“Most people don’t know about this symptom, Dr. Hall says. “When the thyroid is underactive, we don’t create good muscle mass and the muscles don’t function well,” she adds. If you feel pain in your arms, legs and feet for no obvious reason (as in you didn’t fall or hit yourself) your thyroid gland may not be producing enough hormone. Over time, too little of it can damage the nerves that send signals from the brain throughout the entire body.