Primary hyperparathyroidism: Association with coexistent secondary causes of hypercalcemia

Primary hyperparathyroidism is most often diagnosed without confounding medical conditions or medications that may make interpretation of laboratory data more difficult. Occasionally, however, biochemical findings are not what were expected for this diagnosis. In some cases, serum calcium is much higher than anticipated for the level of parathyroid hormone, and in others, parathyroid hormone is lower than it should be for the serum calcium level. These findings suggest that coexisting secondary causes of hypercalcemia may be present. Common factors that stimulate parathyroid hormone secretion, such as vitamin D deficiency, suboptimal calcium intake or absorption, or idiopathic hypercalciuria, must often be ruled out to clarify the cause of hyperparathyroidism. If the workup shows no evident physiological causes to explain the hyperparathyroidism, then it is reasonable to assume that other coexisting causes of hypercalcemia must be present and to evaluate for these.