source: Harvard Health Publishing summary/abstract:
Hyperthyroidism occurs when your thyroid gland makes too much thyroid hormone or your dose of thyroid medicine is too high. The excess hormone “speeds up” virtually every system in your body. The symptoms often include nervousness, palpitations caused by a fast heart rate, feeling hot when others are comfortable, trouble sleeping, and weight loss despite being hungry all the time.
If left untreated, hyperthyroidism does more to your heart than simply making it beat faster. The excess thyroid hormone makes it work harder, too. This can lead to any of several problems:
Abnormal Heart Rhythms
Several heart rhythm disturbances can result from overstimulation of the thyroid. The most common are sinus tachycardia, an abnormally fast heart rate that tops 100 beats per minute, and atrial fibrillation, a disorganized rhythm in the heart’s upper chambers.
High Blood Pressure
In hyperthyroidism, blood vessels relax, lowering diastolic blood pressure (the second, or bottom, number in a blood pressure reading). But an excess of thyroid hormone also increases the force of the heart’s contractions, leading to an increase in systolic pressure (the first, or top, number).