J. Malcolm O. Arnold
Cardiologist, Victoria Hospital, London
Assistant Professor of Medicine, University of Western Ontario
Summary of a talk given to the London Area Chapter of the Thyroid Foundation of Canada
Diseases of the thyroid gland can directly alter the normal function of the heart causing symptoms and resulting in significant complications. To understand how the heart is affected it is first necessary to appreciate how the heart works.
The heart contains muscular chambers which contract and cause blood to circulate around the body. Because of valves within the heart, blood normally circulates in one direction only. Blood is returned to the heart through the veins, and the right atrium and right ventricle of the heart pump blood into the lungs. From the lungs blood is returned to the left atrium and left ventricle from which it is ejected into the arteries which distribute blood to the different organs in the body.
With respect to thyroid disease it is important to understand two principles. One, as the heart itself is called a muscle it requires oxygen to work and receives oxygen through special arteries called coronary arteries. If there is disease in these coronary arteries causing a blockage within the lumen and reduction in coronary blood artery blood flow, the heart muscle then works with an inadequate oxygen supply and heart pain or "angina" can be produced. Two, in order for the heart to beat in a coordinated fashion and expel blood smoothly and efficiently, the heart muscle is stimulated to contract in a synchronized fashion by specialized tissues within the heart which conduct electrical impulses. The impulse normally starts at the top of the right atrium and spreads down through the heart.
Symptoms and Signs
Increased levels of thyroxine released from the thyroid gland stimulate the heart to beat more quickly and more strongly. Initially this may produce a fast heart rate which is called a tachycardia. This is observed by a nurse or physician but is usually not noticed by the patient. However, if the fast heart rate becomes severe then palpitations may be observed by the patient. This is an awareness of the heart beating within the chest. Occasionally it can be noticed by normal individuals and may be caused by excessive exercise or drinking too much caffeine. However, if it occurs at rest and is a fast prolonged heart rate, then it may be abnormal. Palpitations may occur in other types of heart disease, but, if caused by an overactive thyroid gland, they do not necessarily mean that a more serious underlying heart disease is present. In some patients, prolonged stimulation of the heart with thyroxine may cause an incoordination of the conduction of electrical impulses within the heart and atrial fibrillation may ensue. This is where the impulses arising in the right atrium, rather than be conducted normally into the ventricles, form a short circuit within atria and rapidly go round in circles causing incoordinated atrial contraction and loss of regular stimulation of the ventricle with an irregular heart beat.
Prolonged stimulation of heart contraction can cause some increase in blood pressure which is called systolic hypertension. The diastolic blood pressure, that is the lower of the two blood pressure numbers, is not normally increased. The increased contraction of the heart with increased cardiac output causes a pulse that is easily felt at the wrist and contributes to warm sweaty hands.
The first essential is to correct the hyperthyroidism. The decision whether to use drugs, radioactive iodine therapy, or thyroid surgery, depends on the nature of the thyroid problem. At the same time as treating the hyperthyroidism, specific treatment for angina may be commenced with a drug which slows the heart rate such as a "beta blocker", eg. propranolol. There is a wide variety of drugs similar to propranolol and all are equally effective. These drugs are also useful to decrease other symptoms of hyperthyroidism such as finger tremor and anxiety. Heart failure is usually treated in the standard way with heart medications to improve heart function such as digoxin and drugs to increase excretion of water by the kidneys, diuretics, eg. furosemide. We do not fully understand the direct actions of thyroxine on the heart, but it appears that some patients may be relatively resistant to normal doses of digoxin and it may be necessary to measure blood digoxin levels and increase the dose to obtain a desired effect.
Symptoms and Signs
These tend to be the opposite to those mentioned above for an overactive thyroid gland and consist of slow heart rate and low blood pressure. Neither of these signs usually produce symptoms in patients. Prolonged hypothyroidism causes metabolic changes in the body and may produce elevated levels of cholesterol. We are aware that some types of elevated cholesterol levels may produce or aggravate narrowing of the coronary arteries. However, as heart rate and blood pressure are also lowered, the complications of angina or heart attack are relatively uncommon.
In severe prolonged hypothyroidism the heart muscle fibres may become diseased with the development of a weak heart producing heart failure. There is often a collection of fluid around the heart and this is called a pericardia effusion but rarely produces any symptoms. As discussed above there may be an increased risk for coronary artery disease, but the importance of this is not fully understood. As with hyperthyroidism, the occurrence of heart complications is more likely to occur in patients who have underlying heart disease caused by factors other than their thyroid problem.
If heart disease is present then hypothyroidism needs to be corrected slowly. If thyroxine replacement is commenced at doses given to otherwise healthy people, then the demands on the heart may be quickly increased with resultant symptoms of angina or heart failure. Thus it is often necessary to begin thyroid replacement at half or even one quarter of the normal replacement dose. Depending on the patient's response and the lack of heart symptoms, this dose may be increased over a period of weeks or months to the normal thyroid replacement dose.
Thyroid disease may directly effect the heart especially whenever the thyroid gland is overactive. This may produce symptoms of palpitations, heart pain, or heart failure. Similar symptoms may occur with an underactive thyroid if it is treated too rapidly with thyroid replacement therapy. Symptoms of heart disease are much more likely to occur in patients who have underlying heart disease from another cause. Permanent changes in the heart are unusual in patients with normal healthy hearts, unless the thyroid disease is particularly severe and left untreated for very long periods of time.
Copyright ©Thyroid Foundation of Canada/La Fondation canadienne de la Thyroïde.
Reprinted from thyrobulletin, Vol. 8, No. 2.