In a 46-year-old adult with serum calcium of 7.7 mg/dL and 25‑hydroxyvitamin D (25‑OH vitamin D) of 12 ng/mL, is the diagnosis rickets?

No, This is Not Rickets

In a 46-year-old adult with these laboratory findings, the diagnosis is osteomalacia, not rickets. Rickets is specifically a disease of growing bones in children, characterized by defective mineralization at the growth plates (physes), while osteomalacia is the adult equivalent affecting mature bone 1.

Key Diagnostic Distinctions

Age-Based Terminology

• Rickets occurs only in children with open growth plates, typically manifesting before puberty 1
• Osteomalacia is the correct term for adults with the same underlying pathophysiology of defective bone mineralization 1
• The distinction is anatomical: rickets affects the physis (growth plate) which closes after skeletal maturity, while osteomalacia affects mature cortical and trabecular bone 1

Laboratory Findings in This Case

Your patient demonstrates:

• Severe hypocalcemia (7.7 mg/dL; normal 8.5-10.5 mg/dL)
• Severe vitamin D deficiency (25-OH vitamin D of 12 ng/mL)

These findings are consistent with calcipenic osteomalacia due to vitamin D deficiency 1.

Clinical Context

Defining Vitamin D Deficiency

• Severe deficiency is defined as 25(OH)D levels ≤12 nmol/L (approximately 5 ng/mL), at which rickets or osteomalacia may be present 1
• Your patient's level of 12 ng/mL falls well within the deficiency range where osteomalacia is expected 1
• Levels below 15 ng/mL are associated with severe secondary hyperparathyroidism and radiographic bone abnormalities 1

Expected Clinical Features

Look for these specific manifestations of osteomalacia:

• Bone pain, particularly in the spine, pelvis, and lower extremities 1
• Proximal muscle weakness affecting hip and shoulder girdle 1
• Pathologic fractures or pseudofractures (Looser zones) on imaging 1
• Elevated alkaline phosphatase as a marker of increased bone turnover 1
• Secondary hyperparathyroidism due to chronic hypocalcemia 1

Treatment Approach

Immediate Management

Treat with ergocalciferol 50,000 IU weekly for 12 weeks, then monthly thereafter for severe vitamin D deficiency with 25(OH)D levels in this range 1.

Calcium Supplementation

• Provide 1200 mg elemental calcium daily (total from diet and supplements) for adults 1
• Calcium carbonate should be taken with food; calcium citrate can be taken anytime and is preferred if the patient takes proton pump inhibitors 1
• Divide doses to no more than 600 mg at a time for optimal absorption 1

Monitoring Strategy

• Recheck 25(OH)D after at least 3 months of weekly dosing to assess response 1
• Target serum 25(OH)D level of 30-60 ng/mL for optimal bone health 1
• Monitor serum calcium during repletion to avoid hypercalcemia 1
• Follow alkaline phosphatase as a marker of healing 1, 2

Common Pitfalls

Terminology Errors

• Never use "rickets" for adult patients - this is a fundamental diagnostic error that can cause confusion in medical records and communication 1
• The pathophysiology is identical, but the anatomical manifestations differ based on skeletal maturity 1

Treatment Considerations

• Do not use calcitriol or other 1-hydroxylated vitamin D steroids to treat nutritional vitamin D deficiency - use native vitamin D (ergocalciferol or cholecalciferol) 1
• Active vitamin D preparations bypass the body's regulatory mechanisms and increase hypercalcemia risk 1

Differential Diagnosis

While vitamin D deficiency is the likely cause, consider:

• Malabsorption syndromes (celiac disease, inflammatory bowel disease, gastric bypass) 1
• Chronic kidney disease affecting vitamin D metabolism 1
• Medications causing vitamin D deficiency or increased catabolism 1