Sequential serum thyroglobulin determinations, 131I scans, and 131I uptakes after triiodothyronine withdrawal in patients with thyroid cancer

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 (<10 ng/ml). After 2 weeks off T3, 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 <5 ng/ml and <50%), decreased in 1, and increased in 5 subjects. In 44 patients whose measurements were free of interference from anti-TG, 106 scans and serum TG determinations were performed after stopping thyroid hormone replacement. Scan evidence of metastases was always (22 scans) associated with high serum TG levels (>10 ng/ml), but high serum TG levels did not distinguish metastases from residual thyroid tissue. The serum TG level was greater than 10 ng/ml in 5 of 28 instances (18%) of negative scans, while scans were positive in 14 of 32 instances (44%) where TG levels were less than 2 ng/ml