A 32-year-old lady with a history of bulimia nervosa was noted to have a raised adjusted calcium of 2.94mmol/L associated with high parathyroid hormone (PTH) 17.2pmol/L. On review, she had an apparent hypercalcaemia for at least three years, and also had a chronic, severe alkalosis with a bicarbonate up to 81.9mEQ/L. Ionised calcium during that time had actually been low, down to 1.03mmol/L.
This case highlights the effects of alkalosis on calcium, as more albumin is available for binding to ionised calcium. This results in a low ionised calcium, which triggers PTH release and overall leads to raised adjusted calcium levels. Clinicians may misdiagnose a similar patient with primary hyperparathyroidism and treatment would cause worsening of true hypocalcaemia.