The thyroid produces a hormone that regulates the body's metabolism. The thyroid hormone increases the rate at which the body's cells use energy. Thyroid hormone levels affect the body's temperature, mood, heart rate, energy levels and muscle function. If the thyroid gland produces too much thyroid hormone, it causes hyperthyroidism. Too little thyroid hormone causes hypothyroidism. Both kinds of thyroid disorders can cause high blood pressure (also known as hypertension), although they do so using different mechanisms and affect blood pressure in different ways.
Hyperthyroidism can be caused by the immune system stimulating the thyroid gland or by a tumor that causes increased thyroid hormone production. Hyperthyroidism can also be caused temporarily by certain infections that attack the thyroid, causing a transient increase in thyroid hormone being released. One of the effects of excess thyroid hormone is that it causes the heart to beat faster. As the heart beats faster, more blood is pushed through the arteries. This causes both the systolic and diastolic blood pressure to increase (hypertension).
Abnormally low levels of thyroid hormone can also cause elevated blood pressure. Hypothyroidism can be caused insufficient iodine intake or by various conditions that impair the thyroid's production of thyroid hormone. It can also be caused by a problem in the pituitary gland that causes it to not stimulate the thyroid. One result of hypothyroidism is that the arteries are less pliable, a condition that is also known as increased vascular resistance. These stiffer arteries can cause an increase in diastolic blood pressure (the blood pressure when the heart is at rest).