A 77-year-old man was admitted to our hospital complaining of general fatigue. Serum sodium was 116 mEq/l and serum antidiuretic hormone (ADH) was elevated. Radiologic examination revealed nodules in the brain as well as in both adrenal glands. Based on the findings of fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET), we had considered that the cause of the hyponatremia was syndrome of inappropriate secretion of antidiuretic hormone (SIADH) due to active extrapulmonary tuberculosis. Against our expectations, the patient's condition got worse just after he began antituberculous therapy; we finally diagnosed Addison's disease by additional hormonal tests. His condition recovered immediately with the administration of high-dose hydrocortisone, and the tuberculous lesions became smaller with antituberculous medications. Although tuberculous Addison's disease has been decreasing markedly in recent years, we have to consider the possibility of adrenal insufficiency when hyponatremia is observed in patients with active tuberculosis or those having a past history of tuberculosis.