Associations Between Change in Total and Free 25-Hydroxyvitamin D With 24,25-Dihydroxyvitamin D and Parathyroid Hormone.

Jun 23, 2018The Journal of clinical endocrinology and metabolism

Changes in total and free 25-Hydroxyvitamin D may relate to levels of 24,25-Dihydroxyvitamin D and parathyroid hormone.

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Abstract

At baseline, mean total 25-hydroxyvitamin D level was 16.6 ng/mL among participants.

A rise in free 25-hydroxyvitamin D was associated with an increase in 24,25-dihydroxyvitamin D after 4 weeks.
A decrease in parathyroid hormone (PTH) was observed alongside an increase in free 25-hydroxyvitamin D at 4 weeks.
From 4 to 16 weeks, increases in both free and total 25-hydroxyvitamin D were linked to higher levels of 24,25-dihydroxyvitamin D.
During the same period, no changes in PTH levels were associated with rises in either form of 25-hydroxyvitamin D.

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CONTEXT: The physiologic role of free 25-hydroxyvitamin D [25(OH)D] in humans is unclear.

OBJECTIVE: To assess whether rise in total vs free 25(OH)D is associated with 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.

DESIGN: 16-week randomized controlled trial.

INTERVENTION: 60 μg (2400 IU)/d of D3 or 20 μg/d of 25(OH)D3.

SETTING: Academic medical center.

PARTICIPANTS: 35 adults age ≥18 years with 25(OH)D levels < 20 ng/mL.

MAIN OUTCOME MEASURES: 24,25(OH)2D, 1,25-dihyroxyvitamin D [1,25(OH)2D] and PTH.

RESULTS: At baseline, participants [D3 and 25(OH)D3 groups combined] were 35.1 ± 10.6 years. 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, rise in only free 25(OH)D was associated 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). Between 4 and 8 weeks, and again from 8 to 16 weeks, rises in free and total 25(OH)D were associated with 24,25(OH)2D increase; in contrast, rise in neither total nor free 25(OH)D was associated with PTH decrease during these time periods.

CONCLUSIONS: Early rise in free 25(OH)D during treatment of vitamin D deficiency was more strongly associated with changes in biomarkers of 25(OH)D entry into target kidney and parathyroid cells, suggesting a physiologic role of free 25(OH)D in humans.

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Associations Between Change in Total and Free 25-Hydroxyvitamin D With 24,25-Dihydroxyvitamin D and Parathyroid Hormone.

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