A 67-year-old woman developed isolated adrenocorticotropin deficiency (IAD), which manifested as lethargy, a 20-kg body weight loss, hypoglycemia, and parkinsonism, and began corticosteroid replacement. Her symptoms resolved rapidly, and her weight returned to normal within six months. However, she then developed slowly progressive type 1 diabetes mellitus (T1D) with co-existing Hashimoto thyroiditis, and commenced insulin therapy. To our knowledge, this is the first reported case of parkinsonism associated with IAD. In addition, because diabetes mellitus, including T1D, could be latent in patients with untreated IAD, careful assessment of glucose metabolism is needed after commencing corticosteroid replacement until weight regain is achieved.