The adrenal system can become stressed to the point where inputs to the adrenal gland signals for less cortisol. Addison's Disease, which is diagnosable, results in less cortisol than the body needs, causing as one of many side-effects general fatigue. But "adrenal fatigue", where the adrenal gland "shrivels up" because it is working too hard, is mostly a myth.

Several other factors are in play, and the hormones in your body are controlled by several glands and linked in a complex system of signals. Rarely does only one thing contribute to symptoms.