The main reason for muffled voice after thyroid surgery is damage to the recurrent laryngeal nerve. Injury to the recurrent laryngeal nerve can occur for two reasons: First, when the thyroid gland was separated during surgery, the recurrent laryngeal nerve was pulled or clamped, but not severed, which may result in transient hoarseness after surgery. In the second case, post-operative tissue repair scarring may cause transient hoarseness due to compression of the recurrent laryngeal nerve. In both cases, since the laryngeal nerve is not severed and not substantially damaged, it usually returns to normal after physical therapy or local rehabilitation for about three months at most. If one side of the laryngeal nerve is severed during surgery and hoarseness occurs, only then will permanent hoarseness occur, which is basically difficult to recover from. Therefore, after thyroid surgery, hoarseness is a normal complication that gradually returns to normal within three months. If the laryngeal nerve is still not recovered after three months or more, the laryngeal nerve is severed and damaged.