Polypoid rectal lesion composed of ectopic cutaneous, subcutaneous, and thyroid tissues: a case report

Heterotopic thyroid tissue (HTT) represents a rare congenital anomaly resulting from aberrant embryological migration of the thyroid primordium. Most lesions arise within the head‑and‑neck region along the thyroglossal duct. HTT occurring as an intraluminal polypoid rectal lesion is extremely sparsely reported in clinical literature. Moreover, concurrent occurrence of HTT together with ectopic skin and subcutaneous tissue within the gastrointestinal tract is highly uncommon and creates substantial diagnostic challenges for clinicians and pathologists. A 79‑year‑old female presented to our emergency department with 4 h of fresh bright‑red hematochezia. She had no prior history of thyroid disease or gastrointestinal malignancy. Colonoscopy identified a single short‑pedicled, pale‑coloured polypoid lesion with a smooth surface, situated 10 cm from the anal verge and measuring approximately 1.5 cm in diameter. Endoscopic mucosal resection (EMR) was carried out to achieve complete excision. Histopathological assessment showed that the lesion comprised cutaneous stratified squamous epithelium and subcutaneous adipose tissue, underneath which lay nests of benign‑looking thyroid follicles. Immunohistochemical staining revealed strong positive expression of thyroid transcription factor‑1 (TTF‑1), thyroglobulin (Tg), and PAX‑8, confirming the thyroid origin of the heterotopic tissue. Focal cytokeratin (CK) positivity and negative CD10 staining were also observed. There was no cellular atypia or malignant morphological feature. The patient recovered uneventfully from the procedure and stayed asymptomatic throughout follow‑up. This composite heterotopic lesion appears rare in terms of both anatomical location and mixed tissue composition. The pale, smooth-surfaced endoscopic appearance may serve as a subtle auxiliary diagnostic clue. Given its rarity and nonspecific clinical presentation, accurate diagnosis typically relies on comprehensive histopathological evaluation and immunohistochemical testing. Endoscopic resection appears to be a feasible treatment option for symptomatic benign lesions of this type. Individualized long-term surveillance may be considered to monitor for potential recurrence or pathological transformation.