Hi. I'm a doctor, though not an endocrinologist. Tachycardia is a typical symptom of hyperthyroidism. When the heart rate is as high as you have indicated in a later post, the patient has thyrotoxicosis, which may sometimes be accompanied by agitation and exophthalmos (very prominent eyes). Continuous severe tachycardia leads to heart failure as the cardiac muscle gets exhausted. Beta blockers are used to control the tachycardia until definitive treatment for the hyperthyroidism kicks in. Tapazole (methimazole) is one of the standard treatments for hyperthyroidism and beta blockers are standard at the beginning of treatment. In most cases, the cardiac symptoms resolve with effective treatment of the hyperthyroidism, which takes a few weeks. The sooner treatment commences, the better. That is all I will say on the subject as I do not think an Internet forum is the right place for your family member to be getting advice. He or she needs to see an endocrinologist as soon as possible.