2 Answers

it might slow down  your thyroid and your heart beat too slow; and that could cause you to faint !  ....or worse !

Thyroid medicine is used when your thyroid hormones are too high- it lowers them hopefully to normal levels...Going back to normal can cause weight gain, feeling tired or fatigued...etc. go to webmd medicines and type in your medication and it will give you all of the symptoms 7/or put hyperthyroidism and hypothyroidism in your search engine (you would be going from a hyperthyroid state to a hypothyroid state with too much medicine- but may have some of the symptoms when regulating to a normal state)  After that I would call your doc and explain your symptoms and length of time you have experienced them (or if they have gotten worse) and see what your doc thinks