Paradoxical changes in iodine-131 scintigraphic findings in advanced follicular thyroid cancer

We present the findings on iodine-131 (131I) scintigraphy, thallium-201 (201Tl) scintigraphy and quantitative thyroglobulin (QTG) estimation in two patients with follicular carcinoma of the thyroid with extensive metastases. The lesions were initially seen on 131I scintigraphs, but were not subsequently visualized with scanning doses of 131I (5 mCi), while retaining their ability to produce increasing amounts of thyroglobulin and take up 201Tl. Implications in choosing the appropriate diagnostic tests in the management of differentiated thyroid cancer are discussed.