One Solid Differential: Thyroid Cancer as a Hypereosinophilic Syndrome Trigger

Hypereosinophilic syndrome (HES) is characterized by blood or tissue hypereosinophilia leading to organ damage or dysfunction. HES is most commonly secondary or idiopathic, but may be primary (myeloid) or familial. Here, we review a complex case of HES and discuss important diagnostic and therapeutic considerations.A 19-year-old female with no significant past medical history presented to our hospital with respiratory failure, fevers, and generalized malaise. Three months prior to admission, she developed daily fevers, drenching night sweats, and 15 lbs of weight loss. One month prior to presentation, she was admitted with fevers and pancytopenia. Extensive hematologic, rheumatologic, and infectious workup revealed no infectious or rheumatologic trigger. She was found to have severe splenomegaly and a large thyroid nodule on CT imaging. Bone marrow biopsy revealed 42% eosinophils with relative myeloid hyperplasia but was otherwise normal. Fevers and pancytopenia resolved during admission without treatment, and she was discharged with close monitoring. An outpatient PET-CT completed two weeks later revealed only increased splenic and thyroid nodule uptake.Prior to her current admission, she began to develop rapidly progressive eosinophilia and a resurgence of fevers. She presented to a local hospital, where she rapidly decompensated, was intubated, and transferred to our intensive care unit (ICU). Extensive evaluation revealed concern for HES with multi-organ dysfunction, including neurologic, cardiac, and respiratory involvement. Eosinophil count prior to admission was 5,230/mm3, rising to 34,874/mm3 despite pulse-dose steroids. Benralizumab was started, but she had a slow response with ongoing organ dysfunction. Natural killer (NK) cell activity was undetectable. She was transitioned to mepolizumab with a rapid clinical response and was extubated shortly afterwards.Following discharge, she underwent a thyroid biopsy, which was concerning for malignancy. Surgical thyroidectomy confirmed papillary and follicular thyroid cancer. Given suspicion of a possible malignant trigger of HES, her mepolizumab was discontinued two months after resection with close monitoring. Three months after discontinuation, her eosinophil count remained undetectable.This case highlights solid organ malignancies as an unusual but recognized trigger for HES. Additionally, benralizumab activity depends on NK cell function; given markedly reduced NK cell function in some critically ill patients, mepolizumab may be a preferred agent for severe HES cases.