Endocrine Abstracts

Introduction: The group-specific component (GC) gene, located on chromosome 4q12-q13, encodes the vitamin D binding protein (DBP). It is a highly polymorphic protein and 80-90% of vitamin D (VD) metabolites bind to a single site of DBP. DBP deficiency is a recessive condition, which causes low VD levels and features of rickets.

Case Report: A 17.5-year-old female, originally from Pakistan, was referred for persistent VD deficiency despite treatment. She previously had rickets. At 11.3 years, she presented with abnormal biochemistry (Table1). Already on calcium supplements, she commenced VD 20,000units twice weekly for 10 weeks, then 800units daily. Her bone profile improved, but VD remained undetectable. She was re-referred at 16.8 years with bone pain, headaches and undetectable VD levels. She was treated with 25,000units daily for 8 days, then 800 units daily. With no improvement (Table2), intramuscular VD (300,000units) was given at age 17, with continued oral maintenance dosing. Again, the VD level remained undetectable, but bone biochemistry normalised. The 1,25 dihydroxy-VD was also undetectable. This raised the rare possibility of DBP deficiency and levels were found to be undetectable. Genetic analysis is pending. Further VD level monitoring would be uninterpretable, but optimising calcium and VD intake improved her symptoms.

Conclusion: This case highlights the importance of considering DBP deficiency in low VD levels despite treatment and improved bone biochemistry. Delay in diagnosis led to unnecessary high doses of VD supplementation and blood tests. The patient responded to supplementation both clinically and biochemically. In such patients, regular VD supplementation is necessary, and the response should be monitored clinically with bone biochemistry, rather than VD levels.