Anatomy of the Parathyroids

Abstract

The parathyroid glands are usually four in number, two superior and two inferior. Embryologically, the parathyroid glands are derived from the fourth and the third branchial pouches. The two superior parathyroids, which are derived from the fourth branchial pouch, are located at the posterior aspect of the lateral lobes of the thyroid, corresponding to the cricothyroid junction. Positional variations are less frequent with the superior parathyroid glands, and hence they are easily identifiable during neck exploration. The inferior parathyroids, which are derived from the third branchial pouch, follow the origin and descent of the thymus very closely until they reach their eventual destination at the lower poles of the thyroid gland. Considerable anatomic variations can occur in the position of the inferior parathyroids, and they are found in their expected location (lower poles of the thyroid) in only half of all cases. The lower glands are very often found in the thymic tongue; rarely, they may be located in the mediastinum within the thymus or high in the neck adjacent to the carotid sheath because of an arrest in descent. In 2% to 4% of subjects, a fifth (or sixth) parathyroid gland may be present.