BACKGROUND Anaplastic thyroid carcinomas are aggressive malignancies associated with poor clinical outcomes and challenges in diagnosis. While local/regional disease in the neck is the most usual site of biopsy, in some cases, distant metastases may be the site of initial investigation. CASE REPORT A 66-year-old woman with a clinical concern for diffuse metastatic malignancy of unknown primary presented to the Emergency Department (ED) with jaundice and shortness of breath. Recent laboratory test results revealed an elevated CA 19-9. Urinalysis revealed hematuria, proteinuria, and hyperbilirubinemia. She had a computed tomography (CT) scan of the chest, abdomen, and pelvis revealing diffuse involvement of the liver, lungs, adrenals, kidneys, thyroid, pancreas, gallbladder, and brain, but had not yet had a biopsy for definitive diagnosis. An ultrasound-guided liver biopsy was evaluated for cytological features, histological features, and pattern of immunostaining. The cytomorphological histological features were concerning for a high-grade malignancy. Immunohistochemical evaluation revealed that the lesion was positive for CK-AE1/AE3, BRAF, CK7, GATA3, SATB2, PAX8, and TTF-1, but the lesion was not reactive to the following stains: napsin, CK20, CDX2, PCEA, calcitonin, ER and thyroglobulin. The patient was diagnosed with a sarcomatoid anaplastic thyroid carcinoma and died within a few days after diagnosis. CONCLUSIONS This case illustrates that unanticipated specific diagnoses of widely metastatic anaplastic thyroid carcinoma are feasible when integration of patient history, clinical setting, imaging findings, clinical laboratory results, cytomorphology, histomorphology, and results of ancillary immunohistochemical testing are thoughtfully pursued and synthesized.