In secondary hyperparathyroidism, high levels of PTH occur as a
compensation for hypocalcemia rather
than as a primary abnormality of the parathyroid glands. This contrasts with
primary hyperparathyroidism, which is associated with hypercalcemia.
Secondary hyperparathyroidism can be caused by vitamin D deficiency
or chronic renal disease in which the damaged kidneys are unable to produce
sufficient amounts of the active form of vitamin D,
1,25-dihydroxycholecalciferol. As discussed in more detail in the next section,
the vitamin D deficiency leads to osteo-malacia
(inadequate mineralization of the bones), andhigh levels of PTH cause
absorption of the bones.