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Parathyroid glands are small glands of the endocrine system which are located behind the thyroid. There are four parathyroid glands which are normally about the size of a pea. They are shown in this picture as the mustard yellow glands behind the pink thyroid gland. This is their normal color. The sole purpose of the parathyroid glands are to regulate the calcium level in our bodies within a very narrow range so that the nervous and muscular systems can function properly. Although they are neighbors and both part of the endocrine system, the thyroid and parathyroid glands are otherwise unrelated. The single major disease of parathyroid glands is over activity of one or more of the parathyroids which make too much parathyroid hormone causing a potentially serious calcium imbalance. This is called Hyperparathyroidism.
Technically speaking, the four parathyroids arise from the third and forth branchial pouches. This means that they are formed early in embryogenesis along with other organs of the neck.
The lower 2 parathyroids come from the third brachial pouch which is also responsible for producing the thymus gland (not thyroid) which is important for the development of a normal immune system. The thymus eventually sits behind the breast bone (sternum). The upper 2 parathyroids come from the fourth branchial pouch which also produces the thyroid gland.
Normally, as the larger thymus and thyroid glands migrate from the upper neck (where they are formed) into the lower neck and upper chest, they take the small “Passenger” parathyroids with them. The lower parathyroids usually getting stuck in the neck next to the thyroid during this migration and rarely make it all the way into the chest with the thymus. Therefore, under normal circumstances, the lower parathyroids will usually be found just below and behind the bottom of the thyroid. Likewise, the upper parathyroids will be found behind the middle of the thyroid gland. That is to say, normally, all four parathyroids are found around the back side of the thyroid.