16.9: Measurement of serum 1,25‑dihyroxyvitamin D (18b.10)
- Page ID
- 117081
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The active form of vitamin D is 1,25(OH)2D (also called calcitriol) interacts with its nuclear receptor in the intestine, bone, and kidney for most of its functions to regulate calcium and bone metabolism. 1,25(OH)2D also has many other noncalcemic cellular actions that reside in the Paracrine/Autocrine pathway (Norman, 2008; Wacker and Holick, 2013).
1,25(OH)2D in serum is not a useful marker of vitamin D status because it has a short half-life (4–6h) and levels are under stringent homeostatic regulation by factors at the site of 1,25(OH)2D synthesis in the kidney. Hence, it is not surprising that no seasonal variation in serum 1,25(OH)2D concentrations has been reported (Landin-Wilhelmsen et al.,1995).
Synthesis of 1,25(OH)2D in the kidney is stimulated by low serum concentrations of calcium or phosphorus and is inhibited by excess 1,25(OH)2D. In cases of vitamin D sufficiency, a positive relationship exists between serum 1,25(OH)2D and 25(OH)D concentrations (Need et al., 2000), presumably because 25(OH)D is the substrate for 1,25(OH)2D. In vitamin D deficiency, however, this relationship is reversed because with a fall in serum 25(OH)D concentrations there is a rise in the concentration of parathyroid hormone resulting in an increase in the renal production of 1,25(OH)2D ( Figure 18b.2) (Wacker and Holick, 2013). As a result, the circulating concentrations of 1,25(OH)2D often become normal or even elevated. In contrast, serum 1,25(OH)2D levels decrease in renal disease (which affects the enzyme, 25(OH)D-1-α-hydroxylase); levels are very low in anephric patients (i.e., those lacking a functional kidney), and in patients on hemodialysis. Even in normal healthy subjects, concentrations of serum 1,25(OH)2D are in the picomolar range, making analysis difficult (Zittermann, 2003). Serum 1,25(OH)2D concentrations ranging from 60–100pmol/L are normal in adults; concentrations in children tend to be higher. Methods of analysis include HPLC, LC-MS/MS and radioimmunoassay. (Zittermann et al., 2016).