APCU25MPC4 Bradycardia in hyperdrive: a rare case of hyperthyroidism-induced complete heart block

Introduction Complete heart block (CHB) in young patients is uncommon. While congenital or autoimmune causes are often considered, metabolic causes such as thyroid dysfunction are rarely implicated. Hyperthyroidism typically presents with tachyarrhythmias; its association with bradyarrhythmia or CHB is extremely rare.Case presentation A 23-year-old male with Down syndrome and autoimmune polyglandular syndrome (APGS) presented with a generalized tonic-clonic seizure lasting for 10 minutes. There were no signs of infection, trauma, or medication non-compliance. He drowsy, with a pulse rate of 35 bpm. Blood pressure was stable and systemic examination was unremarkable. ECG showed CHB. CT brain was normal. He was managed as symptomatic CHB and scheduled for permanent pacemaker implantation with conduction system pacing (CSP). Thyroid function test (TFT) on admission showed significant hyperthyroidism (TSH < 0.008 mIU/L, T4: 60 pmol/L). Carbimazole 20mg once daily was initiated and treated for 3 weeks while waiting for the pacemaker approval. On the day of the procedure, repeat TFT showed improvement (T4: 22 pmol/L), and repeat ECG revealed restoration of sinus rhythm. An electrophysiology study (EPS) was performed, showing normal AH interval with AH jump—findings consistent with a suprahisian block, which is likely attributed by metabolic in origin. Given the clinical improvement and EPS results, the pacing procedure was withheld and Carbimazole was continued. The patient was discharged with repeat Holter at home. During follow up at our clinic after 2 weeks, he was well and remain asymptomatic, with Holter result shown intermittent first-degree AV block.Discussion Though hyperthyroidism is typically associated with tachyarrhythmias, rare cases may involve conduction disturbances including CHB. The resolution of AV block following improvement in thyroid function and EPS findings supports a reversible metabolic cause rather than intrinsic conduction system disease. This case emphasizes the importance of thorough evaluation before device implantation, especially in young patients with systemic or autoimmune conditions.Conclusion Hyperthyroidism can rarely present with complete heart block. In this case, prompt recognition and correction of the thyroid imbalance reversed the conduction abnormality, avoiding unnecessary pacemaker implantation. This highlights the value of identifying reversible causes before committing to permanent interventions.