The recently cloned sodium-iodide symporter (NIS) represents a key molecule for thyroid function by efficiently accumulating iodide from the circulation into the thyrocyte against an electrochemical gradient. This uptake requires energy, is coupled to the action of Na+/K+-ATPase, and stimulated by TSH, the main hormone regulating thyroid-specific functions. NIS mutations are found in congenital hypothyroidism, and potential defects in the NIS gene, its expression, or function of the NIS protein are currently under investigation in various thyroid diseases. Increased NIS expression has been found in autonomous adenoma and Graves' disease, decreased levels of NIS protein and/or mRNA were observed in Hashimoto's disease, cold nodules, most thyroid cancers and cell lines derived therefrom. Autoantibodies directed against NIS have been identified in autoimmune thyroid disease and blocking antibodies isolated from sera of patients with Hashimoto's disease inhibit NIS function in NIS-transfected CHO cells. NIS mRNA expression can be up-regulated by retinoic acid in human thyroid carcinoma cell lines whereas retinoic acid treatment decreases NIS expression and function in differentiated rat thyroid FRTL-5 cells. Apart from thyrocytes, NIS is also expressed in other tissues known to transiently accumulate radioiodide, albeit at much lower levels, requiring RT-PCR for detection of the transcript. Diagnostic and therapeutic implications of the recent cloning of the human NIS gene such as development of NIS-directed drugs, ligands, antibodies, vaccines, gene therapeutic approaches combining NIS targeting and expression together with the long-established, efficient and safe method of radioiodide therapy are discussed both for application to thyroid related diseases and carcinoma, and non-thyroid benign and malignant diseases. Apart from these therapeutic and diagnostic perspectives the availability of the NIS gene will also open new opportunities to develop sensitive and homologous diagnostic test systems to identify factors involved in autoimmune thyroid disease, evolution of goitre, adenoma and thyroid cancer as well as NIS-directed new drugs. Advanced and sophisticated molecular diagnostic approaches (RT-PCR from fine needle aspirations, screening for mutations, analysis of gene defects) are already developed for NIS and will complement or overcome some established procedures in thyroid diagnostics.