Immunosuppressive therapy of autoimmune hypoparathyroidism in a patient with activating autoantibodies against the calcium‐sensing receptor

Context Activating antibodies directed at the extracellular calcium‐sensing receptor (CaSR) have been described in autoimmune hypoparathyroidism in the setting of isolated hypoparathyroidism or autoimmune polyglandular syndrome type 1. Materials and methods A 34‐year‐old female presented with hypocalcemia (6.0 mg/dL) and hypomagnesemia (1.1 mg/dL) accompanied by low serum PTH (2.4 pg/mL) as well as urinary calcium and magnesium wasting. She was diagnosed with hypoparathyroidism, which was refractory to standard therapy. She was started on 60 mg prednisone and 150 mg azathioprine treatment daily on suspicion of an autoimmune aetiology. The patient was tested for CaSR antibodies. Results The patient was positive for CaSR antibodies of the IgG1 subtype, which stimulated phosphorylation of extracellular signal‐regulated kinases 1 and 2 (ERK1/2) and inositol phosphate (IP) accumulation. Post‐treatment with prednisone and azathioprine, her serum calcium and magnesium normalised, as did her CaSR antibody titre and antibody‐mediated stimulation of ERK1/2 phosphorylation and IP accumulation. Conclusion This is the first demonstration of CaSR antibody‐mediated hypoparathyroidism responsive to immunosuppressive therapy, adding to the evidence that autoimmune hypoparathyroidism can be, in some cases, reversible and not the result of autoimmune parathyroid destruction.