Brown tumours. Coloured X-ray of brown tumours (brown) along the shafts of the tibia bones. The tibia is the larger of the two lower leg bones, The fibula (thinner bone) is also seen. Despite the name, these are not true tumours. They are caused by overactivity of the parathyroid glands (hyperparathyroidism). These glands regulate the body's calcium levels by producing parathyroid hormone. Their overactivity leads to the body acting to remove bone calcium, causing tiny fractures and haemorrhaging. This leads to bone reabsorption and the formation of bone cysts and brown tumours. The condition requires surgery.