From the Guidelines
Hypothyroidism causes bradycardia due to the decreased production of thyroid hormones, which directly influence heart rate and cardiac function, leading to a slower heart rate and decreased cardiac output. In hypothyroidism, the thyroid gland produces insufficient amounts of thyroid hormones (T3 and T4), which normally act on cardiac myocytes to increase heart rate and contractility 1. These hormones work by upregulating beta-adrenergic receptors in the heart and enhancing the heart's sensitivity to catecholamines like epinephrine and norepinephrine. They also directly increase the activity of calcium ATPase in the sarcoplasmic reticulum, affecting cardiac contractility. Without adequate thyroid hormone levels, the heart beats more slowly, resulting in bradycardia (heart rate below 60 beats per minute).
Some key points to consider in the relationship between hypothyroidism and bradycardia include:
- Decreased cardiac output: Hypothyroidism can lead to decreased cardiac output, which contributes to the slower heart rate 1.
- Increased peripheral vascular resistance: Hypothyroidism can also lead to increased peripheral vascular resistance, which can further contribute to the slower heart rate 1.
- Reduced blood volume: Additionally, hypothyroidism can lead to reduced blood volume, which can also contribute to the slower heart rate 1.
- Treatment: Treatment involves thyroid hormone replacement, typically with levothyroxine (synthetic T4), starting at doses of 25-50 mcg daily for elderly or cardiac patients and 75-100 mcg daily for others, with dose adjustments based on TSH levels every 6-8 weeks until euthyroidism is achieved.
It's worth noting that untreated thyroid disease, including hypothyroidism, can lead to impaired maternal cardiac and metabolic function, which could reduce oxygen and nutrient delivery to the fetoplacental unit during pregnancy 1. However, this is not directly related to the question of why hypothyroidism causes bradycardia. The most recent and highest quality study on this topic is from 2020, which highlights the importance of treating thyroid disease to prevent complications such as bradycardia 1.
From the Research
Mechanism of Bradycardia in Hypothyroidism
- Thyroid hormones have a significant impact on heart function, mediated by genomic and non-genomic effects 2.
- In a hypothyroid state, thyroid hormone deficiency results in lower heart rate and weakening of myocardial contraction and relaxation, with prolonged systolic and early diastolic times 2.
- Cardiac preload is decreased due to impaired diastolic function, and cardiac afterload is increased, with reduced chronotropic and inotropic functions 2.
Effects of Hypothyroidism on Cardiac Function
- Subclinical hypothyroidism is associated with subtle changes in cardiac function, including decreased left ventricular ejection fraction and impaired diastolic function 3.
- Levothyroxine treatment can improve cardiac function in patients with subclinical hypothyroidism, with increases in left ventricular ejection fraction and improvements in diastolic function 3.
- However, the benefits of levothyroxine treatment on cardiac function are not universally observed, and may depend on individual patient characteristics, such as age and cardiovascular disease risk 4, 5.
Clinical Implications
- The relationship between hypothyroidism and bradycardia is complex, and may involve multiple mechanisms, including decreased thyroid hormone stimulation of the heart and impaired cardiac function 2, 3.
- Treatment of hypothyroidism with levothyroxine may be beneficial in certain patients, but the decision to treat should be individualized based on patient characteristics and cardiovascular disease risk 6.