Severe hyponatremia in a mild COVID-19 patient: diagnostic challenges of SIADH and adrenal insufficiency.

This case report presents a 57-year-old Japanese woman with mild COVID-19 who developed severe symptomatic hyponatremia and altered consciousness, notably without pneumonia or central nervous system infection. Initial findings, including low plasma osmolality, high urine osmolality, and elevated IL-6 levels, suggested that the syndrome of inappropriate antidiuretic hormone secretion (SIADH) was the primary cause of her hyponatremia. While initial saline infusion offered temporary symptom relief, sodium levels remained unstable, leading to intermittent steroid therapy. Her improvement with steroids, alongside additional hormonal testing, raised the possibility of partial adrenal insufficiency as a complicating factor. Although secondary adrenal insufficiency in COVID-19 cases has been previously reported, its exact contribution to hyponatremia remains unclear. This case underscores the diagnostic complexities in managing hyponatremia associated with COVID-19, as SIADH is often the leading cause but may not fully explain persistent cases unresponsive to standard treatments. The report also emphasizes the importance of considering adrenal insufficiency in similar cases, particularly given COVID-19's potential impact on the hypothalamic-pituitary-adrenal axis. This case highlights the need for further research into COVID-19's effects on hormonal regulation, as such disruptions may play a key role in COVID-19-related electrolyte imbalances.