SAT-801 Hypomagnesemia associated with Teriparatide (ForteoTM) administrationdue to Hypermagnesuria

Abstract Disclosure: I. Ebrahim: None. D.B. Maxwell: None. N.O. Vietor: None. T.D. Hoang: None. M.K. Shakir: None. In high-risk patients with osteoporosis, anabolic agent such as teriparatide, is generally recommended as a first-line therapy. The most frequent adverse events reported with teriparatide are hypercalcemia, often transient, and other nonspefic symptoms. Hypomagnesemia has been observed as an adverse event during the use of teriparatide although the exact pathogenesis is not known. We are reporting 2 patients, treated with teriparatide-induced hypomagnesemia due to increased renal loss of magnesium (Mg). Case 1: A 68-year-old woman was evaluated for management of severe osteoporosis (T-scores: L- Spine -3.2, Total Hip -3.8, Femoral Neck -3.1). Patient had no other risk factors other than early menopause. Lab values: serum calcium 9.8 mg/dL, phosphorus 3.6 mg/dL, Mg 1.8 mg/dL (ref 1.7 to 2.2 mg/dL), creatinine 0.91 mg/dL, PTH 45 pg/mL. LitholinkTM (Lab Corp) performed as part of screening for an increased risk of nephrolithiasis 24-hrs urine: calcium 178 mg, Mg 118 mg (ref 30 - 120). Patient was treated with teriparatide 20 mcg s.c daily. Six months later, lab values: serum calcium 10.1 mg/dL, Mg 0.98 mg/dL, 24-hrs urine Mg 368 mg/24 hrs, fractional excretion of Mg (FeMg) 3.4% (ref 0.5-3%). The patient was treated with magnesium oxide and Teriparatide was continued. Case 2: A 63-year-old man presented for management of osteoporosis (T-scores: L- Spine -3.1, Total Hip -2.9, Femoral Neck -3.2). He also had a history of lumbar vertebral fracture. Lab values: serum calcium 9.1 mg/dL, Mg 1.8 mg/dL, 25-Vitamin D 32 ng/mL, PTH 39 pg/mL, 24-hour urine calcium 210 mg. Patient was treated with teriparatide 20 mcg s.c daily, and 5 months later, laboratory values showed: serum calcium 10.1 mg/dL, Mg 1.0 mg/dL, 24-hour urine Mg 311 mg, FeMg 3.1%. Patient was treated with magnesium oxide and was continued on calcium citrate and Vit D. Two years later, after discontinuing teriparatide, the 24-hour urine Mg normalized (112 mg) . Serum calcium, creatinine, and PTH remained normal throughout the treatment period in both patients. Previous studies have shown that old age and lower baseline serum Mg were associated with teriparatide-induced hypomagnesemia. Although the underlying mechanisms are not known, deposition of Mg into bones due to increased bone metabolism and renal Mg losses due to transient hypercalcemia and its activation of calcium-sensing receptor (CaSR) on the Henle's loop inhibiting paracellular reabsorption of Mg may play a role. Additionally, teriparatide administration may affect the recently reported transient receptor potential channels in the renal tubules. In our 2 patients renal Mg losses could have played a role although, deposition of Mg into bones due to increased teriparatide induced-bone metabolism may also be involved. Monitoring of serum Mg along with other labs should be considered in these patients treated with teriparatide. Presentation: Saturday, July 12, 2025