Parathyroid cyst is a rare disorder of the parathyroid, with a clinical presentation ranging from a nonsymptomatic mass to severe acute parathyroid crisis. We present here a case of a 73-year-old man who had a neck mass for 5 months before presenting to the emergency room with symptoms of severe hypercalcemia. The investigations during his admission led to the suspicion of a hyperfunctioning parathyroid cyst causing the hyperparathyroidism. Total parathyroidectomy was done. The mass was resected and confirmed to be a functional parathyroid cyst. Patient's calcium levels returned to normal after excision. Genetic analysis of the mass showed a single missense mutation c.2164G>A (NM_000215), p.Val722ile (NP_000206) was identified representing the single-nucleotide polymorphism rs3213409 in the JAK3 gene.Parathyroid cysts, although rare, should be considered in a patient with a neck mass and hypercalcemia, We discussed the findings of our genetic analysis.