Tertiary hyperparathyroidism occurs when the parathyroid glands become overactive after a long period of low calcium levels, often due to chronic kidney disease. In this condition, the glands produce too much parathyroid hormone (PTH), leading to elevated calcium levels in the blood. This can result in various health issues, including bone pain and kidney stones. Unlike primary hyperparathyroidism, which is usually caused by a tumor, tertiary hyperparathyroidism is often a response to prolonged low calcium levels. Treatment may involve surgery to remove the overactive glands or managing the underlying kidney disease to restore normal calcium balance in the body.