High grade second degree atrioventricular block in 23-year-old female with subclinical hypothyroidism

Abstract

23-year-old female presented with classic history of vasovagal syncope while helping attend her mother's scalp wound. Physical examination only revealed heart rate was 40bpm. ECG confirmed high grade second degree AV block and intermittent third degree AV block. She has no prior history of syncope or dizziness and no significant family history. She was asymptomatic of her heart block. Her past medical history included Hashimoto's thyroiditis (aged 14y; TSH41mU/L (normal 0.3-5), low free T4 (<5pmol/L; normal 9-19)and free T3 (<1.5pmpl/L; normal 2.6-6). Thyroid peroxidase antibodies>1000iu/mL (normal <12); thyroglobulin antibodies 176iu/mL (normal<34). She was treated with T3 and levothyroxine replacement. Blood tests revealed elevated TSH (5.7mU/L), normal fT4 (9.7pmolL) and fT3 (4.5pmol/L) levels, positive speckled ANA (titre 640) with negative ENA (indicating negative SSA/Ro, SSB/La). Chest radiography revealed no abnormalities. Echocardiography showed normal left ventricular size and function without any valvular dysfunction. Holter ECG showed 2:1 AV block with range from heart rate 38bpm to 84bpm. Exercise treadmill test (10.30mins, 12.6METS) confirmed augmentation of intrinsic atrial and junctional escape rhythm. Cardiac MRI did not reveal evidence of fibrosis or infiltrative disease. Hypothyroidism is an uncommon cause of complete AV block. Given her lack of symptoms attributable to bradycardia or chronotropic incompetence, preserved LV function, lack of mitral valve dysfunction and age - pacemaker implantation was deferred. She will be monitored closely with serial echocardiogram and functional testing. The patient is aware permanent pacemaker will be required in the future.<br />