The parathyroid glands also develop during early embryologic development. There are typically four parathyroid glands; however, supernumerary glands and less than four glands have been reported. They are usually located behind the thyroid. The superior parathyroid glands migrate with thyroid accounting for their more predictable location. The lower parathyroid glands migrate with the thymus and their location can be more variable. On very rare occasion the parathyroid gland can be within the thyroid gland. Each parathyroid gland normally weighs about 30 to 40 mg and measuring about 3-8 mm.

Simply Put:

The parathyroid glands are four tiny glands, located in the neck behind your thyroid gland, that control the body's calcium levels. Each gland is about the size of a grain of rice (weighs approximately 30 milligrams and is 3-4 millimeters in diameter).