---
title: The heart in hypothyroidism
description: How too little thyroid hormone affects the heart, why hypothyroidism slows the pulse and can lead to fluid in the pericardium, how this is investigated, which treatment approaches clinical guidelines describe, and when to call an ambulance.
type: disease
aliases:
  - hypothyroidism
  - underactive thyroid
  - myxedema
  - myxoedema
  - myxedema coma
  - myxoedema coma
  - hypothyroid heart disease
  - pericardial effusion in hypothyroidism
  - autoimmune thyroiditis
  - Hashimoto thyroiditis
sources:
  - tier: guideline
    org: minzdrav-rf
    title: Гипотиреоз
    year: 2024
    url: https://cr.minzdrav.gov.ru/view-cr/531_4
  - tier: guideline
    org: minzdrav-rf
    title: Перикардиты
    year: 2025
    url: https://cr.minzdrav.gov.ru/view-cr/746_2
  - tier: guideline
    org: esc
    title: 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy
    year: 2021
    doi: 10.1093/eurheartj/ehab364
    url: https://doi.org/10.1093/eurheartj/ehab364
  - tier: guideline
    org: minzdrav-rf
    title: Брадиаритмии и нарушения проводимости
    year: 2025
    doi: 10.15829/1560-4071-2025-6669
    url: https://cr.minzdrav.gov.ru/view-cr/160_2
  - tier: reference
    org: msd-manuals
    title: Hypothyroidism (Myxedema) (MSD Manual Professional Version)
    year: 2026
    url: https://www.msdmanuals.com/professional/endocrine-and-metabolic-disorders/thyroid-disorders/hypothyroidism
  - tier: reference
    org: msd-manuals
    title: Hypothyroidism (MSD Manual Consumer Version)
    year: 2026
    url: https://www.msdmanuals.com/home/hormonal-and-metabolic-disorders/thyroid-gland-disorders/hypothyroidism
sources_checked: 2026-10-14
review: null
---

Hypothyroidism is a condition in which the thyroid gland makes too little hormone, so the body is short of thyroid hormones.<Cite n={[1, 5]} /> These hormones are needed by every organ and tissue and affect, among other things, the heart rate.<Cite n={[1, 6]} /> In the heart and blood vessels, hypothyroidism can bring a slower pulse, raised diastolic (lower) blood pressure, fluid in the pericardial cavity (an effusion) and raised blood cholesterol.<Cite n={1} /> Hypothyroidism is one of the potentially reversible causes of a slow heart rhythm.<Cite n={[3, 4]} /> The opposite condition, when there is too much thyroid hormone, is covered in the article on [the heart in thyrotoxicosis](../thyrotoxic-heart-disease/).

<Diagram name="hypothyroid-heart-disease">
  <p slot="normal">
    The diagram shows the thyroid gland and the heart inside its sac, the pericardium. Thyroid
    hormones affect many body functions, including the heart rate.<Cite n={6} /> Normally the
    pericardial cavity holds only a little fluid, which lubricates its layers.<Cite n={2} />
  </p>
  <p slot="disease">
    In hypothyroidism there is too little thyroid hormone, and the pulse slows.<Cite n={[1, 5]} />
    Fluid, an effusion, can build up in the pericardial cavity; the more severe and the longer the
    hypothyroidism, the more fluid there is.<Cite n={[1, 2]} /> It collects slowly, so it rarely
    squeezes the heart.<Cite n={[2, 5]} />
  </p>
</Diagram>

## Why it happens

<Patient>

Most often hypothyroidism is caused by autoimmune thyroiditis: the immune system acts against the cells of the thyroid gland, and over many years the gland gradually stops making enough hormone.<Cite n={1} /> Other common causes are thyroid surgery and radioactive iodine treatment.<Cite n={[1, 5]} /> Hypothyroidism can also happen while taking certain medicines, for example ones that contain iodine.<Cite n={5} />

Hypothyroidism is common. It is found much more often in women than in men, and it becomes more common with age.<Cite n={1} />

When thyroid hormones are lacking, body functions slow down and metabolism is disturbed, including the work of the heart.<Cite n={[1, 6]} /> The pulse becomes slower because both the thyroid hormone level and adrenergic stimulation fall; adrenergic stimulation is the driving effect of adrenaline and similar substances on the heart.<Cite n={5} />

</Patient>

<Student>

Hypothyroidism is found 8–10 times more often in women than in men.<Cite n={1} /> Hypothyroidism is thyroid hormone deficiency.<Cite n={[1, 5]} /> The main change at the cellular level is reduced oxygen consumption, oxidative phosphorylation and ATP synthesis: the cell runs short of energy.<Cite n={1} /> The most common cause of primary hypothyroidism is autoimmune (Hashimoto) thyroiditis; next comes hypothyroidism after radioiodine therapy or thyroid surgery.<Cite n={[1, 5]} /> Drug-induced hypothyroidism can occur with lithium preparations and iodine-containing medications.<Cite n={5} />

A decrease in both thyroid hormone and adrenergic stimulation causes bradycardia.<Cite n={5} /> ESC lists hypothyroidism among the extrinsic (metabolic) causes of both sinus bradycardia or sinus node dysfunction and atrioventricular junction disturbances.<Cite n={3} /> The Russian Ministry of Health bradyarrhythmia guideline names hypothyroidism as a potentially reversible cause of sinus bradyarrhythmias.<Cite n={4} />

Among endocrine diseases, hypothyroidism is the most common cause of hydropericardium; its extent depends on the degree of thyroid hormone deficiency and the duration of the disease; cases have been described in older patients with subclinical hypothyroidism.<Cite n={2} /> The heart can look enlarged on examination and imaging, partly because of dilation but chiefly because of pericardial effusion.<Cite n={5} /> In secondary (central) hypothyroidism the heart is small, and serous pericardial effusions do not occur.<Cite n={5} />

</Student>

## Symptoms

<Patient>

The signs of hypothyroidism are not specific and develop gradually.<Cite n={[1, 6]} /> The most common are tiredness, sleepiness, feeling cold, dry skin, hair loss, a puffy face, constipation, poor memory, a hoarse voice and low mood.<Cite n={1} />

In the heart and blood vessels there may be a slower pulse, raised lower (diastolic) blood pressure, fluid in the pericardium and raised cholesterol.<Cite n={1} /> Sometimes these very signs come to the fore, and the illness is first taken for a different one: [high blood pressure](../hypertension/), [dyslipidaemia](../dyslipidemia/) or a [pericardial effusion](../pericardial-effusion/).<Cite n={1} />

If fluid has collected in the pericardium, there may be breathlessness, chest pain, weakness, swelling and a slow pulse.<Cite n={2} /> People with hypothyroidism, including subclinical hypothyroidism, have a markedly higher risk of coronary heart disease and heart failure.<Cite n={2} /> If hypothyroidism is not treated, over time it can lead to [heart failure](../heart-failure/).<Cite n={6} />

</Patient>

<Student>

The Russian Ministry of Health hypothyroidism guideline describes conditions that hypothyroidism can mimic ("masks"); the cardiological masks are diastolic hypertension, dyslipidaemia and hydropericardium.<Cite n={1} /> In hydropericardium due to hypothyroidism, the clinical picture includes breathlessness, chest pain, weakness, oedema and bradycardia; diastolic hypertension often develops; marked dyslipidaemia, low QRS voltage and QT prolongation on the ECG are typical.<Cite n={2} /> Tamponade is rare because the fluid accumulates slowly.<Cite n={[2, 5]} />

Serum cholesterol is usually high in primary hypothyroidism but not as high in secondary hypothyroidism.<Cite n={5} /> The risk of coronary heart disease and heart failure is markedly increased in both subclinical and overt hypothyroidism, which complicates diagnosis and the choice of management.<Cite n={2} /> According to MSD, patients with subclinical hypothyroidism and TSH above 10 mIU/L are more likely to have hypercholesterolaemia and atherosclerosis.<Cite n={5} />

</Student>

<HeartModel highlight={['sa-node', 'av-node']} />

## How it is diagnosed

<Patient>

Hypothyroidism is confirmed with a blood test for thyroid-stimulating hormone (TSH): the pituitary gland makes it to control the thyroid, and in the most common, primary, form of hypothyroidism it is high.<Cite n={[1, 6]} /> A normal TSH practically rules out a thyroid problem; if needed, free thyroxine (T4) is measured as well.<Cite n={1} /> A TSH test is also worth doing when cholesterol is raised.<Cite n={1} />

The doctor may suspect hypothyroidism from the symptoms and the examination, including a slow pulse.<Cite n={6} /> If a person has a slow heart rhythm and hypothyroidism is suspected, thyroid function is checked, because this cause of bradycardia can be treated.<Cite n={[3, 4]} />

It also works the other way round: fluid in the pericardium is found on a heart ultrasound (echocardiography), and the cause turns out to be the thyroid.<Cite n={2} /> This is why, when an effusion has no clear cause, thyroid function is checked first, even if there are no signs of hypothyroidism.<Cite n={2} /> The size of the effusion is assessed with echocardiography; CT and MRI give fuller information.<Cite n={2} />

</Patient>

<Student>

TSH is the most sensitive test: it is raised in primary hypothyroidism.<Cite n={[1, 5]} /> Subclinical hypothyroidism means raised TSH with free T4 in the normal range; overt hypothyroidism means raised TSH with low free T4.<Cite n={1} /> Reference ranges are not given here. In hydropericardium, work-up starts with TSH; if it is raised, free T4 and thyroid peroxidase antibodies are measured.<Cite n={2} />

ESC 2021: it is essential to differentiate reversible from non-reversible causes of bradycardia; specific laboratory tests, including thyroid function tests, are recommended in patients with clinical suspicion for potential underlying causes of reversible bradycardia (class I, level C).<Cite n={3} /> The Russian Ministry of Health bradyarrhythmia guideline gives a TSH test in patients with bradycardia and suspected hypothyroidism as an example of a useful investigation.<Cite n={4} />

</Student>

## How it is treated

<Patient>

The main thing is to treat the hypothyroidism itself: the missing hormone is replaced with tablets of synthetic thyroid hormone. This is called replacement therapy.<Cite n={[1, 6]} /> Its aim is to reach and keep normal levels of TSH and thyroid hormones in the blood.<Cite n={1} />

People with hypothyroidism and fluid in the pericardium are first of all given replacement therapy.<Cite n={2} /> The earlier hypothyroidism is recognised, the earlier treatment starts and the easier it is to prevent the fluid from squeezing the heart, which is called tamponade.<Cite n={2} />

In older people and people with heart disease the hormone is usually started at small doses and increased gradually, guided by TSH.<Cite n={[1, 6]} /> This matters because too much hormone in older people carries a risk of heart rhythm disturbances.<Cite n={1} /> Because hypothyroidism is a potentially reversible cause of a slow rhythm, it is looked for and treated first; whether a pacemaker is needed is decided by the doctor.<Cite n={[3, 4]} /> Do not start, stop or change medicines on your own.

</Patient>

<Student>

Russian Ministry of Health hypothyroidism guideline: replacement therapy is recommended for all patients with overt hypothyroidism; its aim is normal levels of TSH and thyroid hormones.<Cite n={1} /> In older patients and those with cardiac disease, treatment starts with small doses that are increased under TSH control; titration is especially important because thyrotoxicosis in older people carries a high risk of arrhythmias and fractures.<Cite n={1} /> Specific drugs and doses are not given here. In secondary hypothyroidism, replacement therapy is started only after adrenal insufficiency has been excluded.<Cite n={[1, 5]} />

In hydropericardium due to hypothyroidism, replacement therapy comes first; recommendations on its management are based on clinical observations, and no randomised trials have been done.<Cite n={2} /> ESC: a symptomatic bradyarrhythmia may be related to a transient, potentially reversible or treatable condition (the ESC table of causes of bradycardia includes hypothyroidism); in such cases correction of these factors is required, whereas permanent pacing is not indicated.<Cite n={3} />

</Student>

## When to call an ambulance

A rare, life-threatening complication is myxoedema coma.<Cite n={5} /> It usually develops in people who have had hypothyroidism for a long time, especially if it has not been treated.<Cite n={[5, 6]} /> Its signs are confusion, stupor or coma, a very low body temperature, slowed breathing and seizures.<Cite n={[5, 6]} /> It can be triggered by exposure to cold, an infection, an injury, surgery, and medicines that depress brain function, such as sedatives.<Cite n={[5, 6]} /> Without rapid treatment death is likely; it is treated in hospital, with breathing support if needed.<Cite n={5} /> If a person with known or suspected hypothyroidism has these signs, call your local emergency number (112 in the EU, 911 in the US and Canada, 999 in the UK).

Squeezing of the heart by fluid is rare in hypothyroidism; its signs are described in the article on [cardiac tamponade](../cardiac-tamponade/). When to get urgent help for a very slow rhythm and fainting is covered in the article on [sick sinus syndrome](../sick-sinus-syndrome/). As with any heart condition, call an ambulance for fainting, chest pain or severe breathlessness: this is the site's general safety rule, repeated at the bottom of every page.

<Quiz />