Everything You Need to Know About Vitamin D metabolism for Step 1

Vitamin D questions on Step 1 love to look “simple” (sunlight → strong bones) and then test you on the one enzyme, organ, or regulator that breaks the chain. If you can trace vitamin D from skin → liver → kidney → intestine/bone and map what PTH, phosphate, FGF23, and kidney disease do to that pathway, you’ll crush most calcium/bone metabolism stems.

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Big Picture: What Vitamin D Does (and Why Step 1 Cares)

Vitamin D (calcitriol, $1,25(OH)_2D$) is a steroid hormone that primarily:

• Increases intestinal absorption of calcium and phosphate
• Promotes bone mineralization when Ca/Phos supply is adequate
• Can increase bone resorption (via RANKL) in certain contexts to maintain serum calcium

On exams, vitamin D is often tested in the context of:

• Rickets/osteomalacia
• CKD (renal osteodystrophy)
• Hypo/hyperparathyroidism
• Malabsorption
• Granulomatous disease → hypercalcemia

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Vitamin D Metabolism: The High-Yield Pathway

Stepwise conversion (know the organs + enzymes)

Two lab values that Step 1 loves to trick you with

• 25(OH)D (calcidiol) = storage form and best test for vitamin D deficiency
• 1,25(OH)$_2$D (calcitriol) = active form and can be normal or even high in deficiency (because PTH upregulates 1α-hydroxylase)

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Regulation: Who Turns Calcitriol Up or Down?

Parathyroid hormone (PTH): the “kidney activator”

Low calcium → ↑ PTH → kidney:

• ↑ 1α-hydroxylase → ↑ calcitriol
• ↑ Ca reabsorption (DCT)
• ↓ phosphate reabsorption (PCT) → phosphaturia
• ↑ bone resorption (via osteoblast RANKL → osteoclast activation)

Net: raises serum calcium; lowers serum phosphate.

Phosphate & FGF23: the “calcitriol brakes”

• High phosphate stimulates FGF23 (made by osteocytes)
• FGF23:
  • ↓ 1α-hydroxylase → ↓ calcitriol
  • ↓ phosphate reabsorption in PCT (phosphaturia)

Classic teaching: FGF23 is the body’s “anti-phosphate” hormone and reduces active vitamin D.

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Mechanism of Action: How Vitamin D Works at Target Tissues

Intestine (highest yield)

Calcitriol increases:

• Calcium absorption (via ↑ TRPV6 channels, ↑ calbindin)
• Phosphate absorption

Bone

Vitamin D:

• Supports mineralization by providing Ca/Phos availability
• In conjunction with PTH, can promote resorption (upregulates RANKL via osteoblasts)

Kidney

Calcitriol modestly:

• Increases Ca/Phos reabsorption, but its major “exam effect” is through the gut.

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Deficiency: Pathophysiology You’ll Actually See in Questions

Common causes

• Low sunlight exposure (institutionalized, high latitude)
• Darker skin (melanin blocks UVB)
• Malabsorption (celiac, Crohn, pancreatic insufficiency, bariatric surgery)
• Liver disease (↓ 25-hydroxylation)
• CKD (↓ 1α-hydroxylation → low calcitriol)
• Medications: enzyme inducers like phenytoin, phenobarbital (increase vitamin D catabolism)

What deficiency does to labs (pattern recognition)

Low vitamin D → ↓ intestinal Ca/Phos absorption → low calcium (or low-normal) → secondary hyperparathyroidism → phosphate wasting.

Typical deficiency labs:

• ↓ 25(OH)D
• ↓ Ca (may be low-normal early)
• ↓ phosphate (because PTH wastes phosphate)
• ↑ PTH
• ↑ alkaline phosphatase (high bone turnover/osteoblast activity)

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HY nuance: In early deficiency, calcium may be normal due to compensatory PTH—so the giveaway becomes low phosphate + high PTH + high ALP + low 25(OH)D.

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Clinical Presentations (Step-Style)

Rickets (kids)

Think: failure of mineralization at growth plates.

• Bone deformities: bowed legs (genu varum), knock-knees
• Rachitic rosary (enlarged costochondral junctions)
• Craniotabes, delayed closure of fontanelle
• Delayed growth, delayed tooth eruption

Osteomalacia (adults)

Think: poorly mineralized osteoid.

• Diffuse bone pain, tenderness
• Proximal muscle weakness
• ↑ fracture risk (especially insufficiency fractures)

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Diagnosis: What to Order and How to Interpret It

Best initial test for vitamin D status

• Serum 25(OH)D

Helpful add-ons (often tested together)

• Calcium, phosphate, PTH, alkaline phosphatase
• Consider celiac testing or malabsorption workup if persistent

Imaging clues (less common but very Step 1)

• Rickets: metaphyseal cupping/fraying on X-ray
• Osteomalacia: Looser zones/pseudofractures

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Treatment (and Exam-Relevant “Which Form Do I Give?”)

Nutritional deficiency (most common)

• Vitamin D3 (cholecalciferol) supplementation
• Ensure adequate calcium intake

Malabsorption or severe deficiency

• Higher-dose vitamin D regimens; sometimes calcidiol/calcitriol depending on context
• Treat underlying cause (celiac, pancreatic insufficiency, etc.)

CKD: key twist

• Kidney can’t do 1α-hydroxylation → low calcitriol even if 25(OH)D is okay
• Treat with active vitamin D:
  • Calcitriol (1,25(OH)$_2$D) or analogs (e.g., paricalcitol)
• Also manage hyperphosphatemia (dietary phosphate restriction, phosphate binders)

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Hypervitaminosis D and Hypercalcemia: The “Overdose” Pattern

Excess vitamin D (supplements) → hypercalcemia + hyperphosphatemia because intestinal absorption of both rises.

Findings:

• Nausea, constipation, polyuria, confusion
• Nephrolithiasis
• Shortened QT (hypercalcemia)

Labs:

• ↑ Ca, ↑ phosphate, ↓ PTH

Management is cause-dependent (stop supplements; IV fluids; consider bisphosphonates/calcitonin if significant hypercalcemia).

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High-Yield Associations & Classic Vignettes

1) Chronic kidney disease → low calcitriol → secondary hyperparathyroidism

Mechanism: ↓ 1α-hydroxylase + phosphate retention → ↓ Ca → ↑ PTH
Labs: ↓ Ca, ↑ phosphate, ↑ PTH, ↑ ALP
Buzzwords: “renal osteodystrophy,” bone pain, pruritus, vascular calcifications.

2) Granulomatous disease (sarcoid, TB) → hypercalcemia

Macrophages express 1α-hydroxylase outside kidney → ↑ calcitriol independent of PTH.

Labs: ↑ Ca, ↓ PTH, ↑ 1,25(OH)$_2$D
Clue: hypercalcemia in sarcoidosis with hilar adenopathy.

3) Anticonvulsants (phenytoin/phenobarbital) → vitamin D deficiency

Induce CYP450 → ↑ vitamin D breakdown → osteomalacia/rickets risk.

4) Liver disease affects 25-hydroxylation

Leads to low 25(OH)D and downstream effects.

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Rapid-Fire Step 1 “If You See X, Think Y”

• Best indicator of vitamin D stores → 25(OH)D
• PTH increases calcitriol → stimulates 1α-hydroxylase
• FGF23 decreases calcitriol and increases phosphate wasting
• CKD → can’t activate vitamin D → treat with calcitriol
• Sarcoidosis/TB → ectopic 1α-hydroxylase → hypercalcemia with low PTH
• Vitamin D deficiency → ↑ PTH, ↑ ALP, often ↓ phosphate

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First Aid Cross-References (Where This Lives in FA)

Use these as your quick “flip-to” anchors (edition-dependent headings may shift slightly):

• Endocrine → Calcium homeostasis: PTH vs vitamin D actions (bone, kidney, gut)
• Renal → Chronic kidney disease: secondary hyperparathyroidism / renal osteodystrophy
• Musculoskeletal → Bone disorders: rickets, osteomalacia, osteoporosis differentiation
• Endocrine pharmacology: vitamin D preparations (cholecalciferol, ergocalciferol, calcitriol)

(Exact page numbers vary by edition; search within FA for “1α-hydroxylase,” “FGF23,” “rickets,” and “renal osteodystrophy.”)

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Mini Table: Differentiate Bone Conditions Fast