Being hypothyroid doubles your chances of atherosclerotic heart disease compared to healthy thyroid people because (in part) of the increased difficulty processing lipids (cholesterol and triglycerides) and because of the tendency of mild to moderate hypothyroidism to cause high blood pressure. Elevated lipids and high blood pressure are both key risk factors for heart attacks. Frequently, the slowed metabolism in Hypothyroidism leads to Chronic Fatigue, weight gain, Fibromyalgia pains, and a reluctance to exercise enough.

There is also a decrease in the contractibility of the heart muscle so in cannot work as well, and this actually decreases the rate of heart attacks, in many, until the later. Cardiac conduction defects and arrhythmia's are 2 or 3 times as common in the low thyroid person.  Almost 25% of adults over 50 eventually develop a heart rhythm abnormality called Atrial Fibrillation  Hypothyroidism and nutritional deficits play a major role in this condition, but these basic problems are seldom addressed.

A true tragedy of Modern Medicine is the tendency to put everyone on a dangerous Statin drug (Lipitor, Pravacol, and others) which can lead to Heart Failure, Liver Failure, and Memory Loss or Dementia. There is a lot of money in prescribing Statin drugs: their hazards demand frequent medical supervision, frequent lab work, and they are usually quite expensive. The health problems that they cause bring even more money into the Medical-Pharmaceutical Complex. And, even worse, the only studies that seem to show an advantage to them are the ones funded by drug companies. There has never been strong evidence that this family of unholy chemicals really extends life or improves its quality.

All told, the best thing you can do for your heart is have a healthy level of thyroid hormone: neither too much, nor too little. Notice that I said "healthy" and not "normal." Normal simply means that you fit into a range that covers 95.5% of the population, and has virtually nothing to do with healthy.