Poster Presentation ESA-SRB-ANZOS 2025 in conjunction with ENSA

Introduction

Vitamin D toxicity (VDT) is an uncommon but important cause of hypercalcaemia, increasingly reported due to unregulated supplement use. Management is challenging because of the long half-life and storage of vitamin D in adipose tissue, resulting in prolonged toxicity. 

Case Presentation 

A 68-year-old female presented with two-week history of nausea, fatigue, polyuria, constipation, and confusion. She had been taking herbal vitamin D drops of unknown dose for many years. Investigations revealed severe hypercalcaemia (adjusted calcium 4.74 mmol/L), suppressed PTH (0.8 pmol/L), hypokalaemia (2.9 mmol/L). Serum vitamin D levels were markedly elevated (25(OH)D >1100 nmol/L and 1,25(OH)₂D >480 pmol/L). Extensive work-up excluded malignancy and granulomatous disease (Table 1). The patient received intravenous hydration, potassium replacement, calcitonin, pamidronate, denosumab, and glucocorticoids. Despite these measures, hypercalcaemia persisted for four months, and vitamin D levels normalised only after nine months (Graph 1, Table 2). Hypokalaemia resolved in parallel with declining serum calcium.

Discussion 

VTD is defined as 25(OH)D levels greater than 375 nmol/L,¹ in this case, levels were over three times higher. VDT primarily increases intestinal calcium absorption, leading to hypercalcaemia.² The lipophilic nature of Vitamin D causes slow release from hepatic and adipose stores, prolonging hypercalcaemia despite standard therapies.³ Hypokalaemia, likely due to increased distal tubular calcium delivery and potassium wasting was a notable feature.⁴ Glucocorticoids play a critical role by reducing 1,25(OH)₂D synthesis and intestinal calcium absorption, yet clearance ultimately depends on time.^2,5 

Conclusion

This case highlights the severe and prolonged nature of hypercalcaemia in VDT, which is often refractory to conventional treatments. Clinicians should counsel patients on the risks of non-prescribed vitamin D use and ensure regular biochemical monitoring. Extended glucocorticoid therapy may be required, and patients should be counselled for a prolonged course. Interdisciplinary collaboration between endocrinology and primary care is essential for long-term management of VDT.