PTH and Hypercalcemia: Patients usually develop seizures due to the hypocalcemia, in early life, with hyperphosphatemia, low to undetectable serum PTH levels and normal levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D. On the other hand, gain-of-function mutations in the GCM2 gene cause autosomal dominant hyperparathyroidism type 4 (MIM #617343)9, a disorder characterized by hypercalcemia and elevated or inappropriate PTH secretion by parathyroid glands.