<![CDATA[Introduction: Mixed medullary and follicular-cell derived carcinoma (MMFCC) is an exceptionally rare thyroid malignancy, accounting for less than 0.1% of all thyroid malignancies, defined by the coexistence of morphologically distinct follicular and C-cell-derived populations. Diagnosis is difficult when bone metastasis is the initial presentation, a pattern more typical of follicular thyroid carcinoma that appears only late in medullary disease. This report describes the clinical, radiological, and pathological features of MMFCC. Case Presentation: A 64-year-old male presented with a three-month history of right hip pain caused by a large destructive lytic lesion of the right iliac bone; needle biopsy suggested metastatic thyroid carcinoma. Neck examination was unremarkable and thyroid function tests were normal. Ultrasound identified a 3 cm Thyroid Imaging Reporting and Data System 5 (TIRADS 5) nodule, and fine-needle aspiration (FNA) suggested a follicular neoplasm. After total thyroidectomy, histopathology showed a "mixed intimate" growth pattern of medullary thyroid carcinoma (MTC) with stromal amyloid and linear cracking artifacts, intermingled with papillary thyroid carcinoma (PTC) showing distinct nuclear features. Because immunohistochemical (IHC) markers were unavailable and high-grade features were absent, the diagnosis rested on strong clinicopathologic correlation and characteristic microscopic findings. Conclusions: Bone metastasis as the initial manifestation of MMFCC is rare and mimics more common follicular-cell malignancies. Although WHO criteria ideally require IHC proof of dual lineage, the diagnosis here was reached through careful morphological identification of coexisting MTC and PTC populations together with clinicopathologic correlation. This allowed a dual management strategy despite limited diagnostic facilities. ]]>
