Sodium ions or high plasma osmolality primarily stimulates the synthesis and release of antidiuretic hormone (ADH) or vasopressin from the posterior pituitary. This woman is presenting with signs of diabetes insipidus, a condition caused by lack of ADH, leading to dilute urine and concentrated plasma. Therefore, increased sodium ions in her plasma would be the main stimulating factor for the synthesis of ADH, to increase water reabsorption in the kidneys.

Although hyperkalemia can stimulate ADH release, it is not the primary stimulus. Also, hyperkalemia would present with symptoms such as muscle weakness or cardiac changes, which this patient doesn't have.

Protein does not stimulate the synthesis or release of ADH. Instead, it could increase blood volume or pressure, but this is not related to this patient's condition.

Epinephrine has several effects in the organism, including the increase in heart rate, blood pressure and metabolic rate, but it does not stimulate ADH synthesis or secretion. Moreover, it is typically triggered by stress or sympathetic activation, which is not indicated in this case.