Ive recently been diagnosed with an underactive thyroid. What can I expect. Does it affect wieght loss.?

Im 53 yr old female. My thyroid function tests were great until march this year now they show i have hypothyroidism. Im on thyroxine 25 mg. I lost 20 kgs in the last part of 2009 but now I have regained 3 kgs. Will this problem stop my Weight loss from continuing. What other problems can I expect.