What Causes Weight Gain While On Synthroid?

Hi, I have Hypothyroidism in my family. My sister and mom have it severely. I was lucky enough not to have nay issues. However, in a recent test, my levels were TSH 1.29, T4 4.3 (LOW), and T3 27. One Dr told me I shouldn't worry. I asked my regular Physician at my physical if this was borderline and should be treated. He thought perhaps a low level of Synthroid would help. I went on .25 mcg in June. Been on it since. I have gained about 6 lbs so far. I live a healthy life style. I exercise and eat healthy. I decided today to stop the meds. I have no other symptoms. I do suffer from Thalassemia. Did I make the right decision? Will this hurt my body, taking these unnecessary meds and now stopping them? Will I lose this weight?