Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): This condition is characterized by the excessive release of antidiuretic hormone (ADH) from the posterior pituitary gland or another source, leading to water retention and hyponatremia. The patient's recent history of fatigue, malaise, sore throat, and moderate-grade headaches could be indicative of an underlying infection or inflammatory process that might trigger SIADH. The low urine osmolarity in the context of hyponatremia suggests an inappropriate concentration of the urine given the serum osmolality, which is consistent with SIADH.

Cerebral Salt Wasting (CSW): Although less common than SIADH, CSW can occur, especially in patients with cerebral disorders. It involves the renal loss of sodium due to a disturbance in the sympathetic nervous system's control over renal function. The patient's history of falls and generalized weakness could suggest a cerebral component to his illness.
Adrenal Insufficiency: This condition, characterized by the inadequate production of adrenal hormones, can lead to hyponatremia. The patient's symptoms of fatigue, malaise, and generalized weakness are consistent with adrenal insufficiency. However, the normal glucose level and the absence of other specific findings (like hyperkalemia) make this diagnosis less likely but still plausible.
Hypothyroidism: Severe hypothyroidism can cause hyponatremia due to decreased cardiac output and subsequent increase in ADH secretion. The patient's fatigue, malaise, and generalized weakness could be consistent with hypothyroidism, although specific thyroid function tests would be needed to confirm this diagnosis.

Pituitary or Suprasellar Tumors: Although rare, tumors in the pituitary or suprasellar region can cause SIADH or CSW, leading to hyponatremia. Given the patient's age and symptoms, it's crucial not to miss a potentially treatable cause of his condition.
Subarachnoid Hemorrhage or Other Cerebral Pathologies: These conditions can lead to SIADH or CSW. The patient's history of falls and headaches necessitates careful consideration of cerebral causes, despite the normal CT and CTA head findings, as these may not always detect subtle or early changes.

Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD): A rare condition characterized by an inappropriate concentration of urine due to a genetic mutation affecting the kidneys' response to ADH. This would be an unusual cause of hyponatremia in an elderly patient without a known family history.
Reset Osmostat: A condition where the osmoregulatory mechanism is "reset" to maintain a lower osmolality, leading to hyponatremia. This is a rare cause and typically seen in patients with chronic illnesses or malnutrition.