SUN-311 - Eikenella Corrodens-Associated Thyroid Cyst Infection Presenting as Suspected Recurrent Malignancy

Background
The global incidence of thyroid cancer is increasing, largely driven by incidental detection of papillary thyroid carcinoma via ultrasound-guided fine-needle aspiration (FNA). Infected thyroid cysts are uncommon, we report an unusual case of Eikenella corrodens thyroid cyst infection mimicking malignancy.

Case
A 90-year-old woman with a history of grade I papillary thyroid carcinoma, diagnosed 9 years ago and treated with right thyroid lobectomy, presented to the clinic for routine surveillance. She denied any thyroid-related or systemic symptoms and was afebrile. Examination was unremarkable. Thyroid imaging revealed a 1.4-cm cystic lesion in the left thyroid bed and an enlarged, abnormal left level III lymph node measuring 1.9 cm. Laboratory evaluation showed a thyroid-stimulating hormone level of 1.3 µIU/mL, a serum thyroglobulin level of 3.3 ng/mL, and a thyroglobulin antibody level of 52 IU/mL, raising concern for recurrent thyroid malignancy. FNA biopsy revealed an inflamed squamous-lined cyst in the left thyroid lesion and atypia of undetermined significance in the lymph node.
Both biopsies were negative for malignant cells. Culture of the cyst fluid grew Eikenella corrodens, which was sensitive to penicillin. Neck MRI showed no evidence of abnormal anatomical communication with the thyroid gland. The patient was treated with amoxicillin–clavulanate. Repeat thyroid imaging demonstrated complete resolution of the thyroid cyst however, the lymph node remained persistently lobulated without definitive sonographic features of malignancy.

Discussion
Thyroid infections are rare and typically occur in the setting of preexisting thyroid disease, anatomical anomalies, immunosuppression, or advanced age. Squamous-lined thyroid cysts are infrequent, arising from embryonic remnants or metaplastic changes. Infections of such cysts are uncommon, given the thyroid glands’ intrinsic resistance conferred by encapsulation, high iodide content, dual blood supply, and rich lymphatic drainage.
Thyroid cyst infections may occur via direct inoculation or anatomical communication between the oral cavity and neck structures. They are typically poly-microbial, although mono-microbial infections have been reported.
Eikenella corrodens is part of the normal oral flora. A paediatric case of acute suppurative thyroiditis caused by Eikenella corrodens via a fistula has been reported. Thyroid infections can cause fever and local symptoms, as in Capnocytophaga canimorsus–associated thyroid cyst infection after dog bite; our patient was asymptomatic with no inoculation history.
This represents the first reported isolated thyroid cyst infection caused by Eikenella corrodens in the absence of anatomical abnormalities.
Isolated thyroid infections are rare, may mimic thyroid malignancy, and require aspiration with culture for diagnosis and targeted therapy.

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