Endocrine Abstracts

JOINT1769

Serum phosphate (P) is typically reduced or low-normal in patients affected by primary hyperparathyroidism (PHPT) because of the lower expression of sodium phosphate cotransporter type2a protein in the proximal renal tubule driven by elevated PTH levels. However, current guidelines on PHPT management do not specify a P level threshold or offer recommendation regarding this parameter. To our knowledge, few studies have explored the relationship between P levels and the clinical and biochemical features of PHPT. We retrospectively evaluated 425 consecutive patients, 350 females and 75 males, affected by sporadic PHPT referred to our outpatient clinic from 2018 to 2023. Eighty-six (20%) patients were classified as symptomatic and 339 (80%) as asymptomatic according to the Guidelines from the Fifth International Workshop. Hypophosphatemia (HypoP), defined as phosphate (P) levels <2,5 mg/dL, was found in 245/425 (57%) patients and it was mild (2-2,5 mg/dL), moderate (1-1,9 mg/dL) and severe (< 1 mg/dL) in 183 (75%), 62 (25%) and 0 cases, respectively. P levels were significantly lower in males than in females (2.14±0.43 vs. 2.50±0.43), in patients with symptomatic vs. asymptomatic PHPT (2.34±0.49 vs. 2.46±0.44) and in those with nephrolithiasis (2.31±0.44 vs. 2.50±0.44), but not in patients with osteoporosis at any site (lumbar, femoral sites and 1/3 distal radius) or fragility fractures. Patients were further divided into two groups based on the presence or absence of hypoP. According to the above data, hypoP patients were significantly more frequently males, symptomatic and stone formers, but not osteoporotic. Moreover, patients with hypoP had a more severe biochemical phenotype namely significantly higher PTH (82 vs. 65 ng/l), ionized calcium (1,51 vs. 1,43 mmol/l), total serum calcium (11,1 vs. 10,7 mg/dL) and 24-h urine calcium excretion (360 vs. 244 mg) and significantly lower 25(OH) vitamin D levels (28 vs. 32 mg/l). No significant correlation was found between P levels and bone turnover markers (alkaline phosphatase, osteocalcin and S-CTX), T-score at any site or trabecular bone score TBS values. All patients, except one, with moderate hypoP met at least one surgical criterion. In conclusion, hypoP is associated with a higher risk of nephrolithiasis but not osteoporosis in PHPT patients. The question of whether moderate hypoP could serve as a reliable, cost-effective, and easily accessible criterion for parathyroidectomy, particularly in cases where evaluating all systemic complications is challenging, remains an open question.