SUN-763 - Late Presentation of Familial Isolated Hypoparathyroidism in Adulthood

Introduction: Autoimmune and genetic types account for a minority of cases of hypoparathyroidism, with familial isolated hypoparathyroidism (FIH) often manifesting in childhood. We present a genetically confirmed case of adult-onset FIH Type 1 identified in a middle-aged adult with no prior surgical or autoimmune history.

Case: A 47-year-old Hispanic male with asthma, hypertension, and prediabetes presented to the emergency department for shortness of breath and was admitted for asthma exacerbation. On admission, vital signs were significant for hypertension and tachypnea. Laboratory evaluation showed potassium 2.9 (3.5-5.3 mEq/L), calcium 5.1 (9.2-11.0 mg/dL), phosphorus 5.5 (2.7-4.5 mg/dL), creatinine 1.6 (0.6-1.2 mg/dL), an elevated D-dimer, and an electrocardiogram showing prolonged QTc. A chest CTA excluded pulmonary embolism and echocardiogram revealed mildly decreased ejection fraction. The patient was treated with albuterol and prednisone which resolved asthma exacerbation. However, the patient continued to have hypocalcemia despite continued IV calcium repletion. He denied family history of calcium disorders or prior neck surgery. He denied numbness or tingling. A focused physical exam was negative for Chvostek's sign and Trousseau's sign. Further work-up showed ionized calcium 2.5 (4.65-5.28 mg/dL), parathyroid hormone (PTH) 8 (15-65 pg/mL), vitamin D 25-hydroxy 28.3 (30-96 ng/mL), 24-hour urine calcium test 166.4 (<=300 mg), phosphorus 6.0 (2.7-4.5 mg/dL), serum creatinine 1.4 (0.6-1.2 mg/dL) and GFR 62.4 (>60.0 mL/min). The patient was discharged on calcium carbonate and vitamin D supplements along with asthma medications. During the outpatient endocrinology follow-up, the hypocalcemia persisted with low PTH and high phosphorus which prompted to perform genetic testing which identified homozygous pathogenic intronic variants, c.86+4A>T, in the PTH gene, establishing the diagnosis of FIH Type 1. The patient was subsequently managed with oral calcium carbonate and calcitriol which lead to stabilization of calcium levels.

Discussion: Our case revealed a homozygous pathogenic c.86+4A>T intronic variant in the PTH gene, consistent with FIH Type 1. The pathophysiology is believed to be from the disruption of normal mRNA splicing, resulting in defective PTH biosynthesis. Management of FIH focuses on maintaining serum calcium concentrations in the low-normal range through calcium supplementation and calcitriol, with close monitoring to prevent renal complications such as nephrocalcinosis. Recombinant human PTH (rhPTH 1–84) may be considered in refractory cases, though its role in genetic hypoparathyroidism remains under investigation. FIH type 1 tends to present early in life, though milder variants may remain clinically silent until adulthood. This case highlights unusual adult-onset presentation found during evaluation of unrelated disease.

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