[Investigating patients with differentiated thyroid carcinoma and elevated serum thyroglobulin but negative whole-body scan]

Pedro Weslley S Rosário, Flávia Coimbra P Maia, Alvaro Luís Barroso, Saulo Purisch
Arquivos Brasileiros de Endocrinologia e Metabologia 2005, 49 (2): 246-52
Findings of elevated thyroglobulin (Tg) and a negative whole-body scan (WBS) are not uncommon during the follow-up of differentiated thyroid carcinoma. In 12% of our patients submitted to thyroidectomy and radioiodine with Tg >10 ng/ml during hypothyroidism had a negative diagnostic WBS. This finding generally corresponds to a false-negative WBS. Inadequate preparation in terms of iodine exposure and insufficient elevation of TSH should be excluded. Micrometastases which do not accumulate sufficient iodine to be detected by low radioiodine activity and the loss of the capacity to express the sodium/iodine symporter explain many cases. In patients with elevated Tg, metastases can be identified after the administration of a therapeutic radioiodine dose, with this procedure being indicated in cases with Tg >10 ng/ml during hypothyroidism or >5 ng/ml after recombinant TSH, after exclusion of lung and cervical macrometastases. In the present study, 5 of 7 patients with these criteria showed ectopic uptake on post-therapy WBS. If the post-therapy scan is negative or reveals discrete uptake in the thyroid bed, other methods (e.g. FDG PET) can be performed, and the physician should not insist on radioiodine therapy. If WBS detect lymph node metastases, surgery is indicated, while in cases of diffuse lung metastases radioiodine is indicated until the occurrence of a negative WBS or normalization of stimulated Tg levels. Patients with a positive post-therapy scan may show a significant reduction in Tg, with even complete remission in some cases after radioiodine, but the impact of this treatment on mortality remains controversial.

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