A 58-year-old man was referred to our hospital for darkened skin, general fatigue and weight loss. His adrenocorticotropic hormone and cortisol levels indicated a primary adrenal insufficiency. ¹⁸Fluorodeoxyglucose positron emission tomography/CT showed bilateral enlargement of the adrenal glands, with ¹⁸fluorodeoxyglucose accumulation. Loop-mediated isothermal amplification assays of bronchoalveolar lavage fluid were positive for Mycobacterium tuberculosis. The patient was diagnosed with tuberculous Addison’s disease and treated with antituberculosis agents, including rifampicin. The patient’s fatigue worsened gradually after initiation of rifampicin, and the dosage of hydrocortisone was increased. Serum cortisol level monitoring at 2 hours after administration of hydrocortisone was shown to be clinically useful for determining the optimal dose, especially with concurrent use of rifampicin.