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If left untreated for a long time, hypothyroidism can cause variety of heart conditions including atherosclerosis, cardiac arrest, and even death.

Hypothyroidism often comes with heart issues. Thyroid problems have to be treated because heart complications that come with the disease can be quite serious. If someone has a weak heart, hypothyroidism can make the existing heart problem even worse. Hypothyroidism can even lead to developing new heart conditions in people with no history of coronary diseases.

Several large-scale studies with more than a million people involved have found that both types of underactive thyroid gland – subclinical and overt type – can lead to a variety of heart-related issues that may result in cardiac arrest or even death if left untreated. This can be applied to people with no diagnosed hypothyroidism but who have increased levels of thyroid-stimulating hormone.

Bradycardia (low heart rate)

The thyroid gland uses its hormones to regulate our heart rate. If the level of thyroid hormone is messed up as in the case of hypothyroidism, the heart has difficulties pumping efficiently and becomes weak over time. Also, in hypothyroid people, the heart muscle can't relax well in between heartbeats.

Bradycardia is having 60 or fewer heartbeats per minute. In healthy people, this number is anywhere from 60 to 100, depending on age, sex, and several other factors. People who already have heart issues in combination with underactive thyroid gland are at high risk of atrial fibrillation, as well as developing premature ventricular contractions.

Bradycardia may lead to a lack of oxygen in the organs, as well as some crucial nutrients. It may also cause complications such as heart failure.


It would be logical that slow metabolism caused by decreased thyroid activity would lead to low blood pressure (hypotension), but in fact it's the opposite. What happens is that arteries are rigid in people who have an underactive thyroid, which makes the blood pressure spike up. Over the years, hypertension can lead to atherosclerosis, increased risks of stroke, and heart failure.

You have to bear in mind that hormone replacement therapy sometimes fails to treat hypertension. This might be due to rigid arteries and lack of their elasticity.

Increased cholesterol

Patients who have been struggling with an underactive thyroid for years may experience slower metabolism and their blood can increase lipid levels, including LDL or the “bad” cholesterol and triglycerides. Increased lipid levels can lead to stiffened arteries, which then leads to increased accumulation of plaque within the arteries. All these problems eventually lead to increased risk of cardiac problems such as failure, atherosclerosis, and heart attack.

Arterial sclerosis

Research has found that underactive thyroid is strongly linked to getting atherosclerosis or coronary artery disease regardless of age, sex, or any associated disease such as high blood pressure, obesity, or diabetes mellitus.

While decrease in thyroid hormone improves certain conditions such as coronary artery disease (CAD) and angina – its accompanying discomfort – increase in the levels of bad cholesterol is found to speed up the underlying CAD.

Doctors established that underactive thyroid can be linked to various heart problems, and it's common in people with atherosclerosis, especially women.

Cardiac arrest

Underactive thyroid has the ability to affect the health of your heart. It can either worsen the symptoms of a well-regulated heart condition or cause cardiac arrest in people who don't even know about their underlying thyroid condition.

There's actual research supporting the connection between subclinical underactive thyroid and greater risk of developing cardiac arrest in middle-aged people with thyroid-stimulating hormone over 7.0 mIU/L. Another research discovered that overactive thyroid may lead to increased risk of mortality in patients with heart failure with ejection fraction that is reduced under 35 percent.

Edema (excess fluid)

Edema or swelling caused by extra water trapped in body's tissues is a common result of aggravating heart disease. Underactive thyroid can also cause a form of edema known as myxedema, which is actually buildup of abnormal molecules in the skin, which mostly results is swollen face, arms, and legs.

This type of edema is common occurrence in people who don't treat underactive thyroid for a long time. Severe condition like this can cause various types of dangerous medical emergencies, known as myxedema crisis. Most common signs and symptoms of myxedema crisis are:

  • respiratory problems such as hypoventilation
  • decreased sodium levels
  • extremely low body temperature (hypothermia)
  • low oxygen in the body
  • confusions and convulsions
  • trauma
  • increased concentrations of carbon dioxide (hypercapnia)
  • seizures
  • deep unconsciousness
  • death

Hormone replacement therapy and heart health

Therapy with levothyroxine usually gives good results. It tends to improve the levels of bad cholesterol and triglycerides, decreases blood pressure, improves heart rate, and delays the advancement of coronary artery disease.

Patients who have certain type of cardiomyopathy show better cardiac contractility after trying hormone replacement therapy. Levothyroxine can cause myocardial ischemia or various forms of arrhythmias, but this rarely happens. It's recommended to begin your therapy with small doses of the hormone, and increase it gradually.

When it comes to subclinical type of underactive thyroid, research hasn't shown major differences in blood pressure and cholesterol levels after the treatment with levothyroxine, but people have experienced certain benefits from the therapy.

One research found that levothyroxine used by people with increased levels of thyroid-stimulating hormone (which is anything above 4 mIU/L) increases chances of heart failure. One British research suggests that levothyroxine decreases amount of ischemic diseases in people between ages of 40 and 70, but doesn't seem to work in people over 70 years of age.

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