Carbimazole is a drug rightly used for treatment of hyperthyroidism. Weight gain has been reported as one of the possible side effects. Thyroid hormones control metabolic rate and thus a decrease in thyroid hormones will imply a decrease in metabolic rate and thus more reserves stored as fats. You do not have to stop carbimazole unless your doctor tells you so; thus to deal with your weight gain, you only have to consider a change in diet. You need to eat more of fruits and vegetables, reduced consumption of food rich in carbohydrate, reduce fatty foods etc. You should see your doctor so that he can advice on diet based on the food stuff in your area. However, a better indicator of weight disorder is the body mass index( weight divided by the square of your height in metres). Ideally it should be below 25.