Osteitis fibrosa is a complication of hyperparathyroidism (excess parathryroid hormone production) in which bone becomes softened, deformed and may develop cysts.

There are four parathyroid glands in the neck. They produce parathyroid hormone, which helps control calcium levels in the body. Sometimes they produce too much parathyroid hormone (hyperparathyroidism). In these cases, one gland may develop a growth (80% of cases);or all 4 glands may enlarge (15% of cases). Rarely, parathyroid cancer may cause the condition.

Hyperparathyroidism is most common in women over 60. Genetic changes cause it, and it sometimes runs in families. Neck radiation increases the likelihood of developing this disorder. Parathyroid hormone increases bone "turnover" (metabolism) and may soften it.

Before 1950, about half of people diagnosed with hyperparathyroidism had osteitis fibrosa cystica. Now it is uncommon (less than 10% of people with hyperparathyroidism) because patients are diagnosed earlier and have milder elevations of calcium. People with severe hyperparathyroidism, or those who have parathyroid cancer, are more at risk for developing osteitis fibrosa cystica.