Having an under active thyroid can affect you in many different ways, and there are also other various things such as para thyroid, pituitary gland, hypothalamus etc that can affect thyroid levels. It is very important part of maintaining homeostasis in the body, and can cause some rather negative effects such as fatigue, low heart rate, depression, temperature intolerance, weight gain and effect food metabolism.

I think it would be best if you continued to get your thyroid tested, best go to a different doctor as the one your going to doesn’t sound like they’re treating you very well. Hopefully they will take a course of action that would involve taking your thyroid levels again, and if they aren’t normal, sending you for more tests, then to an endocrine specialist that should be able to work out the best method of treatment and proper drug dosages.

Lastly if the blood tests that showed your thyroid levels to be below normal were taken when you were sick (with something else such as a virus) then that could be the cause of them being low at that time.