aberrant formation or incomplete differentiation of either of the two small, paired endocrine glands, usually found embedded in the connective tissue capsule on the posterior surface of the thyroid gland, which secrete parathyroid hormone (PTH), a hormone that regulates calcium and phosphorous metabolism; in mouse, the parathyroids develop with the thymus from two common parathyroid/thymus primordia originating from the third pharyngeal pouch endoderm; the third pharyngeal pouches are formed at E9.5-E10 and are patterned into dorsal/anterior parathyroid and ventral/posterior thymus domains; the third pouch endoderm proliferates to form bilateral parathyroid/thymus common primordia at E11-E11.5; each primordium separates into one parathyroid gland and one thymus lobe at E12.5-E13.5, which then migrate to their eventual adult locations by about E14.5; in the adult mouse, the parathyroids are located near or embedded within the thyroid gland, and the thymus is situated in the anterior chest cavity