Diagnostic value of circulating cell-free mtDNA in patients with suspected thyroid cancer: ND4/ND1 ratio as a new potential plasma marker

•We analyzed mtDNA ND1, ND4, and nDNA GAPDH levels in plasma and blood cells from patients with thyroid cancer and controls.•Both the thyroid nodule and thyroid cancer patients had significantly increased ND1 levels.•The ND4/ND1 ratio in the thyroid cancer group was higher than the thyroid nodule group (P < 0.05), and significantly higher than the normal control group (P < 0.01).•Plasma levels of nuclear DNA (GAPDH) in the thyroid cancer group were also higher compared to normal (P < 0.05).•Our results suggest the ND4/ND1 ratio has the potential to be a new detection indicator in thyroid cancer.