Endocrine Abstracts

Thyroid hormones are evolutionarily conserved iodine-containing regulators of metabolism and development (reviewed by Mullur et al. (2014)). A key process in synthesis of the effectors thyroxine and tri-iodothyronine is the uptake of iodide from the bloodstream into thyroid follicular cells by the sodium-iodide symporter channel (NIS) encoded by the solute-carrier gene SLC5A5. Perturbation of NIS activity by environmental chemicals can lead to rapid and potent changes in thyroid hormone signalling with concomitant harmful effects. Accordingly, an assay to assess the impact on iodide symport by small molecules is a valuable tool for identifying potential endocrine disruptors. The FRTL-5 cell line has been used to study NIS function for more than 40 years. They were isolated by growing thyroid explants in low serum media with a defined additive cocktail to maintain thyroid characteristics along with proliferative capability. They depend critically on presence of thyrotropin, without which proliferation and other thyroid functions are lost (Ambesi-Impiombato et al. (1980)). In 2010, a non-radioactive method to quantify iodide uptake by FRTL-5 cells in 96-well plate format was published (Waltz et al. (2010)). Here, modifications to the published method are presented that improve sensitivity (1.5pmol compared to 10pmol), with an optimized incubation period (7.5 minute uptake instead of 60 minutes) and improved linearity with regard to iodide concentration and uptake time. We observe similar IC₅₀ values for the competitive inhibitors sodium perchlorate, sodium tetrafluoroborate and sodium thiocyanate. Finally, we present a modification to the cell culture medium that reduces costs by more than 20-fold through complete elimination of thyrotropin, whilst retaining proliferation and stable thyroid characteristics such as NIS function unchanged for more than 70 passages.