Parathyroid Hormone Threshold Effect and Parathyroid Hormone-to-Vitamin D Ratio as Determinants of Bone Mineral Density in Long-Term Kidney Transplant Recipients: A Cross-Sectional Analytical Study>.

OBJECTIVES: Persistent hyperparathyroidism and calcitriol deficiency coexist in most long -term kidney transplant recipients, yet the combined skeletal effects of these presentations have not been formally quantified. MATERIALS AND METHODS: In this cross -sectional analytical study at People's Hospital 115 (Ho Chi Minh City, Vietnam ), we studied 135 kidney transplant recipients (mean age 51.0 ± 11.3 years; 66.7 % male; mean time posttransplant 71.4 ± 28.1 months ) who underwent simultaneous measurement of serum intact parathyroid hormone, 1,25 -dihydroxyvitamin D, and dual -energy x -ray absorptiometry at 3 skeletal sites. A prespecified piecewise ordinary least -squares model characterized the relationship between parathyroid hormone and bone mineral density; a confirmatory sensitivity analysis used KDIGO -motivated breakpoint of 65 pg /mL for parathyroid hormone. We evaluated parathyroid hormone -to -vitamin D ratio as a prespecified secondary exposure measure. RESULTS: Bone loss (worst -site T score of < -1.0 ) was present in 103 participants (76.3 % ) and osteoporosis in 27 (20.0 % ). The confirmatory piecewise model at 65 pg /mL demonstrated that above -threshold parathyroid hormone was inversely associated with T -score composite ( β = -0.00367 per pg /mL; bootstrap 95 % CI, -0.0066 to -0.0013; P < .001 ), a 100 pg /mL increment above the threshold corresponded to a -0.37 T -score decrement. Body mass index was the strongest predictor (β = +0.129 per kg /m ²; P < .001 ). The ratio of parathyroid hormone to vitamin D (ρ = -0.262; P = .002 ) provided modestly better model fit than parathyroid hormone alone (R ² = 0.247 vs 0.229; change in Akaike Information Criterion = 3.2 ). CONCLUSIONS: Parathyroid hormone exerted a threshold -dependent inverse effect on bone mineral density in long -term kidney transplant recipients, confirmed at the KDIGO lower boundary of 65 pg /mL. The ratio of parathyroid hormone to vitamin D captured the combined mineral burden more efficiently than parathyroid hormone level alone, warranting further prospective evaluation as a monitoring target in posttransplant mineral disease.