Levothyroxine can take 1 to 3 weeks to take effect. Your physician should be checking your thyroid levels every six weeks. It may take a few adjustments to find the dose that is correct for you. Once the right dose is found, you are more likely to lose weight than gain weight. Also, side effects are usually related to the disease not the medication. As you get to euthyroid, or within normal thyroid levels, you should have few to no side effects.