What does release of excess thyroxine cause to the heart?

The thyroid gland secretes the hormone thyroxine, that functions as a regulator of body metabolism and growth. The hormone accelerates protein synthesis, energy utilization and heat production. Too much thyroid hormone (hyperthyroidism), results in tachycardia, an increased heart rate, while too little thyroid hormone (hypothyroidism) causes bradycardia, a reduced heart rate.