Free T3 (fT3)

Free T3 directly controls metabolic rate, body temperature, heart rate, and energy production in every cell. Low levels can cause fatigue, weight gain, cold intolerance, depression, and sluggish metabolism, while high levels may lead to anxiety, weight loss, heat intolerance, and rapid heartbeat. Unlike total T3, free T3 isn't affected by changes in binding proteins from pregnancy, medications, or illness, making it a more accurate measure of thyroid function at the tissue level