The Most common cause of vocal cord palsy is

Thyroid surgery is the most common cause when both recurrent and external laryngeal nerves of one side may be involved. It may also occur in lesions of nucleus ambiguous or that of the vagus nerve proximal to the origin of superior laryngeal nerve. Thus, the lesion may lie in the medulla, posterior cranial fossa, jugular foramen or parapharyngeal space. Ref: PL Dhingra Textbook of Ear, Nose and Throat, Edition 6, page - 301