A study of 645 normal adult parathyroid glands in 160 cadavers revealed that there is a definite pattern of anatomic distribution on the basis of the embryologic development of the parathyroid, thyroid, and thymic glands. The sites of predilection of the upper gland (Parathyroid IV) are, in order of frequency, the cricothyroid junction; the dorsum of the upper pole of the thyroid; and the retropharyngeal space. Those of the lower gland (Parathyroid III) are at the lower pole of the thyroid and the thymic tongue; rarely in the upper, the lateral neck, or the mediastinum. An understanding of the developmental relationship of the parathyroid glands to the thyroid and the thymus is fundamental in the delineation of the embryologic origin of the parathyroid glands. The parathyroid gland, located within the surgical capsule of the thyroid (subcapsular), when diseased, remains in place locally. A gland outside of the capsule (extracapsular) is often displaced into the posterior or anterior mediastinum. A collective assessment of the size, weight, color, shape, and consistency of the parathyroid gland is mandatory in the determination of its normalcy. Frozen section examination for stromal and intracellular fatty content is an added assurance of normalcy. That parathyroid glands sink in saline solution, and fat globules float, may aid in differentiating the two types of tissue. Supernumerary, fused, and intrathyroidal parathyroids, albeit rare, are of surgical importance.
