The most likely diagnosis is hyperosmolar hyponatremia, caused by this patient's significantly elevated plasma glucose level. The patient's elevated plasma osmolality indicates the presence of a hyperosmolar state. Hyperglycemia causes the osmotic translocation of water from the intracellular to the extracellular fluid compartment, which results in a decrease in the serum sodium level by approximately 1.6 mEq/L (1.6 mmol/L) for every 100 mg/dL (5.6 mmol/L) increase in the plasma glucose above 100 mg/dL (5.6 mmol/L). Hyperosmolar hyponatremia can also be the result of exogenously administered solutes such as mannitol or sucrose.

Pseudohyponatremia is caused by significant hyperlipidemia or the presence of paraproteins in the serum. In these situations, the laboratory measurement of plasma glucose is erroneously low, and the plasma osmolality is normal. This patient has a high plasma osmolality, which is not seen in pseudohyponatremia.

Hypothyroidism, adrenal insufficiency, and the syndrome of inappropriate antidiuretic hormone secretion (SIADH) are associated with hypo-osmolar hyponatremia caused by a decrease in the excretion of free water and are not compatible with the patient's hyperosmolality. Although lung cancer is a common cause of SIADH and may be present in this patient with a lung mass, his laboratory studies are not consistent with this being the cause of his hyponatremia.

Reset osmostat refers to a downward setting of the level at which sensors of plasma osmolality trigger the release of antidiuretic hormone and is associated with quadriplegia, tuberculosis, advanced age, pregnancy, psychiatric disorders, and chronic malnutrition. This lowered setpoint leads to stable, mild hypo-osmolar hyponatremia, which is inconsistent with this patient's findings.