Updated project metadata. Although most thyroid tumours are benign, thyroid cancer represents the most common malignancy of the endocrine system, comprising mainly follicular and papillary thyroid carcinomas (FTC and PTC, respectively). Previous studies have shed some light on the molecular pathogenesis of thyroid cancer but there have not been any comprehensive mass spectrometry-based proteomic studies to reveal protein expression differences between thyroid tumours and the molecular alterations associated with tumour malignancy. We applied a label-free quantitative mass spectrometry analysis to compare normal thyroid tissue with the three most common tumours of the thyroid gland: follicular adenoma, follicular carcinoma and papillary carcinoma.