GDF15 and differentiated thyroid carcinoma: GDF-15: growth differentiation factor; NG: nodular goiter; FTH: follicular thyroid hyperplasia; FTA: follicular thyroid adenoma; FTC: follicular thyroid carcinoma; FVPTC: follicular variant of papillary thyroid carcinoma; CPTC: classic papillary thyroid carcinoma; *: significant; NS: non-significant