Redifferentiation therapy with retinoic acid in follicular thyroid cancer

We report on a patient with a follicular Hurthle cell carcinoma in whom distant metastases were initially radioiodine negative or only weakly positive. Redifferentiation therapy with 13-cis retinoic acid induced a significant radioiodine uptake in metastatic tissue. Thyroglobulin (Tg) immunostaining and autoradiography of a bone metastasis in the right femur, which was initially radioiodine negative, proved Tg synthesis, combined with iodine incorporation into tumor cells. Glucose metabolism in metastases was partially increased and partially unchanged after redifferentiation therapy. The distinct increase of serum Tg after retinoic acid treatment was interpreted as a functional sign of redifferentiation.