Can autoimmune thyroiditis, such as Hashimoto's thyroiditis, cause heart flutters in patients with hypothyroidism?

Can Autoimmune Thyroiditis Cause Heart Flutters?

Yes, autoimmune thyroiditis (Hashimoto's thyroiditis) can cause heart flutters, but the mechanism depends on the thyroid functional state—hypothyroidism typically causes bradycardia rather than palpitations, while overtreatment with levothyroxine or coexisting hyperthyroid phases can trigger tachycardia and arrhythmias including atrial fibrillation.

Understanding the Cardiac Effects by Thyroid State

Hypothyroidism (Untreated Hashimoto's)

The typical cardiovascular manifestations of hypothyroidism are opposite to what causes "flutters":

• Bradycardia is the most common cardiac manifestation, not tachycardia or palpitations 1, 2
• Systemic vascular resistance increases by up to 50%, leading to mild diastolic hypertension 1, 2
• Diastolic relaxation and filling are significantly slowed 2
• Low cardiac output results from bradycardia, decreased ventricular filling, and decreased contractility 1, 2

However, there is evidence of autonomic dysfunction: Children with euthyroid Hashimoto thyroiditis demonstrate disturbed cardiac autonomic regulation with decreased heart rate variability, suggesting the autoimmune process itself may affect cardiac autonomic modulation independent of thyroid hormone levels 3. This autonomic dysregulation could theoretically contribute to palpitations or irregular heartbeats even in the euthyroid state.

Levothyroxine Overtreatment (Most Common Cause of "Flutters")

This is the critical pitfall: Overtreatment with levothyroxine can precipitate the exact cardiac symptoms patients describe as "heart flutters":

• Overtreatment may cause palpitations, tachycardia, and arrhythmias, particularly in elderly patients or those with underlying cardiovascular disease 4
• The FDA explicitly warns that levothyroxine overtreatment may precipitate angina or arrhythmias and increase heart rate, cardiac wall thickness, and cardiac contractility 4
• Atrial fibrillation risk increases significantly with subclinical hyperthyroidism (TSH <0.1 mIU/L), with a 3-fold increased risk over 10 years in patients ≥60 years 1

Transient Hyperthyroid Phases

Hashimoto's thyroiditis can present with transient hyperthyroid phases (hashitoxicosis) early in the disease course:

• During these phases, patients experience typical hyperthyroid symptoms including tremors, nervousness, tachycardia, and palpitations 1
• These episodes are self-limited and typically require only symptomatic treatment 5

Clinical Approach to Heart Flutters in Hashimoto's Patients

Step 1: Check Thyroid Function Tests Immediately

• Measure TSH, free T4, and free T3 to determine current thyroid state 1
• If TSH is suppressed (<0.1 mIU/L), this indicates either overtreatment or a hyperthyroid phase and directly correlates with arrhythmia risk 1
• If TSH is elevated with normal free hormones (subclinical hypothyroidism), bradycardia is more likely than palpitations 1

Step 2: Obtain ECG and Consider Holter Monitoring

• Check specifically for atrial fibrillation, as hyperthyroidism (including iatrogenic) causes AF in 5-15% of patients, with higher rates in those over 60 years 6
• The American Heart Association recommends thyroid function tests in all patients presenting with heart failure or new arrhythmias 1, 2

Step 3: Adjust Levothyroxine Dosing if Overtreatment Detected

• If cardiac symptoms develop or worsen on levothyroxine, reduce the dose or withhold for one week and restart at a lower dose 4
• In patients with arrhythmias or cardiac disorders, minimal TSH elevations may not require dosage adjustment—maintaining TSH in the upper half of the reference range is acceptable 1, 2
• The goal is to avoid both undertreatment (which worsens cardiac function) and overtreatment (which causes arrhythmias) 4

Critical Pitfalls to Avoid

Do not assume all cardiac symptoms in Hashimoto's patients are from hypothyroidism: The autoimmune process itself may cause autonomic dysfunction 3, and overtreatment with levothyroxine is a common iatrogenic cause of palpitations and arrhythmias 4.

Do not delay beta-blocker initiation if hyperthyroidism is evident: If clinical hyperthyroidism (from hashitoxicosis or overtreatment) is causing symptomatic tachycardia, start a beta-blocker immediately while addressing the underlying thyroid dysfunction 6.

Monitor elderly patients and those with cardiovascular disease more carefully: These populations are at highest risk for cardiac complications from both hypothyroidism and levothyroxine overtreatment 1, 4.