Which of the following is the mechanism for development of hyperparathyroidism in renal failure?
(A) Failure to retain calcium in the kidneys
(B) Increased catabolism of osteoid matrix in cancellous bone
© Failure of absorption of calcium in the gastrointestinal tract
(D) Faulty conversion of 25-hydroxy vitamin D, or 25(OH)D, to vitamin D3, or 25(OH)2D
(E) Renal retention of phosphates
The answer is E. When creatinine clearance falls to below 60 mL/minute, phosphates are retained and a relative hyperphosphatemia results. The reciprocal relationship between serum phosphates and serum calcium gives rise to hypocalcemia, to which the parathyroid glands respond by increasing the output of parathormone. Negative calcium balance is thus a part of the pathophysiology of CRF and results in a loss of bone density. Failure of calcium absorption occurs secondary to loss of strogen support in menopause because of the loss of the parathormone-inhibiting effects of estrogen, relevant in Choices C and D but not in renal failure. The postmenopausal state is associated withhyperparathyroidism for a different reason, which in turn contributes to postmenopausal decrease in postmenopausal bone density.