Serum thyroglobulin measurement by immunoassay is used to detect residual or recurrent thyroid cancer after thyroid ablation. However, the usefulness of immunoassay is limited by both the requirement for thyroid hormone withdrawal to attain optimal test sensitivity and interference by antithyroglobulin antibodies. To circumvent these problems, we amplified thyroglobulin messenger ribonucleic acid (mRNA) in peripheral blood using RT-PCR and compared the accuracy of this test to serum thyroglobulin immunoassay in patients with thyroid cancer. Thyroglobulin mRNA was amplified from peripheral blood of 77 patients who had undergone thyroidectomy for well differentiated thyroid cancer, 68 of whom while taking thyroid hormone for TSH suppression. Patient staging was based on the most recent radioiodine scan after thyroid hormone withdrawal. Ten normal control subjects were also studied. Among patients taking T4, thyroglobulin mRNA was detected in 26 of 33 patients with either thyroid bed or metastatic iodine-avid tissue on most recent withdrawal scan (79%), whereas serum thyroglobulin was detected in 12 of these 33 patients (36%; P < 0.001). Thyroglobulin mRNA was detected in 7 of 35 patients (20%) with negative radioiodine scans, 12 of 19 patients (63%) with radioiodine uptake in the thyroid bed, and all 14 patients with metastases, including 2 patients with antithyroglobulin antibodies. Thyroglobulin mRNA was detected in all 10 normal subjects. Epithelioid cells that stained strongly with antithyroglobulin antibodies were identified in blood. Detection of circulating thyroglobulin mRNA is a more sensitive marker of residual thyroid tissue or cancer than immunoassay for serum thyroglobulin, particularly in patients treated with thyroid hormone or who have circulating antithyroglobulin antibodies.