A high or inappropriately normal PTH level suggests PTH mediated hypercalcemia and narrows the differential to Primary Hyperparathyroidism and Familial Hypocalciuric Hypercalcemia, provided vitamin D deficiency and/or very low calcium intake, renal insufficiency, and use of thiazides or lithium have been ruled out. This evidence concerns the gene PTH and familial hypocalciuric hypercalcemia.