6990 Vitamin D Supplementation in Primary Hyperparathyroidism: Effects on 1,25(OH)Vitamin D and FGF23 Levels

Abstract Disclosure: S.G. Pallone: None. M. Ohe: None. L.D. Santos: None. I.O. Nakaguma: None. I.S. Kunii: None. R.E. Silva: None. S.S. Maeda: None. C.M. Brandao: None. J.G. Vieira: None. M. Lazaretti-Castro: None. Background: Primary hyperparathyroidism (PHPT) is a condition characterized by high levels of PTH in the presence of hypercalcemia. Vitamin D deficiency has been associated with more severe presentations. Our aim was to evaluate the effects of Vitamin D supplementation on mineral homeostasis and related hormones in individuals with PHPT compared to controls without PHPT. Materials and methods: Individuals with and without PHPT received 14,000 IU/week of oral vitamin D3 for 12 weeks. At baseline and endpoint, blood samples were collected to measure 1,25(OH)2 vitamin D (1,25(OH)2D), intact Fibroblast Growth Factor 23 (FGF23), 25-hydroxyvitamin D (25OHD), Parathormon (PTH), total calcium (tCa), phosphorus and other biochemical markers. The intact FGF23 was measured using an automated Diasorin LIAISON kit. The 1,25(OH)2D measurement was performed using liquid chromatography and mass spectrometry (LC-MS/MS). Results: 70 PHPT patients and 75 healthy individuals (CTRL) were included, and 55 PHPT and 64 CTRL completed the 12-week protocol. At baseline, tCa, PTH, 1,25(OH)2D, and intact FGF23 concentrations were significantly higher in PHTP, with no difference in 25OHD levels. After the intervention, the 25OHD levels increased significantly in both groups (PHPT: 22.6.9±6.1 to 31.5±6.8; CTRL: 20.3±5.7 to 32.5±6.4 ng/mL, p0.001). There was significative increase in intact FGF23 levels in both groups (PHPT: 47.9±27.1 to 76.3±33.3; CTRL: 40.5±13.9 to 59.8±19.8 pg/mL, p0.001). After the vitamin D replacement, the tubular reabsorption of phosphate (TRP) was significantly reduced in both groups (PHPT: 85.0±7.4% to 83.6±7.75% CTRL: 89.5±4.5% to 88.7±4.8%, p=0.031), without changes in serum phosphorus levels, possibly indicating phosphaturic response to the intervention. Moreover, this decrease of TRP was positively correlated to the increment of FGF23 in the PHPT group. A paradoxical decrease in 1,25(OH)2D levels after vitamin D supplementation was observed in both groups (PHPT: 94.8±34.6 to 68.9±25.3; CTRL: 68.7±23.5 to 56.4±20.7 pg/mL, p0.001). The reduction of 1,25(OH) 2 D was inversely correlated with the elevation of FGF23 in both groups (r=-0.302, p=0.028). In both groups, no changes in tCa were observed (PHPT: from 11.02±0.9 to 11.1±0.8; CTRL: 9.6±0.3 to 9.6±0.3 pg/mL, p=0.153), as the same for urinary calcium (p=0.534) and PTH measurements (p=0.516). Conclusion: The weekly administration of 14,000 IU of Vitamin D3 was safe and the increase in 25OHD after supplementation did not change the serum or urinary calcium levels in adults with and without PHPT. In both groups, it was observed a paradoxical decrease in 1,25(OH)2D levels after supplementation, which was associated with significant increase in intact FGF23 levels. This effect can maybe explain the safety response on calcium levels in face of vitamin D supplementation among PHPT patients. Presentation: 6/2/2024