Sequential Serum Thyroglobulin Determinations, ¹³¹I Scans, and ¹³¹I 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 (<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. · We conclude that 1) low TG measurements while on thyroid replacement therapy do not exclude the presence of metastatic well differentiated thyroid cancer; 2) TG measurements during thyroid replacement therapy are much less sensitive to the presence of residual thyroid tissue than measurements made 2–4 weeks after withdrawing T₃; 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.