Sequential Serum Thyroglobulin Determinations,131I Scans, and131I Uptakes After Triiodothyronine Withdrawal in Patients with Thyroid Cancer*

Abstract

To develop guidelines for the timing and interpretation of thyroglobulin (TG) measurements in patients with well differentiated thyroid cancer, we performed ¹³¹I whole body scans and uptakes with simultaneous serum TG and TSH determinations in 49 patients with this disease. In 15 patients, TG measurements were obtained initially while the patient was receiving T₃ therapy and then were repeated serially within a period of up to 35 days after T₃ withdrawal. In 9 patients with functioning thyroid tissue, the changes in TG paralleled the changes in TSH levels. Before stopping T₃, TG measurements were a poor indicator of function, since 14 of 15 patients had suppressed TG levels (3, TG increased in all 5 patients with scan evidence of metastases, in 2 of 4 subjects with residual thyroid bed uptake, and in 2 of 6 patients with no uptake. In 23 patients, both the scan and serum TG measurements were performed 2 and 4 weeks after stopping T₃. At 4 weeks, compared to 2 weeks, TG levels were nearly the same in 17 (a change of 3; 3) serum TG can be measured 2 weeks post T₃ withdrawal, as little difference in TG levels is found between measurements made at 2 and 4 weeks; and 4) the combination of serum TG and ^I3II whole body scanning is superior to either one alone for detecting residual functioning thyroid tissue.