- Thyrox 125 Tablet is a synthetic version of thyroxine, a hormone that is naturally produced in your body. When your body does not produce enough thyroxine, you get hypothyroidism. Some symptoms of hypothyroidism include tiredness, weight gain, and muscle aches. This medicine raises your hormone level to the right amount and helps you get rid of these symptoms. For most people, this is a lifelong medication and should not be discontinued without talking to your doctor.