PTH and familial hypocalciuric hypercalcemia: Familial hypocalciuric hypercalcemia (FHH) is a rare inherited disorder of the calcium-sensing receptor gene, CASR. FHH can be biochemically similar to PHPT with an elevated calcium level associated with a normal or high PTH level, a normal 25(OH)D level, and a high 1,25-dihydroxyvitamin D level.