Variations in nerve supply of the thyroid gland

(A) The external branch of the superior laryngeal nerve innervates the inferior constrictor and cricothyroid muscles. The external branch travels with the superior thyroid artery until approximately 1 cm before the artery enters the superior thyroid pole and then divides into branches that enter the lateral inferior pharyngeal constrictor muscle and the cricothyroid muscle. A few smaller branches may be seen entering the superior thyroid.
(B) The RLN is associated with the inferior thyroid artery at approximately the junction of the lower and middle thirds of the thyroid gland. On the left, the RLN ascends in the tracheoesophageal groove and crosses deep to the inferior thyroid artery; on the right, the RLN crosses more obliquely and is oriented more laterally than caudally.
While the nerve most often crosses deep to the inferior thyroid artery, documented variations include passing anterior to the artery as well as passing between branches of the inferior thyroid artery. Another variation of the nerve is the nonrecurrent laryngeal nerve, in which the laryngeal nerve branches directly from the vagus nerve. This variation occurs more commonly on the right side.