T3 is triiodothyronine and T4 is tetraiodothyronine. T3 and T4 are collectively known as thyroid hormones. Thyroid hormone is a hormone secreted by the thyroid gland, and the function of thyroid hormone secretion by the thyroid gland is regulated by the hypothalamic-pituitary-thyroid axis. The pituitary gland promotes the secretion of thyroid hormone by secreting thyroid stimulating hormone (TSH). Therefore, the determination of thyroid function needs to be combined with T3, T4 and TSH for a comprehensive judgment, and cannot rely solely on T3 or T4. 3 and T4 are mostly bound to thyroid-binding globulin in the blood, called bound hormone, and a small portion is not bound to thyroid-binding globulin, becoming free hormone. The thyroid hormones that work in the blood are the free type thyroid hormones, FT3 and FT4, and FT3 and FT4 are not affected by plasma albumin and better reflect the secretory function of the thyroid gland.