An overactive thyroid gland (hyperthyroidism) was reportedly the cause of the former president’s atrial fibrillation episode in 1991, noted the Harvard Men’s Health Watch in 2011. “Thyroid hormone is essential to regulating many of our body’s metabolic functions,” said Dr. Campbell. “It also is a ‘stimulant’ hormone. When the thyroid becomes overactive, more stress hormones such as epinephrine and norepinephrine [or adrenaline] can be released. These hormones, along with thyroid hormone, can result in rapid heartbeats and, in the case of the [former] president, rapid atrial fibrillation.” Bush’s doctors used drug therapy — first digoxin and then procainamide to return his heart rate to normal, according to The New York Times.