Thyroid is rarely a cause of significant weight gain, especially when under treatment. More disappointing is the minimal effect on weight loss once thyroid medicine is begun. One study showed an average weight loss of just eight percent of the excess weight after taking thyroid medicine. In other words, a person who has gained 100 pounds would lose just eight of those 100 pounds after the thyroid is under control.
Synthroid is generally considered a much more effective and “stable” medication than natural or armour thyroid. The best test to determine an appropriate dose of either medicine is the TSH test your doctor has ordered. I would follow his or her recommendation and not change the medicine or dose because of your weight. In the meantime, take a closer look at everything else in your life that you can control: diet, exercise, and stress, while continuing to work with your doctor to find an appropriate medicine and dose for you.