Thyroid carcinomas no longer accessible to radio-iodide or TSH-suppressive T4 therapy, due to loss of thyroid-specific functions, might be sufficiently re-differentiated by retinoic acid (RA) to be treated by conventional methods again. To help evaluate the feasibility of RA re-differentiation therapy in thyroid carcinomas, we examined the functionality of RA receptors (RARs/RXRs), central RA signal mediators, in human thyroid-carcinoma cell lines as model systems. [ 3 H]-RA binding assays with nuclear extracts from follicular thyroidcarcinoma cell lines FTC-133 and -238 revealed high-affinity binding sites for RA. Electrophoretic mobility shift and supershift assays using a DR2 (''direct repeat'' 2) RA response element demonstrated DNA-binding of RAR␣, RAR␥, RXR␣ and RXR in nuclear extracts of FTC-133 and anaplastic HTh74 cells. Use of a DR5 RA response element revealed no difference in DNA binding. In supershift assays with a DR4 T3 response element, we found DNA-binding by TR␣1, TR␣2, and TR. Northern-blot analysis showed low expression of RXR mRNA in FTC-133 and of TR␣1 mRNA in FTC-133 and FTC-238 cells. Using RT-PCR, we detected mRNA for RAR␣, RAR, RAR␥, RXR␣, and RXR in the 4 cell lines and in human thyroid-carcinoma samples. RAR mRNA was reduced in FTC-238 cells and RXR mRNA was decreased in anaplastic C643 cells and 9 of 12 tumor samples. Differential RA regulation of RA-receptor-mRNA expression was observed in the various cell lines. Thus, RA and T3 nuclear receptors are present in thyroid-carcinoma cell lines or tissues, albeit with cell-line and tumor-dependent variations; in the cell lines, they were shown to be functional with respect to DNA and/or ligand binding. Int.