It is unfortunate that you are suffering from Hashimoto's encephalopathy. This is a very rare disorder and has been known for the last 45 years or so. It is also called as steroid responsive encephalopathy associated autoimmune thyroid disease (SEART). In this condition, the main feature is raised levels of antibodies and in fact TSH level may even be normal. The disease responds well to steroids but may relapse. The high anti-TPO levels are part of disease and it is possible to have such a high value. With time the anti-TPO levels will come down. The cause of autoimmunity is multi-factorial including genetic background and some unidentified environmental factors. In your case, you have a strong family background of disease. In fact the only role of steroids is to reduce the autoimmunity. The level of encephalopathy varies from mild to severe and only 60% patients have seizures. Due to mild disease in some, it goes undiagnosed. The second part of your question (why symptoms worsen on standing) may be a part of disease but will be best answered by a neurologist. From the point of hypothyroidism, you should keep Thyroxine dose such that the TSH level remains normal.