Sequential changes in serum thyroid peroxidase following radioiodine therapy of patients with differentiated thyroid carcinoma.

To determine the detectability and the time course of serum thyroid peroxidase (TPO) levels before and 1, 2, 4, and 6 months after 131I administration, we evaluated TPO in 13 selected patients with differentiated thyroid carcinoma (DTC) whose sera did not contain antimicrosomal or antithyroglobulin (Tg) antibodies. All patients received 131I therapy 6 or 8 weeks after thyroid surgery for ablation of the postsurgical thyroid remnant. Serum samples were also collected from 10 normal subjects. Measurement of TPO was carried out by using a new commercially available immunoluminometric assay with a sensitivity of 30 pg/mL. Serum Tg was measured by RIA with a sensitivity of 2.6 micrograms/L before and 6 months after 131I administration. In all patients, a standard total body scan was obtained before and 6 months after 131I administration. TPO was undetectable in all sera from normal subjects. However, serum TPO became detectable in all patients with DTC during the study, whereas rescans were either negative or positive and appeared not to be related to the radioiodine dose given, histology of the thyroid tumor, residual thyroid volume, TSH levels, or age of patients. However, a significant negative correlation was present between TPO levels before 131I administration and the time from surgery (r = -0.82, P < 0.001). Six of 13 patients had increased TPO levels 1 month after 131I administration. Serum TPO levels tended to decrease during follow-up in most patients (7 of 10) with a negative rescan. In 3 patients with positive rescans, TPO levels tended to increase during follow-up. Patients with negative rescans had values of serum TPO overlapping the range of values seen in patients with positive rescans, thus demonstrating the inability of TPO assay as a useful marker for following patients with DTC. We found no correlation between Tg and TPO levels measured before and 6 months after 131I administration, thereby excluding TPO levels as a marker for thyroid cancer. Our results suggest that TPO, unlike Tg, does not appear to be a useful marker for following patients with DTC.