SAT225 An Unexpected Case Of Hypocalcemia

Disclosure: M. Nogueira Cordeiro: None. Calcitriol and oral calcium supplementation remains the standard of care in the management of hypoparathyroidism. The targeted serum calcium level for this condition is quite narrow, typically at the lower end of the reference range to avoid hypercalciuria, but above the calcium level from which a patient may develop symptoms. Pharmacokinetic and pharmacodynamic properties vary among different over-the-counter (OTC) calcium salt preparations, therefore it is important for the provider and patient to understand the differences in calcium formulations to meet therapy goals. We present a case of symptomatic hypocalcemia related to a patient unknowingly switching from calcium carbonate to tricalcium phosphate supplementation. A 71 year-old woman with a history of thyroid cancer status post total thyroidectomy complicated by permanent hypoparathyroidism, CKD stage III, nephrolithiasis and hypercalciuria presented for a follow up visit complaining of perioral numbness for approximately one month. Laboratory evaluation revealed hypocalcemia, hyperphosphatemia, hypomagnesemia and a normal 25-OH-vitamin D level: Ca 7.3 mg/dL (8.5-10.5 mg/dL), Phos 5.6 mg/dL (2.5-4.5 mg/dL), Mg 1.1 mg/dL (1.7-2.8 mg/dL), 25-OH-vit D 45 ng/mL (30-60 ng/mL). Prior to this, her serum calcium had been well controlled, avoiding symptomatic hypocalcemia and hypercalciuria, for years. She had remained on her usual dose of calcitriol, but had changed her calcium supplement to 500 mg of tricalcium phosphate three times per day from calcium carbonate 650 mg three times a day. Rather than adjust her calcitriol, she was recommended to stop tricalcium phosphate and restart the previous regimen of calcium carbonate; serum calcium and phosphorus improved within 72 hours (Ca 7.7 mg/dL, Phos 3.8 mg/dL) and two weeks later her calcium level was back at goal (8.2 mg/dL), phosphate level had normalized (4.5 mg/dL) and she was no longer symptomatic. Tricalcium phosphate is not generally recommended for patients with hypoparathyroidism given they are less available and not as well tested. Previous studies have shown conflicting outcomes for serum calcium elevation after an oral load of tricalcium phosphate when compared with calcium carbonate. Elderly individuals might also experience an elevation in phosphate levels after a tricalcium phosphate load. Additionally, the elemental calcium in OTC tricalcium formulations is often less than what is available in similar appearing calcium carbonate or citrate supplements. Although calcium carbonate and citrate are the most commonly prescribed calcium supplements, other preparations are available OTC and it is difficult to tell these apart. Given the narrow goal of serum calcium for patients with hypoparathyroidism, providers should confirm the calcium formulation at each visit and counsel the patient to notify them if any changes are made in their oral calcium supplementation. Presentation: Saturday, June 17, 2023