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Key Clinical Message A 78‐year‐old man was referred to clinic due to a 5‐year history of weight loss, lethargy, and pathology showing hyponatremia. In the year prior, he had a hospital admission for symptomatic hyponatremia. MRI brain during that admission showed a 1–2 mm pituitary lesion of unknown significance. Testing during this presentation revealed hypocortisolism with ACTH deficiency. Progress MRI brain revealed the presence of a Rathke's Cleft Cyst (RC). Medical management with glucocorticoids resulted in symptomatic and biochemical parameter improvement. To our knowledge this is the first reported case of isolated ACTH deficiency in the setting of a RC.