There are two main types of hyperparathyroidism and these are described below.

Primary hyperparathyroidism

This occurs when one or more of the parathyroid glands become overactive and/or enlarged due to a problem that lies within the gland itself. This is the most common form of hyperparathyroidism. The condition is usually caused by a benign tumor or adenoma growing on a single parathyroid gland and causing it to become overactive. This condition is more common among women, particularly after the menopause.

The other two main causes of hyperparathyroidism include enlargement (hyperplasia) of two or more glands and radiotherapy administered to the head or neck. Primary hyperparathyroidism also occurs as a feature of multiple endocrine neoplasia (MEN), a condition characterized by tumors of the endocrine glands. In very rare cases, primary hyperparathyroidism is caused by parathyroid carcinoma.

Patients being treated with lithium for bipolar disorder are also at an increased risk of hyperparathyroidism and hypercalcemia occurs in around 20% of patients who take this medication as a long term therapy. However, in most of these patients, PTH levels are not significantly raised and symptoms of hyperparathyroidism do not manifest.

Secondary hyperparathyroidism

This occurs when another condition such as kidney disease or vitamin D deficiency leads to a low calcium level. The parathyroid gland compensates by producing excess PTH to try and restore the calcium level.

Chronic renal failure is one of the most common causes of secondary hyperparathyroidism. Failure of the kidneys to regulate the calcium and phosphate level properly, can lead to bone disease referred to as renal osteodystrophy.

If the secondary hyperparathyroidism remains untreated, the parathyroid remains permanently overactive and this is referred to as tertiary hyperparathyroidism.