Recurrent hyponatremia and fatigue: Early adrenal insufficiency? (Endocrinology)

Medical Case Details:

A 58-year-old female with a history of hypertension and hypothyroidism (on amlodipine and levothyroxine) has been experiencing ongoing fatigue, dizziness upon standing, and general weakness for the past few months. She had three episodes of hyponatremia over the last two months (serum sodium levels ranging from 125 to 130 mmol/L). There were no signs of fluid loss (no vomiting or diarrhea), and she is not on diuretics. Her blood pressure is relatively low and drops further on standing (105/65 mmHg sitting, 88/58 mmHg standing). Recent labs showed a slightly low morning cortisol. Her thyroid function (TSH) is within normal range on medication. She also reported losing about 3 kg unintentionally over the past few months. Would you consider early adrenal insufficiency here? Is it reasonable to proceed with a Synacthen stimulation test now, or should we first rule out other causes like SIADH? Any input from endocrinology colleagues would be appreciated.