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. The gene discussed is PTH; the disease is Albright hereditary osteodystrophy.