How is thyroid linked to heart disease?
Heart disease can occur with both increased function of the thyroid gland and decreased function of the thyroid gland. When thyroid function is increased, heart rate increases and the work load of the heart increases. In severe cases heart failure may occur. A peculiarity of heart failure associated with increased thyroid function is that it is a high output failure unlike the usual heart failure in which the pumping function of the heart is reduced and low output failure occurs. While in low output heart failure the extremities are cold, in high output failure due to increased thyroid function, the extremities of the limbs are warm. Another important issue with increased thyroid function is that the heart rhythm may become irregular. This condition is known as atrial fibrillation, in which the upper chambers of the heart generate very fast irregular signals, but fail to contract well. Hence the blood stagnates in some parts of the upper chambers (left atrium) and clots may form. If these clots migrate to the blood vessels of the brain, a stroke may result.
Reduced function of the thyroid gland is also associated with heart disease. Cholesterol levels go up when thyroid function comes down. This in turn can enhance the chance of plaque build up in the blood vessels of the heart (coronary arteries). When thyroid function is very low, fluid collects in various parts of the body. This may occur within the covering of the heart known as pericardium. Collection of fluid within the covering of the heart is called pericardial effusion. If it is severe enough to compress the heart, it prevents proper filling of the heart and blood pressure falls.