SAT-343 - Atrial Fibrillation as A Complication of Severe Hypothyroidism: A Case Report

Background: While hyperthyroidism is a well-recognized risk factor for atrial fibrillation (AF), the association between severe hypothyroidism and AF remains underappreciated. Emerging evidence demonstrates a U-shaped relationship between thyroid-stimulating hormone (TSH) levels and AF incidence, with both subclinical hypothyroidism and hyperthyroidism conferring increased risk.

Case Presentation: We report a 77-year-old female with primary hypothyroidism and post-traumatic epilepsy on multiple antiepileptic medications who presented with progressive fatigue, depressive symptoms, and worsening pedal edema after discontinuing levothyroxine for one month. The patient required supraphysiologic levothyroxine dosing (averaging 200 mcg/day versus weight-based requirement of 130 mcg/day), suggesting malabsorption related to co-administered medications. Laboratory evaluation revealed severe hypothyroidism with TSH 52.2 mIU/L, undetectable free T4 (0.42 ng/dL), and free T3 1.5 pg/mL. Echocardiogram demonstrated significant pericardial effusion requiring pericardiocentesis and drain placement. New-onset AF was discovered on telemetry with heart rate 48-73 bpm. She was restarted on levothyroxine 175 mcg daily and remained inpatient until free T4 became detectable. Anticoagulation was initiated, and she spontaneously converted to normal sinus rhythm prior to discharge.

Discussion: Pericardial effusion occurs in 3-37% of hypothyroid patients, with one echocardiographic study documenting 30% prevalence in myxedema. However, AF development in severe hypothyroidism deserves greater attention. Meta-analyses demonstrate subclinical hypothyroidism increases AF risk by 19-23%. Mechanistic studies reveal TSH directly activates TSH receptors on cardiomyocytes, triggering a TSHR/cAMP/PKA/CREBH cascade that alters ion channel expression, prolongs action potential duration, increases automaticity, and promotes electrical remodeling. In a cohort of 2,311 patients with subclinical hypothyroidism, AF prevalence increased significantly with higher TSH levels (32.1% with TSH 4-10 mU/L versus 44.6% with TSH >10 mU/L). This patient's AF was incidentally discovered on post-pericardiocentesis telemetry monitoring. Spontaneous conversion to sinus rhythm following thyroid hormone replacement aligns with observations that restoration of euthyroidism often resolves AF.

Conclusion: Clinicians should recognize AF as a potential complication of severe hypothyroidism, particularly in elderly patients with multiple comorbidities. This case underscores the importance of medication adherence, optimal levothyroxine administration, regular thyroid function monitoring, and achieving euthyroidism for preventing arrhythmic complications.

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