Shieh A, et al. - Experts evaluated if rise in total vs free 25(OH)D is related to the change in downstream biomarkers of 25(OH)D entry into target cells in kidney and parathyroid: 24,25-dihyroxyvitamin D (24,25[OH]₂D) and PTH, respectively. Results demonstrated a stronger association of early rise in free 25(OH)D during treatment of vitamin D deficiency with changes in biomarkers of 25(OH)D entry into target kidney and parathyroid cells, which suggests a physiologic role of free 25(OH)D in humans.

Methods

• Experts conducted a 16-week randomized controlled trial.
• Intervention included 60 μg (2400 IU)/d of D3 or 20 μg/d of 25(OH)D3.
• They included 35 adults age ≥ 18 years with 25(OH)D levels < 20 ng/mL from an academic medical center.
• Main outcome measures included 24,25(OH)₂D, 1,25-dihyroxyvitamin D [1,25(OH)₂D] and PTH.

Results

• As per data, participants (D3 and 25[OH]D3 groups combined) were 35.1 ± 10.6 years at baseline.
• Findings suggested that the mean total 25(OH)D, free 25(OH)D, 24,25(OH)₂D, and PTH were 16.6 ng/mL, 4.6 pg/mL, 1.3 ng/mL, and 37.2 pg/mL, respectively.
• From 0 to 4 weeks, they noted an association of rise in only free 25(OH)D with a concurrent 24,25(OH)₂D increase (P= 0.03, adjusted for change in 1,25[OH]2D and supplementation regimen) and PTH decrease (P= 0.01, adjusted for change in calcium and supplementation regimen).
• Results demonstrated that between 4 and 8 weeks, and again from 8 to 16 weeks, rises in free and total 25(OH)D were correlated to 24,25(OH)₂D increase.
• On the contrary, rise in neither total nor free 25(OH)D was correlated to PTH decrease during these time periods.