Normal is defined as a heart rate between 60 and 100 beats per minute. Less than 60 beats per minute is bradycardia (low heart rate), more than 100 beats per minute is tachycardia (rapid heart rate). A resting heart rate of 90 is on the high side of normal, but depends on many factors - firstly, how fit are you? Do you exercise regularly? If not, that will generally push your resting heart rate up a bit. Other things, such as anxiety, pain or an overactive thyroid can also cause a tachycardia. Having a resting heart rate of 90 beats per minute isn t going to do you any immediate harm, it s not something you need to go running to the doctor about. Have a look at your diet and lifestyle (caffeine can increase heart rate, so can nicoteine and alcohol), do a bit of exercise if you don t already. If you have any other symptoms of an overactive thyroid, such as weight loss, despite a good appetite, sweating, and a resting tremor, see you doctor for a blood test to determine whether this is a problem or not.