Jack51

Hi, here is some information on Hyperthyroidism (overactive) and your heart.

A rapid heart beat, or irregular skipped beats (palpitations) is a common complaint in patients with hyperthyroidism. Some subjects notice that their basal resting heart rate is a bit higher than normal; other patients observe that their heart rate goes up much faster and takes longer to return to normal when they exercise.

In some older patients, or in patients with existing heart disease, hyperthyroidism may increase the risk of developing an abnormal heart rhythm, such as atrial fibrillation. Several large studies have shown that the risk of developing hyperthyroidism can be up to 5-fold greater in subjects with a suppressed TSH, who feel otherwise well, compared to control subjects with a normal TSH.

Indeed, the diagnosis of previously unsuspected hyperthyroidism is not uncommon in older patients with rapid irregular heart rhythms who present to the emergency room. It may be difficult to maintain the heart in normal rhythm until the hyperthyroidism is brought under satisfactory control. For some patients with sustained irregularities in heart rate and rhythm, the possibility of anticoagulation should be considered until the abnormal heart rate and rhythm can be corrected. Shortness of breath in hyperthyroid patients may also be attributable to effects on the lungs, likely as a partial result of weakness in the muscle used for breathing. A direct effect on oxygen exchange in the lungs remains a possibility.
http://www.mythyroid.com/thyroid_disease_and_the_heart.htm