The parathyroid glands, usually four, 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 (Fig. 6.5/5 and 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 brick-red 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 distinguished from lymph nodes on the basis of their paleness and lack of the smooth and glistening exterior. 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 close relationship of the parathyroid glands to the thyroid points to the need for caution in thyroid surgery.
The parathyroid hormone (parathormone) plays a vital role in regulating the absorption of calcium from the gut, its mobilization from the skeleton, and its excretion in the urine. The production of parathormone is largely regulated by the calcium plasma concentration.