Production of the CYP27B1 enzyme is stimulated by PTH, which is secreted in response to a lack of calcium. It is also stimulated by the hypophosphatemic action of FGF23 on renal phosphate excretion, but to a lesser extent. When PTH is suppressed, or FGF23, produced by osteocytes, is stimulated, 1α-hydroxylation is markedly reduced (Liu et al., 2007; Quarles, 2008). Furthermore, calcitriol can act as a suppressor of CYP27B1, although the mechanism is not fully understood.
Calcitriol has its strongest metabolic activity in inducing its own destruction by stimulating the 24-hydroxylase enzyme (now known as CYP24A1;
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3 Overview of Vitamin D ."
Dietary Reference Intakes for Calcium and Vitamin D . Washington, DC: The National Academies Press,
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