Triiodothyronine (T3): is one of the main thyroid hormones your body makes. It helps control your energy levels, metabolism, body temperature, and many other functions. Normal T3 levels mean your thyroid is working well to keep your body’s systems balanced. Low T3 may cause symptoms like fatigue, weight gain, cold intolerance, or brain fog (often linked to hypothyroidism). High T3 may cause symptoms like anxiety, weight loss, rapid heartbeat, or heat intolerance (often linked to hyperthyroidism).