metabolism, energy, growth, and overall body function. Normal Free T4 means your thyroid is producing enough hormone to support your body’s needs. Low Free T4 may lead to symptoms like fatigue, weight gain, constipation, or feeling cold (often seen in hypothyroidism). High Free T4 may cause symptoms like anxiety, weight loss, rapid heartbeat, or feeling hot (often seen in hyperthyroidism).