Hypothyroidism, or underactive thyroid, develops when the thyroid gland does not produce enough of the hormones thyroxine (T4) or triiodothyronine (T3).

Around 5% of people aged 12 years and older in the United States live with hypothyroidism.

Although many people have only mild symptoms, a person with hypothyroidism may experience weight gain, tiredness, depression, fertility problems, and more.

In clinical cases, where a person has a thyroid-stimulating hormone (TSH) level above 10 and a normal free T4 level, doctors also associate hypothyroidism with a slow heart rate and other significant heart problems. Signs and symptoms do not usually occur in subclinical hypothyroidism.

Preventing heart conditions is one of the primary reasons that doctors need to diagnose and treat clinical cases of hypothyroidism.

This article explores hypothyroidism and slow heart rate. Specifically, it looks at how hypothyroidism can affect the heart, some potential symptoms, and when to seek medical attention.

Someone with hypothyroidism measuring their low heart rate using a smart watch.Share on Pinterest
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Yes, hypothyroidism can cause a slow heart rate, or bradycardia, in some individuals. This can occur if a person’s TSH level is significantly elevated.

Thyroid hormones help control heart rate. In people with clinical hypothyroidism, the heart rate is often 10–20 beats per minute (bpm) slower than usual.

Hypothyroidism causes many of the body’s functions to slow down, including the heart. The heart beats unusually slowly in people with clinical cases of hypothyroidism due to the increased levels of TSH.

As a result, over time, the heart weakens and cannot pump as vigorously.

Additionally, the heart muscles do not fully relax following a heartbeat. When this happens, it can prompt diastolic dysfunction. This condition causes the upper heart chambers to become stiffer and restricts the normal process of filling with blood between heartbeats. It can result in heart failure.

Hypothyroidism causes various cardiac symptoms that are particularly likely in people with an underlying heart condition.

Hypothyroidism affects the heart in the following ways:

  • Heart arrhythmias: Hypothyroidism may increase the likelihood of premature ventricular complexes. In rare instances, it can lead to ventricular tachycardia with a long QT interval, wherein the heart begins to beat very quickly.
  • High diastolic blood pressure: Hypothyroidism causes the arteries and blood vessels to stiffen, thereby increasing diastolic blood pressure. Doctors refer to this as diastolic hypertension.
  • Heart failure: People with hypothyroidism may experience heart failure, even with only mild heart disease.
  • Swelling: Doctors call swelling “edema,” and this can occur due to heart failure. Additionally, hypothyroidism can cause a particular type of edema called myxedema. The exact cause of this condition is a buildup of abnormal proteins and other compounds in the fluid that surrounds the cells.
  • Worsening coronary artery disease (CAD): Doctors associate hypothyroidism with an increase in low-density lipoprotein, or “bad,” cholesterol and an inflammatory marker called C-reactive protein. These molecules may worsen CAD.

A person should speak with a doctor if they experience any symptoms associated with hypothyroidism, including:

  • fatigue
  • weight gain
  • intolerance to cold
  • musculoskeletal pain
  • dry skin
  • dry, thinning hair
  • fertility problems
  • heavy menstrual periods
  • depression

If someone is currently living with hypothyroidism and receiving care for the condition, they should speak with a doctor if they notice any new symptoms.

Heart symptoms to look out for include pain, palpitations, and difficulty breathing.

A person may require a medication review and blood tests to ensure that their current dosage of thyroid hormone replacements remains appropriate for their needs.

Doctors depend on the following information to diagnose hypothyroidism:

  • Symptoms: Hypothyroidism can cause a range of symptoms. However, there is not one specific symptom that indicates hypothyroidism. Therefore, a doctor may ask if the person feels different than they usually do.
  • Medical and family history: A person should tell a doctor if they have ever had thyroid surgery or radiation treatment to their neck to treat cancer. In addition, the doctor may ask if the person has a family history of thyroid disease, has experienced any recent health changes, or takes any medications.
  • Results of a physical exam: During a physical exam, the doctor checks the thyroid gland, measures the heart rate, and tests reflexes. They may also examine the skin for dryness, which can occur with hypothyroidism.
  • Blood tests: Doctors use the TSH test to diagnose hypothyroidism. TSH is a hormone that controls the thyroid gland’s production of T3 and T4. Doctors also use the T4 test to measure free and available thyroxine. This is because most T4 in the blood is bound to a protein called thyroxine-binding globulin, which prevents T4 from entering the body’s cells.

Doctors aim to reduce or prevent cardiac symptoms in people with hypothyroidism by replacing the hormones that the thyroid gland does not make. Doctors often prescribe hormone therapy in the form of levothyroxine pills.

Doctors typically test a person’s blood around 6 weeks after they begin taking replacement thyroid hormones. If necessary, the doctor can adjust the dosage until it works for the person.

Most people can control their hypothyroidism by taking thyroid hormone medication, provided they follow the doctor’s instructions. A person must not stop taking their medication without first consulting a doctor. They should also understand that taking too much of a medication can cause serious health problems.

Doctors typically classify a slow heart rate as under 60 bpm.

However, the heart rate may naturally fall below 60 bpm during deep sleep. Also, athletes and physically active individuals may have a slower heart rate.

Other causes of a slow heart rate include:

  • using certain heart medications
  • sustaining damage to the heart due to heart disease or a heart attack
  • having congenital heart defects
  • having obstructive sleep apnea
  • having problems with the heart’s natural pacemaker, or the sinoatrial node
  • having issues with the heart’s electrical impulses between the atria and ventricles

Hypothyroidism is a condition wherein the thyroid gland does not produce enough hormones. It can lead to various problems throughout the body, including causing a slow heart rate and leading to other cardiac problems.

Doctors diagnose hypothyroidism by taking a medical history, performing a physical exam, and running blood tests that measure thyroid hormones.

Most people can manage hypothyroidism by taking medication to replace the hormones that the thyroid gland does not make. These medications are the primary way that doctors treat heart problems in those with hypothyroidism.

Anyone who is experiencing possible symptoms of hypothyroidism or any cardiac symptoms should seek medical advice.