High free triiodothyronine may cause hyperthyroidism symptoms, such as rapid heartbeat, hyper appetite, nervousness and irritability, etc. If there is only a mild elevation of free triiodothyronine, there may be no symptoms. 1. Rapid heartbeat: The increase in thyroid hormone levels will increase the body’s metabolic rate accordingly, which will put the body in a state of excitement and produce symptoms of rapid heartbeat. 2. Excessive appetite: If thyroid hormones are secreted in large quantities, the metabolism will be accelerated and energy consumption will increase, resulting in symptoms of hunger and overeating. 3. Nervousness and irritability: thyroid hormone can make the sympathetic nerves excited, showing nervousness and irritability. If only free triiodothyronine is mildly elevated, it may be asymptomatic. High free triiodothyronine should be judged and treated under the guidance of a doctor.