Here's some information:

Most of the extra weight gained in hypothyroid individuals is due to excess accumulation of salt and water. Massive weight gain is rarely associated with hypothyroidism. In general, 5-10 pounds of body weight may be attributable to the thyroid, depending on the severity of the hypothyroidism. Finally, if weight gain is the only symptom of hypothyroidism that is present, it is less likely that the weight gain is solely due to the thyroid. 

However, since hypothyroidism usually develops over a long period of time, it is fairly common to find that there is no significant weight loss after successful treatment of hypothyroidism. Again, if all of the other symptoms of hypothyroidism, with the exception of weight gain, are resolved with treatment with thyroid hormone, it is less likely that the weight gain is solely due to the thyroid. Once hypothyroidism has been treated and thyroid hormone levels have returned to the normal range of thyroid hormone, the ability to gain or lose weight is the same as in individuals who do not have thyroid problems.