MEN1 and familial hypocalciuric hypercalcemia: Other less common causes of PHPT include parathyroid hyperplasia, often observed in multiple endocrine neoplasias types 1 and 2a (MEN1, MEN2A), and inactivating heterozygous mutations of the CaSR gene, associated with most cases of familial hypocalciuric hypercalcemia (FHH).