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Context: Radioiodine uptake is rarely observed in normal non-lactating breast tissue. Investigation of the in vivoregulation of iodide uptake in breast tissue may be useful for the induction of radioiodine uptake in breast cancer tissue for diagnostic and therapeutic purposes. Case reports: We report the cases of two post-menopausal women who underwent radioiodine therapy for papillary thyroid carcinoma and in whom breast uptake of radioiodine on post-therapy whole body scan (WBS) was observed. Methods and results: In both patients, elevated serum prolactin levels (123 ng/mL in patient 1 and 48 ng/mL in patient 2) were documented at the time when radioiodine uptake in the breast was observed. The hyperprolactinemia was due to prolonged treatment with the anti-dopaminergic neuroleptic risperidone in Case 1, and chronic renal failure in Case 2. When prolactin levels were normalized (by withdrawal of risperidone in Case 1 and with cabergoline in Case 2), breast tissue uptake was no longer evident on WBS. Conclusion: These cases provide the first documented correlation between serum levels of endogenous prolactin and radioiodine uptake by involuted breast tissue in humans. [ABSTRACT FROM AUTHOR]