The parathyroid glands are four small pea-sized glands that sit 2 on either side of the neck behind the thyroid. (see Location of Parathyroid Glands animation). About 10% of the time, the parathyroid glands may not be in their typical locations and sit in what are known as "ectopic" locations (see Ectopic Location of Parathyroid Glands animation). These ectopic parathyroid glands may be difficult to find and parathyroid surgery requires a surgeon with expertise in finding ectopic parathyroid glands.
The parathyroids control the blood calcium levels by making parathyroid hormone (PTH) in response to low blood calcium levels. PTH increases calcium levels by breaking down bones and taking their calcium, increasing calcium absorption from food in the intestines, and decreasing the amount of calcium lost by the kidneys in the urine. In general, the calcium and PTH levels move in opposite directions—as the blood calcium increases, the PTH level decreases and as the blood calcium level decreases, the PTH level increases. Every cell in the body uses calcium to control its basic functions and communications, so it is critical that calcium levels are tightly controlled.
Vitamin D is an important part of how the body controls calcium levels. Vitamin D can either be made by exposure to sunlight or absorbed in food. PTH then activates vitamin D, which helps the body absorb more calcium from food. Unfortunately, up to a third of people may be vitamin D deficient (i.e. do not have enough vitamin D) and therefore may not absorb enough calcium from the food that they eat. The body then makes more PTH in order to raise the calcium levels. However most of this calcium is taken from the bones. This situation leads to a form of secondary hyperparathyroidism in which the PTH level is high but the calcium levels are normal. See Secondary Hyperparathyroidism». Parathyroid disease and vitamin D deficiency are often seen together since vitamin D deficiency is so common and vitamin D deficiency can have effects on the parathyroid glands. Vitamin D deficiency can lead to temporary hypocalcemia (i.e. calcium levels that are too low) after successful parathyroid surgery but may also lead to issues like kidney stones and very high calcium levels if the deficiency is corrected too fast before the operation. For these reasons, it is important that patients seek the advice of parathyroid experts to find a safe way to bring their vitamin D levels back to normal. See Risks of Parathyroid Surgery ».