Typically veins are generally wider than arteries, and therefore the pressure is lower. Capillaries are tiny vessels, so repsent high resistance to the incoming blood, so the rate of blood flow through will be quite low. All blood from the artery eventually passes through capillaries and into the vein.
Other causes of an overactive thyroid can include inflammation of the thyroid due to viruses or bacterial infections. Also, postpartum hyperthyroidism affects women immediately after pregnancy. This is one of the most common postpartum health disorders, and it affects between ten and twenty percent of women.
The most common cause of overactive thyroid, according to the American Thyroid Association, is Graves' Disease - an auto-immune disorder that tends to run in families. Growths on the thyroid, called nodules, can spur excess hormone production, as can thyroiditis, overuse of iodine supplements, or regular use of iodine containing medications.