These forms of PHPT (Table 3) may occur as part of the complex syndromes (eg, multiple endocrine neoplasia [MEN] types 1, 2A, and 4 [MEN1, MEN2A, and MEN4]; and the hyperparathyroidism‐jaw tumor [HPT‐JT] syndrome); or a nonsyndromic isolated endocrinopathy such as familial isolated PHPT (FIHP), neonatal severe hyperparathyroidism (NSHPT), or familial hypocalciuric hypercalcemia (FHH). This evidence concerns the gene RET and familial hypocalciuric hypercalcemia.