Anaplastic Thyroid Carcinoma With Horner Syndrome and Unusual Metastasis Reveals Masked Papillary Thyroid Carcinoma Following Neoadjuvant Therapy.

Background/Objective: Anaplastic thyroid carcinoma (ATC) is a rare thyroid cancer that has the propensity to be hyperactive and is characterized by the dedifferentiation of follicular cells leading to pleomorphism. This report highlights a 68-year-old man's progression of ATC with an unusual presentation and secondary symptoms revealed to be masking an independent tumor cell origin residual papillary thyroid carcinoma following targeted treatment. Case Report: A 68-year-old man presenting with a history of aortic aneurysm and hyperlipidemia was referred for type 2 diabetes mellitus complications. He had a successful aneurysm repair; however, follow-up imaging revealed a thyroid mass, cystic lesions in the pancreas and liver, and extensive lymphadenopathy, and he endorsed symptoms of Horner syndrome. Fine-needle aspiration suggested ATC, and sequencing confirmed a Discussion: Normally, comorbid thyroid malignancies arise from identical follicular sequence variants. It is unusual to observe 2 independent origins from comorbid thyroid cancers. In addition, the development of Horner syndrome from thyroid cancer, especially anaplastic, is incredibly rare. Conclusion: This case is a unique depiction of how ATC can present despite uncharacteristic immunohistochemical markers, secondary symptoms, and hidden comorbidities. As an understudied malignancy, it is important to recognize this condition's variability to prevent increased mortality and morbidity.