There are usually four parathyroid glands-two on each side of the neck. It is possible to have more than four, but unlikely to have fewer than four-most likely when three or fewer glands are found, the fourth gland was simply not identified……
They usually are located at the back of the Thyroid gland and there is an upper, deeper gland and a lower, more superficial gland on each side.
The closest thing to a parathyroid gland in size and shape is a split pea or lentil. Often they are described as being like a grain of rice- but this is the wrong shape.
Normal glands are very small, but very capable, with plenty of reserve functional capacity, weighing only 30-60uGrams. Half of one gland is usually all you need to make enough PTH.
They can be in other places and this is explained by embryology. As an embryo grows, it also develops and transforms from a relatively primitive entity with gill-like structures in the neck to a miniature human form. In doing so the lungs, thyroid, thymus and parathyroid glands all form, develop and migrate from the jaw region toward their final locations.
It is not rare for parathyroid glands to be “undescended”- located higher in the neck than usual, or “over-descended”- located in the chest. They can also end up inside the thyroid gland or off to the side-along the course of the vagus nerve in the neck. Ultimately , they can be located from the base of skull to the level of the heart. But most commonly they are close to the usual locations.
Function
The parathyroid glands are responsible for maintaining steady levels of calcium in the blood via a homone called parathyroid hormone (PTH). Calcium is important in sending electrical messages in nerves, and in muscle contraction. It is important as a major component of bone.
PTH acts to increase absorption of calcium from the gut and to reduce loss of calcium in the urine. It is also important in the constant process of remodelling of bone- that is breaking down and rebuilding bone. Uncontrolled release of increased amounts of PTH from parathyroid adenomas and hyperplastic glands has the effect of reducing bone mineralisation and the calcium from the bone is released into the blood, raising the serum level.
The kidneys try to manage high calcium levels by releasing more into the urine, which can lead to stone formation.
Ultimately bone density is constantly being reduced while there is excess parathyroid function.
