Vitamin D is a unique hormone rather than merely a vitamin. Unlike most vitamins that must be obtained exclusively from the diet, vitamin D can be synthesized in the skin upon exposure to ultraviolet B (UVB) radiation from sunlight. The kidney plays a pivotal role in converting inactive vitamin D into its biologically active form, making renal function critical for maintaining calcium-phosphate homeostasis, skeletal health, and several extra-skeletal functions.
Vitamin D deficiency is highly prevalent worldwide and is particularly common in patients with chronic kidney disease (CKD). Understanding vitamin D metabolism is therefore essential for nephrologists and physicians involved in managing CKD-mineral and bone disorder (CKD-MBD).
Vitamin D exists primarily in two forms:
* Produced in the skin from 7-dehydrocholesterol following UVB exposure.
* Present in animal-derived foods such as fatty fish, egg yolk, and liver.
* Derived from plants and fungi.
* Produced by UV irradiation of ergosterol.
Both forms are biologically inactive and require sequential activation in the liver and kidneys.
The skin serves as the primary source of vitamin D in humans.
Step 1: UVB Exposure
Ultraviolet B radiation (290–315 nm) converts 7-dehydrocholesterol in the epidermis into pre-vitamin D3.
Reaction:
7-Dehydrocholesterol + UVB → Pre-vitamin D3
Step 2: Thermal Isomerization
Pre-vitamin D3 undergoes spontaneous thermal conversion to cholecalciferol.
Pre-vitamin D3 → Vitamin D3 (Cholecalciferol)
Vitamin D3 then enters the circulation bound to Vitamin D Binding Protein (DBP).
The first activation step occurs in the liver.
The enzyme 25-hydroxylase (CYP2R1) converts vitamin D into 25-hydroxyvitamin D [25(OH)D], also known as calcidiol.
Equation
Vitamin D3 + O₂ + NADPH
↓ CYP2R1 (Liver)
25-Hydroxyvitamin D [25(OH)D]
25(OH)D is the major circulating form and is measured to assess vitamin D status.
* Vitamin D3: 1–2 days
* 25(OH)D: 2–3 weeks
The kidney performs the crucial second hydroxylation step.
25(OH)D is filtered by the glomerulus and reabsorbed by proximal tubular cells through megalin-mediated endocytosis.
Within proximal tubular mitochondria, the enzyme 1-alpha hydroxylase (CYP27B1) converts 25(OH)D into the biologically active hormone 1,25-dihydroxyvitamin D [1,25(OH)₂D], also known as calcitriol.
25(OH)D + O₂ + NADPH
↓ CYP27B1 (Kidney)
1,25(OH)₂D (Calcitriol)
This step represents the rate-limiting stage in vitamin D activation.
The kidney tightly regulates calcitriol production.
Stimulators
Parathyroid Hormone (PTH)
Low serum calcium stimulates PTH secretion.
PTH ↑ → CYP27B1 ↑ → Calcitriol ↑
Hypophosphatemia
Low phosphate increases calcitriol synthesis.
PO₄ ↓ → CYP27B1 ↑
Fibroblast Growth Factor-23 (FGF-23)
Produced by osteocytes.
FGF-23 ↑ → CYP27B1 ↓
FGF-23 ↑ → CYP24A1 ↑
Hyperphosphatemia
Elevated phosphate suppresses calcitriol synthesis.
Calcitriol
Negative feedback suppresses its own production.
Calcitriol acts through the Vitamin D Receptor (VDR), a nuclear transcription factor.
Intestinal Effects
Increases absorption of:
* Calcium
* Phosphate
* Magnesium
Calcium absorption:
10–15% without vitamin D
30–40% with adequate vitamin D
Promotes:
* Osteoblast differentiation
* Bone mineralization
* Skeletal growth
At high concentrations, calcitriol can stimulate osteoclast-mediated bone resorption.
Suppresses:
* PTH synthesis
* Parathyroid gland hyperplasia
Vitamin D receptors are found on:
* T lymphocytes
* B lymphocytes
* Macrophages
* Dendritic cells
Emerging evidence suggests immunomodulatory functions, although definitive clinical benefits remain under investigation.
Both calcidiol and calcitriol are degraded by the enzyme 24-hydroxylase (CYP24A1).
Equations
25(OH)D
↓
CYP24A1
24,25(OH)₂D
1,25(OH)₂D
Calcitroic acid
Biliary excretion
This pathway prevents vitamin D toxicity.
Several vitamin D formulations are available clinically.
Nutritional Vitamin D
* Cholecalciferol 60,000 IU sachet
* Cholecalciferol capsules
* Ergocalciferol preparations
Active Vitamin D
* Calcitriol 0.25 μg
* Calcitriol 0.5 μg
* Intravenous calcitriol
Selective VDR Activators
* Paricalcitol
* Doxercalciferol
Hypervitaminosis D is usually iatrogenic.
Common Causes
Excessive Supplementation
Most common cause worldwide.
Examples:
* Repeated high-dose injections
* Multiple 60,000 IU sachets consumed daily
* Manufacturing errors
Prescription Errors
Incorrect interpretation of deficiency treatment schedules.
Factitious Vitamin D Intake
Self-medication with over-the-counter preparations.
Granulomatous Disorders
Extrarenal production of calcitriol:
* Sarcoidosis
* Tuberculosis
* Fungal infections
Activated macrophages express CYP27B1.
Lymphoma
Certain lymphomas produce excess calcitriol.
CYP24A1 Mutations
Reduced degradation of vitamin D metabolites.
Biochemical Features of Hypervitaminosis D
Laboratory Findings
* Hypercalcemia
* Hyperphosphatemia
* Suppressed PTH
* Elevated 25(OH)D
Typically:
25(OH)D >150 ng/mL
Neurological
* Confusion
* Lethargy
* Weakness
Gastrointestinal
* Nausea
* Vomiting
* Constipation
Renal
* Polyuria
* Polydipsia
* Nephrocalcinosis
* Acute kidney injury
Cardiovascular
* Arrhythmias
* Hypertension
The kidney is the principal site of calcitriol synthesis.
As CKD progresses:
Loss of Functional Nephrons
Reduced CYP27B1 activity
Reduced calcitriol production
Hypocalcemia
Secondary hyperparathyroidism
Suppresses calcitriol production.
Elevated FGF-23
Further inhibits 1-alpha hydroxylase.
Reduced Megalin Function
Decreases tubular uptake of vitamin D metabolites.
CKD Stages 1–3
Treat nutritional deficiency.
Preferred agents:
* Cholecalciferol
* Ergocalciferol
Target:
25(OH)D >30 ng/mL
Correction of nutritional deficiency alone may be insufficient.
Patients often require:
* Calcitriol
* Alfacalcidol
to suppress secondary hyperparathyroidism.
Nutritional vitamin D should still be corrected.
However, active vitamin D therapy is often necessary because diseased kidneys cannot efficiently convert 25(OH)D to calcitriol.
Options include:
Treatment must be individualized based on:
* PTH
* Alkaline phosphatase
Vitamin D metabolism exemplifies the intimate relationship between the skin, liver, and kidney. While sunlight initiates vitamin D synthesis and the liver performs the first hydroxylation step, the kidney serves as the master regulator by converting 25-hydroxyvitamin D into biologically active calcitriol through 1-alpha hydroxylase. Loss of this renal endocrine function is a hallmark of CKD and contributes significantly to CKD-MBD, secondary hyperparathyroidism, and skeletal complications.
Management of vitamin D deficiency in CKD therefore differs fundamentally from that in the general population. While nutritional vitamin D replacement remains important, many patients with advanced CKD require active vitamin D analogues that bypass the failing kidney. Understanding these pathways enables clinicians to optimize mineral metabolism, prevent complications, and improve outcomes in patients with chronic kidney disease.
For a nephrology audience, I would also suggest adding a one-page figure showing the Skin → Liver → Kidney → Intestine/Bone/Parathyroid axis, along with FGF-23 and PTH regulation, which makes the article much more impactful for publication or teaching purposes.
Dr. Balaji Kirushnan Senior Consultant Nephrologist Kauvery Hospital, Chennai.
