Vitamin D Deficiency and Excess

Summary

Vitamin D imbalances can occur as deficiency or excess. Vitamin D deficiency is usually caused by low sun exposure. Importantly, infants are at risk due to low sun exposure and low levels of Vitamin D in breast milk. The clinical effects of Vitamin D deficiency mainly stem from decreased calcium and phosphate levels, leading to poor mineralization of bone, which presents as rickets in children and osteomalacia in adults. Other lab findings include increased alkaline phosphatase (ALP), a marker for compensatory bone resorption to restore calcium levels. Likewise, increased PTH levels are also observed, thought to be a compensatory response to hypocalcemia.

Vitamin D excess is rare, and mainly results from overconsumption. However, one rare but commonly tested cause is granulomatous disease, since epithelioid macrophages may express α-1 hydroxylase (usually only found in the kidney), which converts vitamin D to its active form, calcitriol. Symptoms of Vitamin D excess include hypercalcemia, GI symptoms, and altered mentation.

Key Points

• Vitamin D Deficiency
  • Causes
    • exclusive breastfeeding without supplementation
      • breast milk itself contains very low levels of vitamin D
    • low sun exposure (especially with pigmented skin)
    • chronic kidney disease (CKD)
      • lack of 1-alpha-hydroxylase needed to activate vitamin D
    • fat malabsorption (e.g. gastric bypass)
      • Vitamin D is fat-soluble and is absorbed in the GI with fat
    • poor dietary intake and prematurity are other causes
  • Effects
    • Hypocalcemia (↓ calcium) and Hypophosphatemia (↓ phosphate)
      • impaired Vitamin D-dependent GI absorption
      • hypocalcemia may present with Chvostek’s or Trousseau’s sign (hypocalcemic tetany)
    • Impaired bone mineralization/fractures
      • Rickets in children
        • bow legs (genu varum), rachitic rosary line, kyphosis, and harrison’s groove are common signs
      • Osteomalacia in adults
      • Biopsy shows unmineralized osteoid (matrix) around trabeculae
    • Muscle weakness/pain also seen
  • Other labs
    • Increased alkaline phosphatase (ALP)
      • marker for compensatory bone resorption to restore calcium levels
    • Increased parathyroid hormone (PTH)
      • compensatory response to hypocalcemia
• Vitamin D Excess
  • Causes
    • overconsumption (food faddist)
    • Granulomatous disease (eg. TB, CGD, sarcoidosis)
      • increased activation of vitamin D by epitheloid macrophages expressing 1-alpha-hydroxylase (usually only in kidney)
  • Effects
    • Hypercalcemia
      • hypercalciuria also seen
    • muscle weakness, constipation/loss of appetite, mental status changes (stupor) also seen

Find Vitamin D Deficiency and Excess and other Vitamins among Pixorize's visual mnemonics for the USMLE Step 1 and NBME shelf exams.