<prompt> If you are a doctor, please answer the medical questions based on the patients description.Hi I am a 23 year old female, 5 7, 12 st. I was diagnosed with Chronic Fatigue Syndrone 10 years ago. A few weeks ago I found out that I have an abnormally fast heart rate for someone of my age, I thought I might have Postural Tachycardia Syndrone a friend had sent me an article about it, my doctor and I had never heard of it but seems to fit my symptoms. So I went to the doctor and discussed it with her my heart rate sitting was 108bpm and when I stood rose to 140bpm, she said come back to the nurse and have her do it again and an ECG, so I did the following week, ECG was fine, this time my heartrate was 72 laying down then only rose to 73 when I stood. So I am now thinking that it is not POTS- although the doctor had put me back onto 80mg of propranolol, could this make the difference?. Do you have any ideas of what this could be? I don t know whether this is relevant but my brother had SVT a couple of years ago, other than that no known heart problems in my family.  <response>: Hello! Welcome on Chat Doctor! I carefully passed through your question and would explain that your symptoms could be related to different causes: - an orthostatic intolerance. Although your heart rate changes are not typical of postural tachycardia, it is necessary performing a Head Up Tilt test to exclude this possibility. - a metabolic disorder chronic anemia, thyroid dysfunction, a Chat Doctor. For this reason, I would recommend performing some blood lab tests: - complete blood count-thyroid hormone levels-cortisol plasma levels-vitamin D plasma levels-blood electrolytes. I dont think your symptoms are related to your family history. You should discuss with your doctor on the above issues. Hope to have been helpful! Kind regards, 