Addison’s disease causes skin darkening (hyperpigmentation) due to low cortisol levels, which trigger increased production of ACTH—a hormone that also stimulates melanin production in the skin, leading to darker patches on areas like knuckles, elbows, and inside the mouth. This hormone imbalance is a key feature of the condition. You should seek medical evaluation if you notice unexplained darkening along with symptoms like fatigue, weight loss, dizziness, or low blood pressure, as early diagnosis and treatment are important to prevent serious complications.