FIHP refers to autosomal dominant PHPT occurring as an isolated endocrinopathy, in the absence of clinical features associated with the known PHPT syndromes (e.g., MEN1, MEN4, HPT-JT).58 Distinguishing FIHP from syndromic PHPT such as MEN1 is difficult as PHPT is frequently the first manifestation of disease. Here, MRS2 is linked to familial isolated hyperparathyroidism.