1. What is the HPA axis?
In order to truly understand chronic fatigue as it relates to adrenal dysfunction, we must first define the hypothalamic-pituitary-adrenal, or HPA, axis. This endocrine (hormone-secreting) system is crucial for the body’s ability to stabilize blood pressure, regulate the immune system, produce the precursors to sex hormones, and handle stress. Most people are familiar with the adrenal glands through their relationship to the “fight or flight” response. But how do these processes happen?
Think of the HPA axis as a line of workers in a factory. At the beginning of the conveyer belt is your hypothalamus, the guy in charge. The hypothalamus is a region of the brain which is able to convey messages from neurons, or brain cells, to endocrine, or hormone-producing cells. Think of hormones as short messages that are passed from worker to worker in the assembly line. The responsibility of the hypothalamus is to pass a hormone message to the next guy in line, the pituitary gland. The pituitary, or “master gland” as it is sometimes called, has the ability to produce hormonal memos and send them to any of the downstream workers on the conveyer belt. These hormonal messages are a prompt to “make more product,” and are sent to other endocrine glands, like the thyroid, ovaries, testes, and adrenal glands.
The adrenal glands, the last stop on the HPA axis conveyer belt, are responsible for making a variety of different steroid hormones. Hormones involved in the stress response, like epinephrine/norepinephrine (adrenaline) and cortisol, are produced in response to the perception of the hypothalamus that there is a STRESSOR. Stressors can be immediate and dangerous, like being in a car accident, or more insidious and long-term, like worrying about a final exam. Unfortunately, your brain doesn’t have an accurate way to tell the difference between the two, and stress hormones are produced all the same.
In a normal situation, when stress hormones are secreted by the adrenal glands, the hypothalamus has the ability to gauge when “enough is enough,” and will shut down the whole process. This is called a “negative feedback loop.” As you will see in the next article, however, the HPA axis can be “fooled” into continuing this process indefinitely, leading to what many people know as “adrenal fatigue.”