RECURRENT SYNCOPE AS AN ISOLATED PRESENTATION OF SEVERE PRIMARY HYPOTHYROIDISM

Case Presentation: A 30-year-old male with no prior medical history presented to the emergency department after two witnessed episodes of transient loss of consciousness. The first episode occurred in the morning when he was found on the floor with facial bruising and mild confusion but denied prodromal symptoms such as dizziness, palpitations, or chest pain. There was no bowel or bladder incontinence, tongue biting, or focal neurological deficit. Later that evening, he experienced another similar episode while sitting in a car, noted by family as transient unresponsiveness with brief abnormal eye and arm movements lasting one to two minutes. Post-episode confusion raised concern for seizure activity.Initial laboratory evaluation, including complete blood count, metabolic panel, D-dimer, and troponin, was unremarkable. EKG showed sinus rhythm at 61 bpm without ischemic or conduction abnormalities. CT head revealed no acute findings, and both MRI brain and EEG were normal. Echocardiogram demonstrated normal systolic and diastolic function with no valvular abnormalities. Extensive cardiac and neurologic workup ruled out structural or epileptic causes, narrowing the differential to metabolic or endocrine etiologies. Continuous telemetry revealed intermittent sinus bradycardia with a nadir heart rate of 47 bpm. Thyroid studies showed a markedly elevated TSH of 326 mIU/L and low free T4 and T3 levels, consistent with severe primary hypothyroidism. The patient was started on oral levothyroxine, resulting in complete resolution of symptoms and normalization of heart rate on follow-up.

Discussion: Syncope is an uncommon and often underrecognized manifestation of severe hypothyroidism. Proposed mechanisms include cardiac conduction abnormalities such as severe sinus bradycardia, intermittent atrioventricular block, and, rarely, ventricular tachyarrhythmias, each leading to transient cerebral hypoperfusion. The absence of classic hypothyroid symptoms often delays recognition and prompts extensive neurologic and cardiac evaluations. Reported cases describe similar presentations of hypothyroidism with bradyarrhythmia-induced syncope or seizure-like episodes, all resolving with thyroid hormone replacement. This highlights the importance of maintaining a broad differential diagnosis that includes endocrine causes when evaluating unexplained syncope, particularly in patients with otherwise unremarkable neurologic and structural cardiac findings.

Conclusions: Severe hypothyroidism should be considered in the differential diagnosis of unexplained syncope, particularly when bradycardia is present and neurologic findings are unremarkable. Early diagnosis and thyroid hormone replacement can prevent recurrence and avoid unnecessary testing.Clinical Pearls:Severe hypothyroidism can present solely with syncope, even without classic symptoms.Thyroid testing is a simple, low-cost step in evaluating recurrent, unexplained syncope.Prompt hormone replacement results in rapid symptom resolution and prevents unnecessary neurologic workup.