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Syndrome of inappropriate antidiuretic hormone (SIADH) secretion is an endocrinological disorder which occurs when there is an evidence of continued antidiuretic hormone (ADH) secretion in the absence of an appropriate osmotic volume stimulus. Most commonly it is caused by different types of cancer or medications (eg. diuretics, antidepressants, antipsychotics...). Secondary adrenal insufficiency is caused by the insufficient adrenocorticotropic hormone (ACTH) secretion and cortisol production. In the hypocortisolism, the inhibitory mechanism on ADH secretion vanishes causing increased ADH secretion and leading to SIADH-like state. Both conditions, SIADH and hypocortisolism, manifest in euvolemic hyponatremia, however, its cause can be misdiagnosed and result in an inadequate treatment and potentially fatal outcome. Our case is about fifty-seven-year-old male patient who was treated with chemoradiotherapy for hematological malignancy of the oropharynx and who presented with severe hyponatremia se-veral times that almost ended fatally. Considering his malignant disease, patient was diagnosed with SIADH as a cause of hypona-tremia and was treated with hypertonic saline fluids and fluid intake restriction. However, after having suffered Covid-19 infection the patient was given glucocorticoids according to the protocol at the time. The sodium levels finally corrected, and the secondary adrenal insufficiency has been declared as the real cause of hyponatremia. SIADH is often thought to be the cause of hyponatremia in patients with malignant disease. However, SIADH represents a diagnosis of exclusion. Before making a final diagnosis, thyroid and secondary adrenal insufficiency must be ruled out. Moreover, secondary adrenal insufficiency imitates SIADH and results in euvolemic hyponatremia but the management and potential outcomes of these two conditions differ significantly. In patients who have been previously treated for malignancies with the treatment options that increase the risk of adrenal insufficiency (chemotherapy, neck or skull base radiotherapy, immunotherapy), adrenal reserve must be evaluated. If introduced, the glucocorticoid therapy should be carefully deescalated in these patients because of the risk of previously unrecognized adrenal insufficiency. This especially applies to conditions that include glucocorticoid therapy in the treatment protocol, such as Covid-19 infection.

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