Radioiodide uptake and turnover in a pseudo-medullary thyroid carcinoma

A mass in a woman's neck, with no sequestration by Tc-99m imaging, was accompanied by an elevated serum calcitonin on one occasion. Light microscopy of a biopsy was consistent with medullary thyroid carcinoma. Electron microscopy showed secretory granules similar to those found in normal C-cells and in medullary carcinoma of the thyroid. The neck mass (and pulmonary and hepatic metastases likely from the principal mass) concentrated radioiodide (I-131). Subsequent immunoperoxidase examination of the tissues showed them to contain thyroglobulin but not calcitonin. The tumor was thus likely of follicular-cell origin. Reports of radioiodide uptake in medullary thyroid carcinoma may be correct, but each case will have to be re-examined with attention to tissue markers such as thyroglobulin and calcitonin. These markers might more correctly classify the origin of the tumor. The possibilities of cell interconversion and of dual origin are also discussed. Whole-body turnover of radioiodide was quantified as well as that in the pulmonary lesions. Rapid removal of radioactivity was present, suggesting that agents reducing iodide turnover might have therapeutic value in these cases. At five days after radioiodide administration, a biopsy specimen showed that the tumor-to-blood ratio (per gram) was greater than 1.