HTN is the medical abbreviation for hypertension, according to Stedman's. Blood pressure is measured by the amount of resistance against blood flowing through the arteries. When more force is required to move blood through narrowed arteries, this is called high blood pressure or hypertension, according to Mayo Clinic.
People can have hypertension without having any symptoms, states Mayo Clinic. Even if there are no symptoms present, uncontrolled high blood pressure still causes damage to the heart and blood vessels.
For a small group of people, hypertension is caused by some underlying condition, such as kidney disease, thyroid dysfunction or pregnancy, according to Heartland Cardiovascular Center. In approximately 90 percent of people, there is no obvious cause for hypertension.