SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion): This condition is commonly seen in patients with head trauma, especially those with subdural bleeds. The hyponatremia in SIADH is characterized by euvolemia, low serum osmolality, high urine osmolality, and high urine sodium levels. The clinical presentation, combined with the laboratory findings of low serum sodium (123 mEq/L in this case), supports SIADH as the most likely diagnosis.

Cerebral Salt Wasting (CSW): This condition is also associated with head trauma and can present similarly to SIADH with hyponatremia. However, CSW is characterized by volume depletion, which can be subtle. Laboratory findings include low serum sodium, low serum osmolality, and high urine sodium. Distinguishing between SIADH and CSW can be challenging, but the presence of volume depletion and a higher urine output in CSW can help differentiate it from SIADH.
Adrenal Insufficiency: This can occur in the context of head trauma, especially if there is pituitary involvement. Adrenal insufficiency can lead to hyponatremia due to the lack of aldosterone, leading to impaired renal sodium retention. Laboratory findings would include low cortisol levels, and potentially low aldosterone levels, along with hyponatremia.

Acute Kidney Injury (AKI): While less common as a primary cause of hyponatremia in this context, AKI can develop in patients with head trauma due to various factors, including hypovolemia, medications, or direct kidney injury. AKI can lead to hyponatremia through various mechanisms, including the inability to excrete free water. Laboratory findings would include elevated creatinine levels, and the clinical context would dictate the need for further evaluation.
Thyroid Dysfunction: Both hypothyroidism and hyperthyroidism can lead to hyponatremia, although the mechanisms differ. Hypothyroidism can cause decreased cardiac output and potentially lead to hyponatremia through mechanisms similar to heart failure. Hyperthyroidism can increase free water clearance, potentially leading to hyponatremia. Laboratory findings would include abnormal thyroid function tests.

Pituitary Apoplexy: A rare condition that can occur in the setting of head trauma, leading to sudden pituitary dysfunction. This can result in acute adrenal insufficiency, among other hormonal deficiencies, potentially leading to hyponatremia. The diagnosis would be supported by imaging findings of pituitary hemorrhage or infarction, along with appropriate laboratory and clinical findings.
Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD): A rare genetic disorder that leads to an inappropriate concentration of urine despite low serum osmolality, resulting in hyponatremia. This condition would be considered in patients with a family history or when other causes of hyponatremia have been ruled out. Laboratory findings would include hyponatremia, low serum osmolality, and inappropriately high urine osmolality.