What causes hyperparathyroidism?

In more than 80% of all people with primary hyperparathyroidism, it is caused by an adenoma. This is a growth made out of parathyroid gland tissue. Adenomas are often directly inside one of the four parathyroid glands. This can increase the size of the gland overall. Sometimes parathyroid adenomas grow elsewhere in the neck area – for example, inside the thyroid gland – or in the chest. But that's rare.

Normally, the parathyroid glands make bigger or smaller amounts of the parathyroid hormone based on what the body needs: If there's enough calcium in the blood, they make less of the hormone. If the calcium levels drop, the glands make more of the hormone again. But adenomas make more of the parathyroid hormone regardless of what the body needs.

In rare cases, people have more than one adenoma, which may then be inside different parathyroid glands. It is also rare for all four parathyroid glands to be enlarged and all produce too much parathyroid hormone. If that happens, though, it's called parathyroid hyperplasia.

It is not known exactly why some people develop adenomas. Experts believe that genes play a role. This is especially true when all four parathyroid glands are enlarged, but it also applies to other, rare forms. These include medical conditions where there are similar growths in other glands as well – such as the pancreas, the pituitary gland or the adrenal glands. There is also a rare, inherited form of hyperparathyroidism that starts shortly after birth and, if left untreated, can greatly affect the child's bone growth and other things.

These mainly include medications with lithium in them that can be prescribed for the treatment of mental health conditions such as bipolar disorder. Experts also believe that exposure to radiation increases the risk of hyperparathyroidism.

Hyperparathyroidism is only rarely caused by parathyroid cancer: in less than 1% of all cases. Like adenomas, cancerous tumors make too much parathyroid hormone in an uncontrolled way.

When people talk about hyperparathyroidism caused by cancer, they often mean something known as paraneoplastic syndrome. But this is not the same as parathyroid cancer. Instead, the syndrome can be caused by certain cancer tumors in other parts of the body, such as the breast or the lungs. These tumors make a neurotransmitter (a type of chemical messenger) that is similar to parathyroid hormone and has the same effect: People with paraneoplastic syndrome also have high calcium levels and may experience other typical hyperparathyroidism symptoms, such as belly pain. But a blood test can be done to see whether there are higher levels of the “real” parathyroid hormone or the similar neurotransmitter.

Various medical conditions cause calcium levels in the body to go down. A typical example is chronic kidney disease: If the kidneys don't work properly, important things in the blood (such as calcium and phosphate) get out of balance. The parathyroid glands react to this by producing more parathyroid hormone to make up for the lack of calcium, among other things. Calcium is then released from the bones. Because of this, people with kidney disease may have weaker bones that break more easily.

Other medical conditions lead to a lack of calcium in the blood because not enough calcium gets into the body: For example, if the digestive tract is chronically inflamed or if it's a lot shorter after surgery, the body has a harder time absorbing nutrients such as calcium. A vitamin D deficiency can also lead to a calcium deficiency. The body needs vitamin D in order to absorb calcium properly in the bowel. One possible cause of a vitamin D deficiency is liver disease. In order for vitamin D to act as a hormone, it must be activated in the kidneys first. Because of this, kidney damage can also cause problems associated with a vitamin D deficiency – like a calcium deficiency – and lead to hyperparathyroidism.

There are also rare forms known as tertiary and quaternary hyperparathyroidism. In tertiary hyperparathyroidism, a different medical condition causes the parathyroid glands to continuously produce more parathyroid hormone. Over time, the glands stop reacting to whether or not there is actually enough calcium in the blood – similar to what happens with an adenoma. Quaternary hyperparathyroidism is a condition where an adenoma has led to calcification (hardening) of the kidney tissue. As a result, the kidneys can no longer help to make vitamin D – and the lack of vitamin D then causes the parathyroid glands to make more parathyroid hormone.