Measuring serum Tg and performing a diagnostic whole body scan (DxWBS) has become the standard for follow-up of patients with differentiated thyroid carcinoma. The primary aim of this study was to determine whether recombinant human TSH (rhTSH)-stimulated Tg alone is sufficiently sensitive to identify residual cancer in patients with no clinical evidence of disease and undetectable or very low serum Tg levels during thyroid hormone (TH) therapy. A secondary aim was to investigate the frequency

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... f tumor in such patients. One hundred and seven consecutive patients, aged 10.9 -85.3 yr (median, 36.3), at the time of initial surgery who had Tg levels on TH therapy that were undetectable (95% <0.5 ng/ml) or low (4% 0.6 ng/ml, 1% 1.0 ng/ml) and who underwent rhTSHstimulated testing 10 months to 35 yr (median, 3.5 yr) after initial thyroidectomy and 131 I ablation were retrospectively studied. Many (50%) were at high risk of tumor recurrence, and 5 had distant metastases during the course of their disease. In response to rhTSH, Tg ranged from 0.5 or less to 17.9 ng/ml, remaining at 0.5 ng/ml or less in 68 (64%) patients and increasing to levels between 0.6 and 2 ng/ml in 19 (18%) others and to levels higher than 2 ng/ml in 20 (19%) patients. Eleven patients (10%), all of whom had rhTSH-stimulated serum Tg levels above 2 ng/ml, were found to have persistent tumor in lung (4 patients), lymph nodes (5 patients, 3 with cervical central compartment, 1 bilateral cervical, and 1 with mediastinal nodes) identified by fine needle cytology, surgical pathology, posttherapy whole body scans, or computed tomography and, in two patients, with high serum Tg values alone (4.6 and 7.0 ng/ml after rhTSH and, respectively, 28.5 and 70.6 ng/ml after TH withdrawal), although in neither could the tumor site be identified. Thirteen patients (12%) were treated with surgery or 131 I, and in some cases both, as a result of the rhTSH studies; 10 had tumor, 1 had residual uptake in the thyroid bed visible on rhTSH-stimulated diagnostic whole body scan (DxWBS), and 2 had high serum Tg levels, presum- Results Patient characteristics One hundred and seven patients, 88 (82%) females and 19 (18%) males, aged 10.9 -85.3 yr (median, 36.3) at the time of thyroidectomy, were given rhTSH 10 months to 35 yr (me-Mazzaferri and Kloos • Diagnostic 131 I Whole Body Scanning with rhTSH