Endocrine · Year 2 · from Endocrine

Case 1: Vitamin D Deficiency with Secondary Hyperparathyroidism

Patient Demographics

  • Age: 68 years
  • Sex: Female
  • Occupation: Retired nurse, now homebound

Chief Complaint

"I've been having bone pain and feeling weak."

History of Present Illness

A 68-year-old woman presents with a 6-month history of progressive diffuse bone pain, particularly in her back, hips, and legs. She describes generalized weakness and difficulty rising from a chair. She has had two falls in the past 3 months without significant injury. She reports limited sun exposure as she has been mostly homebound caring for her ill husband. Her diet is restricted due to lactose intolerance, and she avoids dairy products. She does not take any supplements. Past medical history includes Crohn's disease (ileal involvement, currently in remission on mesalamine) and a history of small bowel resection 10 years ago.

Physical Examination

  • Vital Signs: BP 128/76 mmHg, HR 72 bpm
  • General: Thin, elderly woman in mild discomfort
  • Musculoskeletal:
  • Tenderness over lumbar spine, pelvis, and proximal femurs
  • Proximal muscle weakness (4/5 hip flexors, difficulty standing from seated position)
  • Waddling gait
  • Skin: No pallor
  • Neurologic: No paresthesias, negative Chvostek and Trousseau signs

Workup

  • Laboratory Studies:
  • Calcium (total): 8.4 mg/dL (low-normal)
  • Calcium (ionized): 4.2 mg/dL (low-normal)
  • Phosphorus: 2.1 mg/dL (low)
  • Albumin: 3.8 g/dL (normal)
  • 25-hydroxyvitamin D: 8 ng/mL (severely deficient, normal >30)
  • 1,25-dihydroxyvitamin D: 18 pg/mL (low-normal)
  • PTH: 185 pg/mL (elevated, normal 15-65)
  • Alkaline phosphatase: 245 U/L (elevated)
  • Creatinine: 0.9 mg/dL (normal)
  • 24-hour urine calcium: 45 mg/24hr (low, indicating avid renal retention)
  • Imaging:
  • DEXA scan: T-score -2.8 at lumbar spine, -2.6 at total hip (osteoporosis)
  • X-ray pelvis: Looser zones (pseudofractures) in bilateral pubic rami
  • X-ray spine: Diffuse osteopenia, compression deformity T12

Diagnosis

Severe vitamin D deficiency with secondary hyperparathyroidism and osteomalacia

Pathophysiology

  1. Vitamin D deficiency from limited sun exposure + malabsorption (ileal resection impairs bile salt absorption needed for vitamin D uptake)
  2. Low vitamin D leads to decreased intestinal calcium absorption
  3. Hypocalcemia triggers compensatory PTH elevation (secondary hyperparathyroidism)
  4. Elevated PTH maintains serum calcium by:
  • Increasing bone resorption (leading to bone loss)
  • Increasing renal calcium reabsorption
  • Stimulating 1-alpha-hydroxylase (though substrate limited)
  1. PTH also decreases phosphorus reabsorption (causing hypophosphatemia)
  2. Low calcium and phosphorus impair bone mineralization (osteomalacia)
  3. Looser zones are characteristic pseudofractures of osteomalacia

Treatment

ACUTE REPLETION:

  1. Vitamin D3 (cholecalciferol) 50,000 IU weekly for 8 weeks
  2. Calcium carbonate 1200 mg daily with meals (or calcium citrate if on PPI)
  3. Monitor 25(OH)D and calcium at 8 weeks

MAINTENANCE:

  1. Vitamin D3 2000-4000 IU daily (may need higher doses given malabsorption)
  2. Calcium 1200 mg daily
  3. Target 25(OH)D >30 ng/mL

MONITORING:

  1. Recheck 25(OH)D, calcium, phosphorus, PTH at 8-12 weeks
  2. Expect PTH to normalize as vitamin D replete
  3. Annual DEXA to assess bone density improvement
  4. Fall risk assessment and prevention

ADDITIONAL CONSIDERATIONS:

  1. Evaluate for other fat-soluble vitamin deficiencies (A, E, K) given malabsorption
  2. Consider intramuscular vitamin D if oral absorption inadequate
  3. Address underlying Crohn's disease management

Clinical Pearl

Vitamin D deficiency is extremely common, particularly in elderly, homebound individuals, those with malabsorption syndromes, and those with dark skin at northern latitudes. The appropriate physiologic response to vitamin D deficiency is elevated PTH (secondary hyperparathyroidism), which maintains serum calcium at the expense of bone health. Laboratory hallmarks include low 25(OH)D, elevated PTH, normal or low calcium, low phosphorus, and elevated alkaline phosphatase. In adults, the bone disease is osteomalacia (defective mineralization), characterized by bone pain, proximal myopathy, and Looser zones (pseudofractures) on imaging. Treatment with vitamin D and calcium will normalize PTH and allow bone remineralization. The 25(OH)D level is the best indicator of vitamin D status, as it reflects stores, whereas 1,25(OH)2D may be normal or low despite deficiency.

Clinical Image

Diagram illustrating calcium homeostasis showing the interactions between PTH, vitamin D, and their effects on bone, kidney, and intestine to maintain serum calcium levels.

Image Source: Wikimedia Commons - "Calcium regulation and parathyroid hormone" License: CC BY-SA 4.0 URL: https://commons.wikimedia.org/wiki/File:Calcium_regulation_and_the_parathyroid_hormone.svg

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