One of the diagnostic dilemmas which frequently comes up in the outpatient evaluation of hypercalcemia (and also, I’m told, on boards-type exams) is how to differentiate familial hypocalciuric hypercalcemia versus primary hyperparathroidism–both of which are common causes of hypercalcemia. The short answer: check the urine calcium–it should be low in FHH but normal to high in primary hyperparathyroidism.
Primary hyperparathyroidism, as we are all aware, usually results from either a parathyroid gland adenoma or 4-gland parathyroid hyperplasia. The PTH level is either high or inappropriately normal, and as a result there is constant high urinary calcium reabsorption, constantly high Ca reabsorption from bone, and increased 1,25-OH vitamin D synthesis leading to increased Ca uptake in the gut. The urine calcium concentration is high; hence, the increased risk of nephrolithiasis in these patients. Also, the urine concentrating ability may be diminished in primary hyperparathyroidism.
In contrast, the defect in familial hypocalciuric hypercalcemia is genetic–an autosomal-dominant-inherited mutation in the calcium-sensing receptor gene. Despite hypercalcemia, PTH levels are normal to only mildly elevated. The urine calcium concentration is low (often measured as the urine calcium:creatinine ratio, which in FHH should be