Hypothyrodism is a very common condition characterised by reduction in effective levels of a hormone called thyroxine. It is usually seen in women between the ages of 40-50 (but can be seen at any age) and is almost 10 x commoner in women than in men. It is estimated that about 1 in every 50 people you meet is overtly hypothyroid and it is estimated that there are many more people who are probably sub-clinically hypothyroid meaning that they probably have the process going on but it may not have manifest as significantly abnormal blood tests.
It is also worth noting that often the symptoms come on gradually and are often missed by the patient and the doctor.
As hypothyroidism is exceptionally common and because thyroxine plays a very important role in several processes in the cardiovascular system, I felt it apposite to do a blog on the effects of hypothyroidism on the heart.
The first thing to say is that although, thyroxine is very important in the cardiovascular system, the deficiency of thyroxine does not really cause very prominent cardiac symptoms unless the deficiency is very severe indeed.
Some cardiovascular symptoms that patients with hypothyroidism may manifest include:
Exertional breathlessness and exercise intolerance
A slow heart rate
High blood pressure
Swelling of the legs which is typically non-pitting meaning that when you press the swollen legs with a finger it doesn’t leave a very obvious indentation
Fluid in the sac surrounding the heart which is termed a pericardial effusion. This is particularly important as it can be quite common and when it occurs it can get quite big and cause the patient symptoms of breathlessness and exercise intolerance.
The hormone, Thyroxine, controls several enzymes which regulate how fast the heart beats and how strongly it contracts. Thyroxine is also responsible for how well the heart responds to hormones such as adrenaline. In addition, thyroxine is important because it relaxes our blood vessels and it also promotes the release of a compound called EDRF (endothelial derived relaxing factor) which also helps our blood vessels relax and all these processes can get affected when there is a deficiency of thyroxine.
From a cardiac and circulatory perspective, the simplistic way to understand the effects of hypothyroidism is that everything slows down and everything gets stiffer.
What this means is that hypothyroidism is accompanied with a slow heart rate (bradycardia) and less effective contraction of the heart but the blood pressure actually increases. About 20-40% of patients with hypothyroidism will manifest a higher blood pressure. What is interesting is that the blood pressure goes up because of the stiffness of the blood vessels and this actually results in an increase in the diastolic blood pressure (the lower of the 2 blood pressure numbers) and a reduction in the pulse pressure (the difference between the systolic and diastolic blood pressure). Less blood gets round the body but this doesn’t cause a huge problem because the oxygen requirements of the body are less anyway because of everything slowing down. There was an interesting study in patients who had had thyroid cancer and therefore their thyroid glands had been removed and in those patients when their thyroxine replacement was withheld for 6 weeks their diastolic blood pressures went up by an average of 6/9mmHg.
As the heart rate slows down patients with hypothyroidism may be more prone to developing ventricular ectopics. These are extra but ineffective heart beats that come in when the heart slows down and this may manifest as missed beats or heart flutters.
Another heart rhythm problem that can occur in hypothyroidism is that when the heart slows down, something called the QT interval on the ECG gets longer. In patients with a pre-existing prolonged QT interval or in patients with known heart disease or underlying heart rhythm disturbances, there is a risk of a dangerous heart rhythm disturbances known as Torsades de pointes. Finally, many patients with heart rhythm disturbances are put on an iodine containing drug called Amiodarone and this medication by itself can also cause hypothyroidism.
As mentioned earlier, the contraction of the heart becomes weaker and therefore in patients who already have impaired contraction such as patients with heart failure, the presence of hypothyroidism can make the heart failure worse and also lead to worse outcomes.
In patients who have angina, the symptoms of angina actually get better with hypothyroidism as the demands are diminished. However in the long run, hypothyrodisim could actually accelerate atherosclerosis. This is because of the increase in blood pressure, but also because it is associated with higher cholesterol levels. The total cholesterol, LDL cholesterol, VLDL and triglyceride levels can all be increased. In addition, blood CRP levels which are a marker of inflammation, are increased. Homocysteine levels can also go up.
Hypothyroid patients often get edema, meaning swelling of the legs and also around the eyes. Typically the edema is non pitting and occurs because of accumulation of glycosaminglycans which also cause accumulation of retention of water in the feet.
The treatment is to normalise the thyroxine levels with thyroxine replacement. In patients who have angina it is important to normalise the thyroid levels slowly because there is a theoretically increased risk of worsening angina or even heart rhythm disturbances. Thyroxine replacement will increase the heart rate by about 10 beats per minute. Also it is worth noting that thyroxine replacement will not usually increase ectopic beats.
One other thing worth noting is that in patients who take warfarin, the warfarin dose may need to be increased during hypothyroidism but as the levels normalise the warfarin dose may need to go down again
Hypothyroidism is common and therefore I would recommend that it is important to think about it as being an underlying cause of non-specific symptoms of tiredness, weight gain and low mood in middle aged patients and especially women.
It can be associated with biochemical and haemodynamic changes in the body
Finally, these changes may be reversed with careful replacement.
Hypothyroidism; thyroid; hormones; blood pressure; cholesterol; heart disease