Calcium malabsorption does not cause secondary hyperparathyroidism

We challenge the widespread assumption that malabsorption of calcium per se causes secondary hyperparathyroidism. Serum parathyroid hormone (PTH) does not rise at the menopause despite the fall in calcium absorption, nor is it raised in osteoporotic women with vertebral fractures despite their low calcium absorption. The age-related rise in serum PTH can be accounted for by the age-related fall in serum 25(OH)D and/or decline in renal function with consequent loss of the calcemic action of vitamin D on bone. The reference interval for serum PTH is established in the fasting state when it is at the top of its diurnal cycle and is maintaining serum ionized calcium at the expense of bone to meet the calcium being lost through skin, bowel, and kidneys. There is no evidence that the fasting PTH is influenced by the previous day's intake or absorption of calcium, although it can be lowered by a large evening calcium supplement. Malabsorption of calcium-like dietary calcium deficiency-is a risk factor for osteoporosis because it reduces or prevents the normal food-related daytime fall in PTH and bone resorption, not because it causes secondary hyperparathyroidism.