We evaluated 57 patients (3 males and 54 females, between 15 and 77 years of age, 49 papillary, 4 follicular, and 4 medullary carcinomas) who underwent total thyroidectomy. Blood was drawn at least 6 months after surgery or the administration of radioactive iodine. Of the 53 DTC patients, 21 had evident metastases (16 lung, 1 bone, 2 mediastinum, and 2 lung and bone) detected by either computed tomography or 131 I scintigram. All the patients with metastasis and 21 of 32 patients without metastasis received 5 to 150 (mean: 55.7 mCi) and 5 to 100 (mean: 17.0 mCi) 131 I, respectively. All the patients after total thyroidectomy, except those with medullary carcinoma, received a suppressive dose of thyroxine so that their serum thyroid stimulating hormone (TSH) levels were undetectable. 17 healthy subjects (5 males and 12 females, between 29 and 59 years of age) with no evidence of thyroid disease were also examined as controls. The protocol was approved by the institutional review boards at the participating institutions, and informed consent was obtained.