Most cases of primary hyperparathyroidism are sporadic, but in less than 10%, they are familial inherited in nature which are usually present as an autosomal dominant trait, namely, multiple endocrine neoplasia type 1 (MEN1), MEN2A, MEN4, familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism (NSHPT), autosomal dominant moderate hyperparathyroidism (ADMH), hyperparathyroidism-jaw tumor syndrome (HPT-JT), and familial isolated hyperparathyroidism (FIHPT) [38]. Here, RET is linked to familial isolated hyperparathyroidism.