Antithyroid medicines like carbimazole are used to treat an overactive thyroid gland (hyperthyroidism), which is also known as thyrotoxicosis. When your thyroid gland produces too much thyroid hormone, it can cause many of your body’s functions to speed up, and this leads to symptoms such as losing weight, sweating, feeling irritable or shaky, and diarrhoea. You also may feel tired or worried, and develop menstrual problems. Carbimazole is the most commonly used medicine for hyperthyroidism.