Triiodothyronine (T3), Free: Free T3 is the active form of the thyroid hormone T3 that is not bound to proteins in your blood. Because it is “free,” your body can use it right away to help control energy, metabolism, mood, and body temperature. Normal Free T3 shows that your thyroid is making and converting hormones properly. Low Free T3 may cause symptoms like fatigue, brain fog, weight gain, or feeling cold. High Free T3 may cause symptoms like anxiety, weight loss, palpitations, or feeling overheated.