Magnesium deficiency was corrected with oral magnesium oxide.<h4>Outcomes</h4>The patient remained asymptomatic during follow-up with adequate calcium supplementation.<h4>Lessons</h4>Adult-onset PHP1B should be considered in the differential diagnosis of unexplained hypocalcemia with elevated PTH, even in the absence of Albright hereditary osteodystrophy. Here, PTH is linked to pseudohypoparathyroidism type 1B.