Weight gain after anti thyroid hormone medication increase.

I recently gained about 16 pounds after starting treatment for hyperthyroidism. I started this medication when I was diagnosed last summer and wasn't really working out or watching what I ate because I was continuously losing weight due to my disease. At that point I was around 136 pounds when my weight for the past 4 years has been around 142. After an increase in my dosage this past January I've gained about 16 pounds so at my highest I was around 154. I've started a low carb and exercise fitness regimen for about 5 weeks and have lost less than three pounds when usually when I would go on a South Beach style diet previously I would drop weight within the first two weeks. Does anyone have any suggestions or a similar story? Any help or support would mean the world to me.