Severe hypocalcaemia with elevated PTH and normal vitamin D: a diagnostic pitfall due to chronic negative calcium balance in a vegan patient

Introduction: Secondary hyperparathyroidism is usually associated with chronic kidney disease or vitamin D deficiency. Case description: A 62-year-old man with severe osteoporosis presented with persistent hypocalcaemia and markedly elevated parathyroid hormone levels. Renal function, phosphate and vitamin D levels were normal. Extensive investigations excluded common causes. Twenty-four-hour urinary calcium excretion was profoundly reduced, indicating a chronic negative calcium balance. Dietary assessment revealed a strict vegan diet with low calcium intake and reduced bioavailability. Treatment with calcium citrate and calcitriol normalised serum calcium and reduced PTH levels. Conclusion: A chronic negative calcium balance is an under-recognised cause of secondary hyperparathyroidism. Measurement of urinary calcium is a key diagnostic tool in these patients.