I've been on sythroid for 2 months and gained 9 pounds despite the fact that I strictly follow a Weight Watcher diet (and have remained on it successfully for 6 years...until 2 months ago) and exercise 5 out of 7 days a week. The doctor said it is not from the medication, but I'm getting off of it anyway, as I'm only on it to reduce the size of a nodule.