How do thyroid disorders affect heart health and what treatments are recommended?

Thyroid Disorders and Heart Health: Impact and Management

Both hypothyroidism and hyperthyroidism significantly affect cardiovascular health, with treatment of the underlying thyroid disorder being essential to reverse cardiac manifestations and improve morbidity and mortality outcomes. 1

Cardiovascular Effects of Thyroid Disorders

Hypothyroidism Effects

• Bradycardia: Characterized by heart rate below normal limits, sometimes with "mosquito sign" on ECG (symmetric T waves without ST segment) 1
• Conduction abnormalities: Including atrioventricular blocks 1
• Pericardial effusion: Common in severe hypothyroidism 2
• Diastolic dysfunction: Impaired ventricular relaxation 3
• Increased risk of atherosclerosis: Higher risk of myocardial infarction in subclinical hypothyroidism 3
• Narrowed pulse pressure: May require immediate assessment 1

Hyperthyroidism Effects

• Hyperdynamic cardiovascular state: High cardiac output with low systemic vascular resistance 3
• Tachycardia and palpitations: Increased heart rate and contractility 3
• Atrial fibrillation: 3-5 fold increased risk, especially in those >60 years with TSH <0.1 mIU/L 4
• Increased cardiovascular mortality: Up to 3-fold higher in individuals >60 years with TSH <0.5 mIU/L 4
• Left ventricular hypertrophy: Increased LV mass 3
• Heart failure: Can occur even without intrinsic heart disease 5

Diagnostic Approach

Laboratory Testing

• TSH and Free T4: Essential for diagnosis of thyroid dysfunction 1
• Free T3: May be needed in suspected hyperthyroidism 1
• Additional cardiac workup: ECG, echocardiogram when clinically indicated 1

Risk Stratification

• Age >60 years: Higher risk of cardiac complications, especially with hyperthyroidism 4
• TSH <0.1 mIU/L: Stronger evidence for increased risk of atrial fibrillation compared to TSH 0.1-0.4 mIU/L 4
• Pre-existing cardiovascular disease: Increases risk of adverse outcomes 6

Treatment Recommendations

Hypothyroidism Management

1. Levothyroxine replacement therapy:

   • Starting dose: Based on age and cardiac status 1
     • Age <70 without cardiac disease: 1.6 mcg/kg/day
     • Elderly or with cardiac disease: 25-50 mcg/day
   • Titration: Gradual dose increases with monitoring every 4-6 weeks 1
   • Target: TSH within normal reference range 1

2. Special considerations:

   • Acute symptomatic bradycardia: Consider atropine 0.5-1 mg IV (may repeat to max 3 mg) while initiating thyroid replacement 1
   • Myxedema coma: Requires IV levothyroxine and hospital admission 1, 6
   • Adrenal insufficiency: Must be treated with glucocorticoids before starting levothyroxine to prevent acute adrenal crisis 6

Hyperthyroidism Management

1. Beta-blockers: First-line for cardiac symptom control 1

   • Reduces heart rate and improves diastolic filling 4
   • Decreases atrial premature beats and LV mass index 4

2. Treatment of hyperthyroidism:

   • Options include antithyroid drugs, radioactive iodine, or surgery 7
   • Successful treatment decreases heart rate and cardiac output 4
   • May facilitate reversion of atrial fibrillation to normal sinus rhythm 4

3. Atrial fibrillation management:

   • Rate control: Beta-blockers preferred; nondihydropyridine calcium channel antagonists when beta-blockers contraindicated 4
   • Anticoagulation: Based on CHA2DS2-VASc score, not solely on thyroid status 4
   • Cardioversion: Consider after achieving euthyroid state 4

Monitoring and Follow-up

• TSH and Free T4: Recheck 6-8 weeks after treatment initiation or dose adjustment 1
• Cardiovascular parameters: Monitor heart rate, blood pressure, and cardiac symptoms 1
• Long-term monitoring: Every 6-12 months once stabilized 1
• Diabetes monitoring: Thyroid hormone therapy may worsen glycemic control 6
• Bone health: Avoid over-replacement to prevent decreased bone mineral density 6

Special Considerations

• Subclinical thyroid dysfunction: Even minimal changes in thyroid hormone levels affect cardiovascular function 3
• Amiodarone-induced thyroid dysfunction: May require discontinuation of amiodarone 4
• Acute noncardiac illness: May be associated with transient AF; treat underlying condition 4
• WPW syndrome with AF: Prompt direct-current cardioversion recommended if hemodynamically compromised 4

Pitfalls and Caveats

• Overtreatment with levothyroxine: Can precipitate angina or arrhythmias, particularly in elderly patients or those with cardiovascular disease 6
• Undertreatment of hypothyroidism: May not resolve cardiovascular manifestations 1
• Missing central hypothyroidism: Can occur if only TSH is checked without Free T4 1
• Drug interactions: Levothyroxine absorption affected by many medications; adjust administration as needed 6
• Concurrent adrenal insufficiency: Must be treated before thyroid replacement to avoid adrenal crisis 6

By addressing the underlying thyroid dysfunction with appropriate treatment, most cardiovascular manifestations can be reversed, significantly improving patient outcomes and quality of life.