The most likely diagnosis is vitamin D deficiency. Special populations will have lower levels of vitamin D owing to medical conditions or medication side effects. Obesity has been correlated with lower vitamin D levels possibly related to fat sequestration. Phenobarbital and phenytoin may increase the metabolism of vitamin D to inactive forms. Glucocorticoids can decrease vitamin D metabolism to active forms. Agents that decrease absorption such as orlistat can decrease vitamin D absorption. Malabsorption disorders, including celiac disease and bariatric surgery, can also result in vitamin D deficiency. This patient has multiple risk factors for vitamin D deficiency including age, possible malnutrition, glucocorticoid use, and being home bound. In the face of ongoing vitamin D deficiency, normocalcemia is maintained through increased bone resorption through increased osteoclastic activity.
Antiresorptive drugs, especially the first dose of intravenous bisphosphonates and denosumab, rapidly suppress osteoclastic bone resorption and can precipitate hypocalcemia in these patients. Vitamin D sufficiency should be assessed prior to initiating antiresorptive drugs, especially those administered parenterally.
High baseline bone turnover and abrupt alteration in calcium flux between blood and bone are also features of hungry bone syndrome. However, this syndrome specifically occurs after parathyroidectomy for primary hyperparathyroidism. It is caused by rapid influx of calcium from the blood into the skeleton. In the absence of parathyroidectomy, hungry bone syndrome cannot explain this patient's findings.
Acute hyperphosphatemia from tumor lysis syndrome or phosphorus-containing bowel preparations may cause acute hypocalcemia. Chronic hyperphosphatemia associated with chronic kidney disease can also result in hypocalcemia. However, these conditions are not present and, in the case of chronic kidney disease, cannot account for the patient's rapid clinical deterioration.
In contrast to surgery, hypoparathyroidism due to autoimmunity, radiation, or infiltrative processes develops slowly and serum calcium declines gradually over months to years, which is not consistent with the precipitous drop seen in this patient over 2 days.