The Adrenal glands sit on top of each kidney and regulate many systems of the body, including our fight or flight responses. With frequent over stimulation, such as long term stress, the adrenals become exhausted and unable to contribute adequately or effectively to balancing the systems of the body. Adrenal exhaustion can have a knock on effect on the thyroid, thus slowing the metabolism and creating a cascade of associated issues such as poor sleep, weight gain and chronic fatigue.
"The Cortisol Connection" by Shawn M Talbott explains this process and further information can be found at www.thyroiduk.org.uk. I have found certain supplements (containing bovine or porcine adrenal concentrate) to be useful in strengthening these important glands as a temporary measure. However, returning to full power requires a long term approach that encompasses diet and lifestyle simultaneously.