I have heard that people can develop thyroid trouble after extreme dieting. Pretty sure Elisabeth Hassleback claims that happened to her after being on Survivor. But, possibly they found the problem when she was dieting, and it did not cause it. I would say never do anything very extreme because it is a burden to the body. When people lose weight the body has to adjust to the changes. It is balancing electrolytes, which is part of the reason people lose a lot of water weight at first, hormones change, because some hormones are stored in fat, and metobolism might slow due to less food intake, and nutrition might suffer on a very low calorie diet, and some nutrients have a causal affect with hormomes. Vitamin D affects the parathyroid function for instance. Sometimes when you challenge the body too much, I guess there is a possibility it might cause permanent damage. But, I believe in the bodies ability to heal, I think most of the time when we start doing the right things again, our bodies can go back to normal.