Parathyroid gland: An endocrine organ usually associated with the thyroid gland.Embryologically, the glandular primordia are formed in the endoderm of the third and fourthpharyngeal pouches and are associated with the similarly derived primordia of the thymus.There may be from one to three pairs of the small glands present in individuals of the variousvertebrate classes, although two pairs appear most frequently. They are characteristicallywithin, on, or near the thyroid gland. In response to lowered serum calcium concentration, ahormone is produced which promotes bone destruction and inhibits the phosphorus-conserving activity of the kidneys.Anatomy: Parathyroid glands occur in all vertebrate classes with the exception of the fishes,although cells that appear to be homologs of parathyroid cells are found in cyclostomes at thedorsal and ventral ends of all pharyngeal pouches. Whether they function as parathyroidtissue, however, is unknown.Mechanism of action: Parathyroid hormone has two major target organs, bone and kidney.It acts on bone in several ways with both short-term and long-term effects. Short-termchanges include a rapid uptake of bone fluid calcium into osteoblast cells, which in turn pumpthe calcium into the extracellular fluids. Long-term effects of parathyroid hormone includestimulating the activity and increasing the number of osteoclasts, bone cells which act tobreak down bone matrix and release calcium from bone. All of these effects will result inincreased blood calcium values. Parathyroid hormone also inhibits the renal reabsorption ofphosphate, thus increasing the urinary output of phosphate. Phosphate reabsorption acrossthe renal tubule is dependent upon sodium transport, and parathyroid hormone interferes withthis sodium-dependent phosphate transport in the proximal tubule. There are reports thatparathyroid hormone stimulates calcium uptake into the body across the intestine. However,this is not a direct effect. Parathyroid hormone stimulates the production of the most activemetabolite of vitamin D, 1,25-dihydroxycholecalciferol, during vitamin D synthesis. Thismetabolite of vitamin D directly stimulates the intestinal absorption of calcium.THE PARATHYROID GLAND OF DOG:The parathyroid glands are responsible for controlling the calcium in the body. There are fourparathyroid glands, two on each lobe of the thyroid gland. They are located at the base of the .dogs have four parathyroid glands, two on each side of the neck. The parathyroid glands are attached tothe surface or are embedded within the thyroid glands
. One parathyroid gland is located on the top pole (end) of the thyroid and the other is located on thebottom pole. Normally these glands are about 2 to 3 mm in diameter and are tan colored. The glandsproduce parathyroid hormone which causes the calcium level in the blood to increase. Because manyelectrical systems of the body’s organs such as the kidneys, bowels, muscle, and brain are totallydependant on calcium, a change in the normal level of this important element in the blood can be veryharmful to the pet. In addition, high calcium levels can cause stones to form in the urineTHE PARATHYROID GLAND OF Ox :The parathyroid gland consisted of two portions on each side of the Iaryngeal area. They areofficially designated at Externa III and Interna IV. The external gland was located near thetermination of the common carotid artery. It could be associated with the submandibularsalivary gland. The internal gland lay near or embedded in the thyroid gland. In each case itwas a brownish rounded but firm body which could be separated from the surrounding tissue.
Dissection of the neck of a heifer, showing the superior parathyroid glands.THE PARATHYROID GLAND OF HORSE :Location : accounts differ horse is beleaved to have four, atleast two of which are inconnective tissue on or near the thyroid gland remaining two may be in the chest, close to thefirst pair of ribs or elsewhere on the thyroid.Appearance: obviously hard to identify, but irregularly shaped and quite small, the size of ashirt button.Structure : a single-function gland of paler, less dense tissue than the thyroid.