No, a TSH of 64 does not cause tachycardia—severe hypothyroidism causes bradycardia, not tachycardia.
Understanding the Cardiac Effects of Severe Hypothyroidism
A TSH of 64 mIU/L indicates severe primary hypothyroidism, which produces hemodynamic changes opposite to those of hyperthyroidism. The most common cardiovascular manifestations are bradycardia, mild hypertension (often diastolic), and a narrowed pulse pressure 1. The low cardiac output in hypothyroidism results from bradycardia, decreased ventricular filling, and decreased cardiac contractility 1.
Key Cardiovascular Manifestations in Severe Hypothyroidism
- Bradycardia is the hallmark cardiac finding, not tachycardia, as thyroid hormone deficiency directly reduces heart rate 1
- Systemic vascular resistance increases by as much as 50%, contributing to elevated diastolic blood pressure 1
- Diastolic relaxation and filling are slowed, leading to diastolic dysfunction 2
- Pericardial effusions and nonpitting edema (myxedema) can occur in severe, long-standing cases 1
ECG Findings in Hypothyroidism
The electrocardiographic changes in hypothyroidism include 2:
- Sinus bradycardia (10.3% of cases) as the primary rhythm disturbance
- Low voltage complexes (25% of cases)
- T-wave inversions (23.5% of cases)
- Prolonged QTc interval
- QRS prolongation (2.9% of cases)
When Tachycardia Occurs with Elevated TSH: Alternative Explanations
Ventricular Arrhythmias in Severe Hypothyroidism
While bradycardia is typical, hypothyroidism paradoxically increases the risk of ventricular arrhythmias, not supraventricular tachycardia 3. A recent case report documented polymorphic ventricular tachycardia in a patient with TSH of 142 mIU/L, which resolved completely with levothyroxine treatment 4. This represents a life-threatening arrhythmia requiring immediate defibrillation, not the sustained sinus tachycardia typically implied by the term "tachycardia."
Critical Pitfall: Distinguishing Thyroid States
The evidence consistently shows that tachycardia occurs with hyperthyroidism (low TSH), not hypothyroidism (high TSH) 1. Subclinical hyperthyroidism with TSH <0.1 mIU/L increases heart rate, left ventricular mass, and cardiac contractility 1. Severe, long-standing hyperthyroidism can cause persistent sinus tachycardia or rapid ventricular rates from atrial fibrillation 1.
Clinical Implications for TSH of 64 mIU/L
Expected Cardiovascular Presentation
A patient with TSH of 64 mIU/L should present with 1, 2:
- Bradycardia (heart rate typically <60 bpm)
- Mild hypertension with narrowed pulse pressure
- Possible diastolic dysfunction on echocardiography (30.8% of hypothyroid patients)
- Fatigue, dyspnea, and lower extremity edema from reduced cardiac output
- Possible pericardial effusion (2.94% of cases)
If Tachycardia Is Present: Look for Alternative Causes
If a patient with TSH of 64 presents with tachycardia, investigate other etiologies 1:
- Concurrent infection or sepsis
- Anemia (which can coexist with hypothyroidism)
- Heart failure decompensation from another cause
- Pulmonary embolism
- Medication effects (sympathomimetics, bronchodilators)
- Rare ventricular tachycardia (requires ECG confirmation and immediate treatment) 4
Management Priority
The cardiovascular dysfunction from severe hypothyroidism is reversible with thyroid hormone replacement 1. Treatment should be initiated cautiously, particularly in older patients or those with cardiac disease, starting with levothyroxine 25-50 mcg/day and titrating slowly to avoid precipitating cardiac complications 5. The bradycardia and other hemodynamic abnormalities will normalize as thyroid function is restored 1.