Nowhere near enough information.
For how long has this been your situation?
Are your stools well formed or loose?
Any blood or mucous?
Any pain in the stomach?
Any drastic change of diet or medication?
i have no medical knowledge but if this is common for you maybe your metabolism is high and i mean you can only put out what you put in as well thyroid can affect metabolism
Your underactive thyroid is playing no part in your problem as I presume that you are taking Thyroxine in some form, the dose regulated by annual blood tests.