Metastases to the pituitary gland are rare; cancers that most commonly metastasize to the pituitary are breast and lung cancers. No specific computed tomography or magnetic resonance imaging features reliably distinguish primary pituitary masses from metastases. A combination of a detailed clinical assessment together with specialist endocrine and neuroradiology support is essential to make the rare diagnosis of a pituitary metastasis. We present the case of a man with metastatic lung cancer, initially presenting as hypopituitarism. Subtle features in the history, together with neuroimaging findings atypical for pituitary adenomas, provided clues that the diagnosis was one of the pituitary metastases. Treatment of diabetes insipidus (DI) with replacement antidiuretic hormone (ADH) was complicated by extreme difficulties in achieving a satisfactory sodium and water balance. This was the result of coexistent DI and syndrome of inappropriate ADH secretion perpetuated by the patient's primary lung cancer, a phenomenon not previously described in the literature.