Complex sentence: Parathyroid surgery is usually performed when there is hyperparathyroidism this conditions cause many diseases related with calcium reabsorption, because the principal function of the parathyroid hormone is to regulate it.
Split into simpler sentences: Parathyroid surgery is usually performed when there is hyperparathyroidism. This condition causes many diseases related with calcium reabsorption, because the principal function of the parathyroid hormone is to regulate it.