['To determine whether it is reliable to do thyroglobulin measurements during thyroid hormone substitution (Tg ON) alone or whether it is also necessary to do 131I total body scanning (TBS) and Tg measurements after withdrawal of thyroid hormone substitution (Tg OFF) in the follow-up of patients with differentiated thyroid carcinoma.', 'Retrospective.', 'University Hospital Nijmegen.', '202 Patients (151 females and 51 males, mean age 50.6 years) with differentiated thyroid carcinoma were examined in the period 1970-90. All patients had undergone total thyroidectomy and if necessary 131I ablation. 27 Patients with Tg antibodies were excluded (13.4%). In 175 patients Tg OFF levels were compared with TBS and clinical and radiological data. In 81 of them Tg ON levels were also compared.', 'Specificity of Tg OFF and Tg ON measurement was 83 and 88%. Sensitivity of Tg OFF and Tg ON measurement was 100 and 92%. In detecting local residual thyroid tissue Tg OFF was superior to Tg ON. In detecting metastases Tg OFF and Tg ON were both superior to TBS.'] Is [ Compared to whole body scanning thyroglobulin assay reliable in the follow-up of thyroidectomized patients with thyroid carcinoma ]?