Use of Teriparatide in Hyperphosphatemic Familial Tumor Calcinosis: Evaluating the Interaction Between FGF23 and PTH on the Phosphaturic Effect.

Hyperphosphatemic familial tumor calcinosis (HFTC) is a rare disease characterized by hyperphosphatemia and calcium and phosphorus crystal deposition. It occurs due to the loss of function of FGF23. Herein, we report a case of a 50-year-old woman diagnosed with HFTC (homozygous variant in the GALNT3 gene, c.803_804 C insertion) with a history of ectopic calcifications in the past 30 years. Laboratory tests on admission were as follows: phosphate (P) 7.1 mg/dL (Normal range (NR) 2.5-4.5 mg/dL), FGF23 c-terminal 2050 RU/mL (NR < 150 RU/mL), and intact FGF23 (iFGF23) 18.93 pg/mL (NR 12.0-69.0 pg/mL). Treatment with acetazolamide, sevelamer, and a phosphorus-restricted diet was started, but phosphatemia remained high and calcifications continued to progress. In an attempt to further decrease P, a 36-day cycle of teriparatide (TPTD) 20 mcg twice daily was added, decreasing P from 6.2 to 5.2 mg/dL and increasing the 1.25(OH)2 vitamin D by 34.2%. As urinalysis was not feasible at the end of the 36-day cycle, a second cycle was performed for another 28 days, producing a similar decrease in P (from 6.4 to 5.5 mg/mL) and an evident decrease in the rate of tubular reabsorption of P (from 97.2 to 85.3%), however, accompanied by a worrying increase in calciuria. The use of TPTD 20 mcg twice daily in a patient with genetic resistance to FGF23 (HFTC) was associated with consistent increase in phosphaturia and reduction in phosphatemia, in addition to an increase in calcitriol. The resulting hypercalciuria precludes the therapeutic use of TPTD in HFTC and suggests an important role of FGF23, not only in phosphate homeostasis but also in avoiding any excess of calcitriol.