Background <p>Poorly differentiated thyroid carcinoma (PDTC) is a rare, aggressive thyroid malignancy. While thyroid cancer has been reported in monozygotic twins, the occurrence of PDTC in this context is exceptionally uncommon. Such cases may offer unique insights into the interplay between genetic and environmental factors in thyroid cancer development.</p> Case presentation <p>We report the case of 67-year-old monozygotic twin sisters, both diagnosed with thyroid malignancies with discordant histology. Patient A presented with a 5.5&#xa0;cm PDTC with local invasion and distant metastases. She underwent total thyroidectomy and lymphadenectomy followed by radioactive iodine (RAI) therapy. Patient B had a history of recurrent papillary thyroid carcinoma (PTC) over several years, managed with multiple surgeries and RAI. Despite their identical genetic background, the twins developed distinct thyroid cancer subtypes, with varying ages of onset and clinical courses.</p> Discussion <p>This case highlights the heterogeneous presentation of thyroid malignancies in genetically identical individuals, suggesting a potential role for environmental or epigenetic factors in tumorigenesis. A review of the literature on thyroid cancer in monozygotic twins indicates a general pattern of similar pathology and age at diagnosis, contrasting with the discordant phenotypes seen in our patients. PDTC poses diagnostic and therapeutic challenges due to its aggressive nature and limited response to conventional therapies. Advances in molecular profiling and targeted treatments may help improve management and outcomes.</p> Conclusion <p>The occurrence of discordant thyroid cancer subtypes in monozygotic twins is exceedingly rare and emphasizes the need for further studies exploring genetic and epigenetic contributions to thyroid cancer. Understanding these factors is critical for developing personalized approaches to diagnosis, treatment, and prevention.</p>