Coexisting Thyroiditis and Carditis in a Patient With Lyme Disease: Looking for a Unifying Diagnosis

Background/objective: Lyme disease, the most common vector-borne infection in the United States, causes multisystem inflammation. We describe a patient who presented with symptoms of Lyme disease, carditis, and thyroiditis.

Case report: A 53-year-old woman developed fatigue and dyspnea on exertion 1 month after returning from a trip to Delaware. Her electrocardiogram (ECG) showed first-degree atrioventricular (AV) block with a P-R interval up to 392 milliseconds, in the setting of elevated free thyroxine and undetectable thyroid-stimulating hormone levels. Lyme serology was positive. She was hospitalized and started on ceftriaxone. During the second day of hospitalization, AV block worsened to second-degree Mobitz type II but converted back to first-degree AV block after a few hours. Her 24-hour I-123 thyroid uptake and scan revealed markedly diminished I-123 uptake of 1.2%. On day 4, the P-R interval improved, and she was discharged on doxycycline for 3 weeks. P-R interval on ECG and repeated thyroid function tests were normal after finishing antibiotic treatment.

Discussion: In our patient, known exposure to the vector, a classic rash on the chest, improvement in the symptoms, and normalization of thyroid function tests after antibiotic therapy support Lyme infection as a cause of carditis and painless, autoimmune thyroiditis.

Conclusion: Our case highlights the importance of considering Lyme disease as a cause of painless, autoimmune thyroiditis, especially in patients with concurrent cardiovascular involvement.