THE PARATHYROID GLANDS
Usually four parathyroid glands, small epithelial bodies located close to or embedded within the much larger thyroid, are present. The parathyroid glands also develop from the pharyngeal lining; one pair (parathyroids III or external parathyroid glands) comes from the third pharyngeal pouches, the other (parathyroids IV or internal parathyroid glands) from the fourth pouches (Figure 6-5/5,7).
In the dog, cat, and small ruminants the parathyroid glands generally become recessed or embedded within the substance of the thyroid gland and frequently escape notice in routine dissections. Once exposed, they can be identified by their pale color, which contrasts with the thyroid tissue. In cattle and the horse they are usually located close to the thyroid gland.The parathyroids III are carried down the neck by the developing thymus and come to rest at various levels, generally near the carotid bifurcations but much farther caudally in the horse (in which they may approach the thoracic inlet). They are also not always easily recognizable because they resemble small lymph nodes; however, they are paler and lack the smooth, glistening exterior of these. These glands are usually located at the rostral end of the thyroid gland in the dog and at the caudal end in the cat.
The parathyroid hormone (parathormone) plays a vital role in the regulation of various aspects of calcium metabolism: absorption from the gut, mobilization from the skeleton, and excretion in the urine. The production of the hormone is largely regulated by the calcium plasma concentration. The close relationship of the parathyroid glands to the thyroid points to the need for caution in thyroid surgery.