Brief Anatomy and Embryology
The parathyroid glands, typically four in number, develop from the dorsal extremities of the third and fourth pharyngeal (branchial) pouches and are sometimes therefore designated as parathyroid glands III and parathyroid glands IV. In the course of their development, the parathyroids derived from the third pouches soon become associated with the thymus, which develops primarily from these pouches, while those from the fourth pouches become associated with the developing thyroid gland.
As the thyroid and thymus, with their associated parathyroid glands, move caudally from the region in which they originate, the thymus normally descends beyond the level at which the thyroid halts. The parathyroids derived from the third branchial pouches are therefore typically carried farther than those derived from the fourth branchial pouches. Thus the parathyroids from the fourth pouches are typically located more cranially on the thyroid gland, and are called superior parathyroid glands, while those derived from the third pouches are typically freed from the thymus and become associated with the thyroid gland toward its lower pole, becoming the inferior parathyroid glands.
(Hollinshead Anatomy for Surgeons: Volume I, The Head and Neck, 2d Edition, 1968 Harper & Row Publishers, pp 584-585.)