Isolated and acquired hypoparathyroidism occurs as an autoimmune disorder either alone or in association with other autoimmune diseases. Reduced PTH (parathyroid hormone) secretion due to disorder of Ca-sensing regulation in parathyroid gland is most commonly caused by activating mutations of the CaSR (Ca-sensing receptor) gene. There has been accumulating evidence that it also occurs as a result of activating autoantibodies directed to CaSR. Hypoparathyroidism is one of the most common endocrine manifestation in APECED (Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy) , which is also known as APS (Autoimmune Polyendocrinopathy Syndrome) type I . In APECED, the spectrum of epitope responsible for the hypoparathyroidism is presumably CaSR. It remains unclear how autoimmunity against CaSR occurs and how much patients caused by this disorder exist.