BACKGROUND: The sodium iodide symporter(NIS) is a plasma membrane protein which is respoasibIe for iodide transport into thyroid cell. The cDNA sequence of NIS has recently been cloned from rat and human. Intrinsic ability and its differences in iodide accumulation have been exploited as a useful tool for diagnosis and therapy of thyroid diseases. It is also known that some differentiated thyroid cancers do not take up radioactive iodine at therapeutic dose. METHODS: To understand the expression and regulation of NIS in thyroid tumars, we measured the expressons of human NIS(hNIS), TSH-receptor(R), and thyroglohulin(Tg) mRNAs from papillary thyroid carcinoma(PTC) tissues by reverse transcriptase-polymerase chain reaction (RT-PCR) and RNase protection assay(RPA). RESULT: By RT-PCR analysis, 87% of PTC expressed hNIS mRNA, but the degree of expression were variable. Interestingly, 32% of PTC showed significant level of hNIS expression even though pre-operative technetium thyroid scan of all thyroid tumors were cold but the level was lower than normal control tissues. All of PTC showed the expressions of Tg and TSH-R mRNAs and there was a correlation between hNIS mRNA and TSH-R mRNA(Rsq 0.35, p=0.01). By RPA, the expression of hNIS and TSH-R in normal control tissue were detected with 20microgram and 40microgram of total RNA respectively, but the higher concentrations(> or =60microgram for hNIS and > or =40microgram for TSH-R) were required to detect in PTC, showing that tbe expression of hNIS in FTC was lower than TSH-R expression. CONCLUSION: PTC tends to lose hNIS mRNA expression earlier than TSH-R mRNA and the measurement of hNIS mRNA in PTC may be useful as an indicator of the therapeutic response to radioactive iodine.