Abstract Disclosure: R. Alemzadeh: None. J.D. Ross: None. D. Guillen-Mendoza: None. Background: Pseudohypoparathyroidism (PHP) is a heterogeneous group of disorders due to renal resistance to parathyroid hormone (PTH) resulting in biochemical hypoparathyroidism but with skeletal sensitivity to PTH. Case Presentation: A 15-year-old African American male was incidentally found to have biochemical evidence of severe hypocalcemia during routine evaluation of his bilateral slipped capital-femoral epiphyses. The patient was at age-appropriate stage of puberty with normal height and without any features of hereditary osteodystrophy. However, he had history of mild developmental delay requiring school accommodations. On physical exam, there was some mild dyscoordination with finger and toe-tapping and a Myerson’s sign, negative tetany signs. The biochemical screening revealed markedly low serum calcium (4.9 mg/dL) with elevated phosphorus (7.8 mg/dL) and PTH (1314 pg/mL) levels. His hypocalcemia was accentuated by the presence of severe 25 hydroxy-vitamin D deficiency (7.5 ng/mL) with mild hypomagnesemia (1.4 mg/dL). He had normal renal and thyroid function tests with normal growth factors. There were minimal plasma cyclic AMP responses to a single dose of intravenous teriparatide (Ellsworth-Howard test) consistent with diagnosis of pseudohypoparathyroidism. The skeletal survey showed bowing and under-tubulation of the radius, ulna, tibia, and fibula as well as slipped capital femoral epiphysis bilaterally. His brain CT scan showed symmetrical severe dense calcifications in the basal ganglia, bilateral frontal, parietal and temporal subcortical white matter, pons and right dentate nucleus. Genetic testing did not reveal any GNAS mutations. The patient was initially treated with parenteral calcium followed by high-dose oral calcium supplements and calcitriol. His coexisting hypovitaminosis D and hypomagnesemia were corrected with oral high-dose cholecalciferol and magnesium sulfate, respectively. Conclusions: An incidental finding of severe asymptomatic hypocalcemia and elevated PTH without hereditary osteodystrophy phenotype raised the clinical suspicion of pseudohypoparathyroidism type 1B. This was associated with cerebral calcifications with increasing risk of worsening learning disabilities and minimal neurological findings. Presentation: Thursday, June 15, 2023