Bone damage in hyperparathyroidism. Coloured magnetic resonance imaging (MRI) scan of a section through the head of a 27-year-old patient with hyperparathyroidism, showing osteosclerosis and bone erosion at the base of the skull. Hyperparathyroidism is the overactivity of the parathyroid glands, resulting in the excess production of parathyroid hormone (PTH), which regulates blood calcium levels. It does this by triggering bone resorption. Therefore, raised PTH levels are harmful to bone as they trigger excessive resorption of bone tissue.