Triiodothyronine, or T3, and whether low T3 is serious or not depends on the circumstances and varies from person to person, and needs to be evaluated in conjunction with other tests. Thyroid deficiency, pituitary disease, Hashimoto’s thyroiditis, permanent damage to the thyroid gland, etc., can lead to low T3, and symptoms of hypothyroidism such as fear of cold, weight gain, slow heart rhythm, and fatigue may appear. If low T3 is caused by iodine deficiency, anti-thyroid drugs, etc., iodine supplementation or discontinuation of the drug can generally return to normal, which is not considered serious. If the patient has a low T3, it is recommended that the patient go to the hospital for timely and standardized treatment to avoid delaying the condition.