Hyperparathyroidism: An Overview

The four parathyroid glands, normally located behind the four poles of the thyroid gland, secrete parathyroid hormone (PTH), which regulates extracellular calcium levels. Hyperparathyroidism, which can be primary, secondary, tertiary or quaternary, results in excess production of PTH. Excessive PTH secretion may be due to hyper-function in the glands themselves, in which case it is referred to as primary hyperparathyroidism and leads to hypercalcemia (elevated calcium levels). In these cases, oversecretion of PTH is due to adenoma, hyperplasia or, rarely, carcinoma of the parathyroid glands. It may also occur in response to low calcium levels, as encountered in various situations such as chronic kidney disease (CKD) and Vitamin D deficiency; this is referred to as secondary hyperparathyroidism. Secondary hyperparathyroidism can also result from malabsorption (chronic pancreatitis, small bowel disease, malabsorptiondependent bariatric surgery) in that the fat soluble vitamin D cannot get reabsorbed or rarely, long-term lithium treatment. Tertiary hyperparathyroidism is seen in patients with long-term secondary hyperparathyroidism which eventually leads to hyperplasia of the parathyroid glands and a loss of response to serum calcium levels. Quaternary hyperparathyroidism is rare and may be observed after surgery due to primary hyperparathyroidism, when it has led to renal damage that now again causes a form of secondary (quaternary) hyperparathyroidism that may itself result in autonomy (quintary) hyperparathyroidism. Additionally, quaternary hyperparathyroidism may develop from hungry bone syndrome after parathyroidectomy.