Comprehensive guidance on the diagnosis and management of primary mesenchymal tumours of the thyroid gland.

Most primary thyroid tumours are of epithelial origin. Primary thyroid mesenchymal tumours are rare but are being increasingly detected. A vast majority of thyroid mesenchymal tumours occur between the fourth and seventh decades of life, presenting as progressively enlarging thyroid nodules that often yield non-diagnostic results or spindle cells on fine needle aspiration biopsy. Surgery is the preferred mode of treatment, with adjuvant chemoradiotherapy used for malignant thyroid mesenchymal tumours. Benign thyroid mesenchymal tumours have excellent prognosis, whereas the outcome of malignant thyroid mesenchymal tumours is variable. Each thyroid mesenchymal tumour is characterised by its unique histopathology and immunohistochemistry. Because of the rarity and aggressive nature of malignant thyroid mesenchymal tumours, a multidisciplinary team-based approach should ideally be used in the management of these tumours. Comprehensive guidelines on the management of thyroid mesenchymal tumours are currently lacking. In this Review, we provide a detailed description of thyroid mesenchymal tumours, their clinical characteristics and tumour behaviour, and provide recommendations for the optimal management of these tumours.