How does Hashimoto's thyroiditis (Hashimoto's) affect heart rate and what is the recommended management?

Hashimoto's Thyroiditis and Heart Rate

Hashimoto's thyroiditis typically causes bradycardia (slow heart rate) when it progresses to hypothyroidism, with the most common cardiovascular manifestation being a reduced resting heart rate along with mild hypertension and narrowed pulse pressure. 1

Cardiovascular Effects of Hashimoto's-Related Hypothyroidism

Primary Heart Rate Changes

• Bradycardia is the hallmark cardiac finding in hypothyroidism resulting from Hashimoto's thyroiditis 1
• The low cardiac output in hypothyroidism is caused by three mechanisms: bradycardia, decreased ventricular filling, and decreased cardiac contractility 1
• Systemic vascular resistance can increase by as much as 50%, and diastolic relaxation and filling are slowed 1

Additional Cardiovascular Manifestations

• Mild hypertension (often diastolic) and narrowed pulse pressure commonly occur 1
• Pericardial effusions and nonpitting edema (myxedema) can develop in severe, long-standing hypothyroidism 1
• Clinical heart failure is rare because cardiac output is usually sufficient to meet the lowered systemic demands in hypothyroidism 1

Increased Cardiovascular Risk

• Patients with Hashimoto's thyroiditis have a 1.44-fold increased risk of developing coronary heart disease compared to the general population (adjusted HR = 1.44,95% CI = 1.05-1.99) 2
• This risk is particularly significant in women and subjects younger than 49 years 2
• Combining Hashimoto's with hypertension or hyperlipidemia further increases CHD risk (adjusted HR = 2.06 and 1.83, respectively) 2

Management Recommendations

Thyroid Function Testing

• Thyroid function tests are recommended in all patients presenting with heart failure due to the relative ease of diagnosis and availability of definitive treatments 1

Levothyroxine Treatment Strategy

• First-line treatment is synthetic levothyroxine to normalize TSH levels 3
• Initial dosages should be tailored: lower starting doses (25-50 mcg) should be used for elderly patients with known cardiovascular disease, while young, healthy patients can start with the full dose of 1.6 mcg/kg 1
• TSH monitoring should be performed 6-8 weeks after initiating levothyroxine or changing the dose, then annually once TSH is at goal 1, 3

Cardiovascular Protection Through Treatment

• Treatment with levothyroxine for more than 1 year reduces the risk of CHD to levels comparable with the general population (adjusted HR = 0.84,95% CI = 0.47-1.52) 2
• Hashimoto's without T4 treatment or treatment for less than 1 year is associated with higher CHD risk (adjusted HR = 1.55 and 2.42, respectively) 2

Critical Dosing Considerations

• Minimal TSH elevations may not require dosage adjustment in patients who feel well, particularly those with arrhythmias or other cardiac disorders 1
• When serum TSH is in the upper half of the reference range and patients continue to note symptoms, it is reasonable to increase levothyroxine to bring TSH into the lower portion of the reference range 1
• The rapidity of dosage adjustment depends on the patient's age and medical comorbidities 1

Common Pitfalls to Avoid

• Overtreatment with levothyroxine can cause tachycardia, palpitations, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, and cardiac arrest 4
• About 1 in 4 patients receiving levothyroxine are maintained unintentionally on doses sufficient to fully suppress TSH, which increases cardiovascular risk 1
• Monitor for cardiac arrhythmias during surgical procedures in patients with coronary artery disease receiving suppressive levothyroxine therapy 4
• If cardiac symptoms develop or worsen, reduce the levothyroxine dose or withhold for one week and restart at a lower dose 4