I Am On Levothyroxin 50 Mg. I Was On 25

I am on Levothyroxin 50 mg. I was on 25 but Dr. increased it. My question is I am losing weight about 1/2 pound a month. I didn't mind at first but want to know how much is too much. I am 86 and this is my first experience with thyroid problem. I am now 8 pounds lighter and at my age I am worried when it will level off.