VITAMIN D - Complete Reference

Harrison's Principles of Internal Medicine, 22nd Edition (2025)

1. NATURE AND CLASSIFICATION

2. SYNTHESIS AND FORMATION

2a. Cutaneous Synthesis (Endogenous Route)

• UV-B radiation (wavelength 290-320 nm) causes a photochemical cleavage of the B-ring of 7-dehydrocholesterol (provitamin D₃) in the skin, forming pre-vitamin D₃, which undergoes thermal isomerization to vitamin D₃.
• Cutaneous production is decreased by:
  • Melanin (competes for UV-B photons)
  • High SPF sunblocks
  • Aging (efficiency of synthesis declines)
  • Northern latitudes / reduced solar exposure

2b. Dietary Absorption

• Vitamin D is absorbed in the small intestine as part of the fat-absorption pathway (chylomicrons).
• Terminal ileal disease impairs enterohepatic circulation of vitamin D metabolites.

3. METABOLISM - THE THREE-STEP ACTIVATION PATHWAY

Step 1 - Transport

Step 2 - 25-Hydroxylation in the Liver

Nephrotic syndrome reduces DBP (urinary loss) and greatly shortens the half-life of 25(OH)D.

Step 3 - 1α-Hydroxylation in the Kidney (Rate-Limiting, Regulated Step)

Step 4 - Inactivation (24-Hydroxylation)

• CYP24A1 (24-hydroxylase) converts both 25(OH)D and 1,25(OH)₂D to 24,25(OH)₂D and 1,24,25(OH)₃D respectively - biologically inactive, water-soluble metabolites excreted in bile.
• This is the primary catabolism pathway and is upregulated by 1,25(OH)₂D itself.
• Loss-of-function mutations in CYP24A1 → impaired catabolism → elevated 1,25(OH)₂D → hypercalciuria and nephrocalcinosis (idiopathic infantile hypercalcemia).

4. MECHANISM OF ACTION - THE VITAMIN D RECEPTOR (VDR)

• 1,25(OH)₂D acts via the nuclear Vitamin D Receptor (VDR), a member of the steroid/thyroid hormone receptor superfamily.
• The VDR forms a heterodimer with the Retinoid X Receptor (RXR).
• This VDR-RXR complex binds to Vitamin D Response Elements (VDREs) in the promoter regions of target genes.
• VDR is expressed in most tissues - intestine, kidney, bone, parathyroid, muscle, immune cells, brain, breast, colon, prostate, keratinocytes.
• Notable exception: Alopecia is seen with mutant VDRs but NOT with vitamin D deficiency - VDR effects on hair follicle are ligand-independent.

5. PHYSIOLOGIC FUNCTIONS

5a. Classical Functions - Mineral Ion Homeostasis

• Increases transcriptional expression of TRPV6 (apical calcium entry channel), calbindin D-9K (cytosolic calcium ferry), and PMCA1b (basolateral Ca²⁺-ATPase).
• Net effect: markedly increases active calcium absorption.
• Also increases phosphate absorption via NaPi-IIb co-transporter.

• Increases calcium and phosphate reabsorption.
• Suppresses PTH synthesis in parathyroid glands (negative endocrine feedback).

• Stimulates osteoblast differentiation and mineralization (provides adequate Ca²⁺ and PO₄³⁻ substrate).
• At pharmacologic doses: stimulates osteoclast activity (bone resorption).
• Regulates normal maturation of the enthesis (bone-tendon attachment site).

• 1,25(OH)₂D directly suppresses PTH gene transcription.
• This is the rationale for using calcitriol in secondary hyperparathyroidism of CKD.

5b. Non-Classical (Pleiotropic) Functions

Harrison's note (22E): Despite compelling observational data, recent placebo-controlled trials did NOT show therapeutic benefit of vitamin D supplementation for cancer prevention, cardiovascular disease, type 2 diabetes, depression, or tuberculosis infection.

6. ASSESSMENT OF VITAMIN D STATUS

7. DISORDERS OF VITAMIN D

7a. VITAMIN D DEFICIENCY

• Muscle soreness, weakness, bone pain
• Hypocalcemia, hypophosphatemia, secondary hyperparathyroidism
• Rickets in children; Osteomalacia in adults
• Increased risk of fractures

• Nutritional deficiency: Vitamin D₃ 600-800 IU/day (National Academy of Medicine recommendation); higher doses for treatment of established deficiency
• Glucocorticoid-induced osteoporosis prevention: Calcium 1000-1200 mg/day + Vitamin D 600-800 IU/day + weight-bearing exercise

7b. RICKETS (Pediatric)

1. Calcium-deficiency rickets - nutritional vitamin D deficiency (most common globally)
2. Phosphopenic rickets - FGF23-mediated phosphate wasting (see hereditary forms below)
3. Vitamin D-Dependent Rickets (VDDR):

• Craniotabes (softening of skull bones)
• Rachitic rosary (costochondral junction enlargement)
• Harrison's sulcus (indentation along diaphragm insertion)
• Bowing of weight-bearing long bones
• Widened, frayed, cupped metaphyses on X-ray
• Reduced growth, delayed tooth eruption
• Some cases: symptomatic hypocalcemia and markedly elevated PTH

7c. OSTEOMALACIA (Adult)

• Low/normal serum calcium
• Low/normal serum phosphate
• Elevated ALP
• Elevated PTH (secondary)
• Low 25(OH)D (in nutritional form)

• Looser's zones (pseudofractures) - pathognomonic - horizontal lucent bands perpendicular to bone cortex
• Coarsened trabecular pattern

• Nutritional vitamin D deficiency
• Malabsorption
• Defective vitamin D metabolism (drug-induced, renal osteodystrophy)
• Phosphate-wasting syndromes (oncogenic osteomalacia, X-linked hypophosphatemia)
• VDDR types I and II

7d. HEREDITARY HYPOPHOSPHATEMIC RICKETS (FGF23-Mediated)

1. Inhibiting renal tubular phosphate reabsorption (downregulates NaPi-IIa/IIc)
2. Suppressing renal 1α-hydroxylase (CYP27B1) → reduces 1,25(OH)₂D

7e. VITAMIN D-RELATED HYPERCALCEMIA

• Requires chronic ingestion of >10,000 IU/day in otherwise healthy adults
• Mechanism: elevated 25(OH)D (>100 ng/mL) itself has biologic activity at high levels, causing increased intestinal Ca²⁺ absorption and bone resorption
• Note: 1,25(OH)₂D may NOT be elevated (unlike in granulomatous disease)
• Treatment: stop vitamin D, restrict calcium intake, hydration; glucocorticoids (40-100 mg/day prednisone) are effective; fat stores may sustain toxicity for weeks after cessation

• Macrophages in granulomas express 1α-hydroxylase that is induced by interferon-γ and TNF-α
• This extra-renal hydroxylase lacks normal negative feedback from calcium and 1,25(OH)₂D
• Results in unregulated overproduction of 1,25(OH)₂D → hypercalcemia
• Treatment: glucocorticoids (suppress macrophage 1α-hydroxylase activity); also hydroxychloroquine

7f. DEFECTIVE VITAMIN D METABOLISM - DRUG-INDUCED

7g. VITAMIN D IN CHRONIC KIDNEY DISEASE (CKD)

• Reduced functioning renal mass → reduced 1α-hydroxylase activity → low 1,25(OH)₂D
• Results in: hypocalcemia → secondary hyperparathyroidism → renal osteodystrophy
• Subtypes of renal osteodystrophy: osteitis fibrosa cystica (high-turnover), adynamic bone disease (low-turnover), mixed
• Treatment: calcitriol or active vitamin D analogues (paricalcitol, doxercalciferol); phosphate binders; dialysis

8. RECOMMENDED DAILY INTAKE

9. VITAMIN D PREPARATIONS USED CLINICALLY

10. SUMMARY FLOWCHART OF VITAMIN D AXIS

7-Dehydrocholesterol (skin)
        ↓ UV-B (290-320 nm)
   Vitamin D₃/D₂
        ↓ Liver CYP2R1 (not tightly regulated)
   25(OH)D [Calcidiol] ← main serum marker, half-life 2-3 wk
        ↓ Kidney CYP27B1 (tightly regulated)
               ↑ PTH, ↑ low PO₄
               ↓ FGF23, ↑ Ca²⁺, ↓ 1,25(OH)₂D
   1,25(OH)₂D [Calcitriol] ← active hormone
        ↓ VDR-RXR heterodimer → VDRE → gene transcription
   ↑ Intestinal Ca²⁺ & PO₄ absorption
   ↑ Renal Ca²⁺ & PO₄ reabsorption
   ↑ Bone mineralization
   ↓ PTH synthesis
        ↓ CYP24A1 (catabolism)
   24,25(OH)₂D / 1,24,25(OH)₃D → excretion (bile)

• Harrison's Principles of Internal Medicine, 22nd Edition (2025), Chapter 421: "Vitamin D Deficiency and Disorders of Vitamin D Metabolism" (Block 46, pp. 3313-3320)
• Harrison's 22E, Chapter 37: "Vitamin and Trace Mineral Deficiency and Excess" (Block 37)

Recent Literature Note: Recent placebo-controlled trials (including VITAL, D-HEALTH, USPSTF 2021) have not confirmed benefits of vitamin D supplementation for cancer, cardiovascular disease, type 2 diabetes, or respiratory infections in individuals who are not severely deficient - consistent with Harrison's 22E assessment. The evidence remains strongest for bone health and fall prevention in elderly, vitamin D-deficient populations.This is a complete, research-grade breakdown sourced directly from Harrison's Principles of Internal Medicine, 22nd Edition (2025). Here's a quick orientation to the material above:

1. Synthesis & Metabolism (Section 2-3) - The three-step activation (skin → liver → kidney), the enzymes (CYP2R1, CYP27B1, CYP24A1), their genes, regulation by PTH/FGF23/calcium, and protein binding via DBP. This is pure biochemistry and is exam-critical.
2. Mechanism of Action (Section 4) - VDR-RXR heterodimer, VDREs, ligand-independent VDR action in hair follicles - these are molecular biochemistry concepts.
3. Functions (Section 5) - Classical (mineral ion homeostasis) and pleiotropic effects, with the important caveat that RCTs have not confirmed non-skeletal benefits of supplementation.
4. Disorders (Section 7) - All disorders organized by pathomechanism:
   • VDDR Type I (CYP27B1 mutation) vs. Type II (VDR mutation)
   • FGF23-mediated phosphopenic rickets (XLH, ADHR, oncogenic osteomalacia)
   • Drug-induced deficiency (phenytoin, barbiturates, rifampin - via CYP induction)
   • CYP24A1 loss-of-function → hypercalcemia
   • Sarcoidosis hypercalcemia - the unregulated extra-renal 1α-hydroxylase mechanism
5. Clinical preparations (Section 9) - Helps understand which diseases need active calcitriol vs. standard vitamin D.