I want to know the reason why I have hypothyroidism. How can I find out the cause? May 2012 I weighed 123 by nov 2012 I weighed 153...What went wrong?

A few possibles. Hypothyroidism may be caused by pregnancy, viral infection, auto-antibodies, other insult, hereditary, or spontaneous causes. You could be tested for some of these causes. Typically, low thyroid will cause weight gain, constipation, depression, dry skin, brittle hair. Whatever the cause, start on replacement hormone therapy while you are investigating, so that you may start to feeling more normal.