I've been taking levothyroxine sodium 0,50 mcg for several months, i used to take only 0,25 mcg and i'm now having symptoms of faintiness and dizziness. i feel better when i don't take the medication. what should i do? i can't quit it completely?

I think you wrote the wrong doses. Your previous dose was 25 mcg (0.025 mg) and current dose is 50 mcg (0.05 mg). If you had been appropriately tested before the dose was changed then you definitely have hypothyroidism and you need thyroid replacement therapy. The current dose may be too high (or not) so you need to be re-tested, do not stop your med, see your doctor ASAP. Take care.