Hypothyroidism and PR Prolongation

We report a case of hypothyroidism associated with PR interval prolongation; this is a rare but known association.

Case report: An 80 year old man presented to the emergency department complaining of fatigue and dizziness. Due to chronic mental illness, he had not filled his prescription of 175 micrograms of levothyroxine for over 3 months. He had no other prescriptions, and was not taking beta blockers or other cardiac medications. Review of systems was limited by confusion. His speech was slow, and there was a prolonged relaxation phase of his deep tendon reflexes. Vital signs were normal with the exception of heart rate of 55 bpm. All electrolytes and magnesium were normal, and serial enzymes were negative for myocardial infarction. There was no recent outdoor activity and no history of tick bite. TSH was 58 μIU, with free T4 of 0.1 μg/dl.

An EKG showed sinus bradycardia with an old left bundle branch block and PR prolongation of 480 ms; it was misread by the computer as atrial fibrillation. A prior EKG when the patient was euthyroid had prolongation of only 300 ms. He was treated with an initial “loading dose” of 400mcg of levothyroxine and 100mg of hydrocortisone, and admitted for observation. He rapidly improved with resumption of his levothyroxine at the original dose. One week later, his TSH had dropped to 42 μIU, his T4 was 0.6 μg/dl, and his PR interval had returned to his baseline of 300 ms.

Discussion: There are few reported cases of heart block associated with hypothyroidism, and most are 2nd or 3rd degree; PR prolongation is rarely reported. Our patient had a prolonged PR interval at baseline, but there was a clear correlation between his TSH level and his PR interval. Typical causes of first degree block include intrinsic AV nodal disease, myocardial infarction, hypervagotonia, electrolyte disturbances, hypermagnesemia, lyme disease, aortic ring disease, and drugs; these drugs include calcium channel blockers, beta blockers, and digitalis glycosides. Hypothyroidism is also a potential cause of first degree heart block and should be considered in the differential diagnosis.

This abstract was submitted as part of an Associate's competition amongst all residents in the state of Massachusetts, and was selected to be presented as a poster at the 2003 Annual Meeting of the ACP. The abstract was required to conform to certain size and space restrictions. 

Thanks to Stewart Babbott MD, for his assistance in editing this abstract.

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