SUN-778 Out of Sight, Out of Control: A Four-Decade Journey with a Rare PTH-resistance Disorder

Abstract Disclosure: T. Prazeres: None. S. Tariq: None. T. Rave: None. H.D. Sacoto: None. R. Belokovskaya: None. A. Franco-Akel: None. Pseudohypoparathyroidism (PHP) is a rare disorder that affects around 0.79 per 100,000 individuals. It is characterized by organ resistance to parathyroid hormone (PTH) due to a GNAS gene mutation. This can result in low calcium, high phosphorus, and high PTH levels at a young age, which can lead to bone fractures, seizures, or tetany. We present a 42-year-old female known to have PHP after presenting with hypocalcemic seizures at the age of 2. During childhood, she had multiple pathologic fractures and a bilateral hip fracture requiring surgery at the age of 9. She was initially treated with calcitriol but had inconsistent care for more than 10 years. When the patient returned to care, she presented with asymptomatic severe hypocalcemia - 6.7 mg/dL [8.4 - 10.5 mg/dL] and 25-hydroxy vitamin D (VitD25OH) deficiency - 17.2 ng/mL [30 - 80 ng/mL], markedly elevated PTH - 657 pg/mL [15 - 65 pg/mL], hyperphosphatemia - 5.0 mg/dL [2.5 - 4.5 mg/dL], subclinical hypothyroidism (TSH 4.37 uIU/mL [0.27 - 4.20 uIU/mL] with normal free T4). She was clinically euthyroid, with a regular menstrual history, and denied seizures, paresthesia, arthralgia, recent fractures, or a family history of similar disorders. Her BMI was 29.6 kg/m2, with a normal stature and phenotype, no dysmorphism, or signs of osteodystrophy. Treatment was resumed with calcium carbonate, calcitriol, and ergocalciferol, and the patient was referred to genetic medicine for further testing. On follow up VitD 25OH levels improved, and TSH normalized without any treatment, however, she had persistent severe hypocalcemia - 7.7 mg/dL and elevated PTH - 525 pg/mL, even with escalating therapy. She is currently being treated with a total daily amount of 7500 mg of calcium carbonate (elemental calcium 3000 mg), 1 mcg of calcitriol, and 4000 IU of VitD25OH. This case underscores the importance of consistent follow-up in PHP, where gaps in care can lead to severe, persistent biochemical imbalances and increased therapeutic complexity. It reminds us that hypocalcemia should be considered in cases of seizure and bone fractures. The latter is associated with such severe hypocalcemia that very few cases are described, making this case even more unique. Lastly, a detailed history, clinical features, and laboratory tests help suggest that this case can be a type 1B PHP, however, a genetic workup is essential to confirm the definitive diagnosis and provide management guidance. Presentation: Sunday, July 13, 2025