Disorders of Calcium and Phosphate Metabolism Practice Test 2026 – The Complete Guide to Exam Mastery!

Low vitamin D reduces intestinal calcium absorption, so serum calcium falls. The parathyroid glands respond to this hypocalcemia by secreting more PTH, a state referred to as secondary hyperparathyroidism. Elevated PTH tries to restore calcium by increasing bone resorption, enhancing renal calcium reabsorption, and boosting production of active vitamin D, but in vitamin D deficiency the available substrate limits full normalization. PTH also drives phosphate excretion, leading to low phosphate, which further impairs bone mineralization and contributes to rickets. So the expected PTH level is elevated.