The Inhibitery Effect of of Cultured Rat Iodide on TSH-stimulated Thyroid Cells (FRTL--5)* Growth

NVe studied the inhibitory effect of iodide on thyroid growtlk iR FRTL-5 cells, which pyeserve many thyroid functions, When the amount of iodide in medium exceeds IO-6 M, TSH-stimulated [3H]thymidine incorporation into DNA was sigRificantly inhibited. Methimazole (O.:5 mM> reversed the suppression, as was observed in other functions such as thyroid hormone synthesis and secretion. Cell numbers were also monitored to ensure that [`'H]thymidine incorporation refiected cell multiplication. Iodide iRhibited TSKstimulated cAMP formation and also 8-bromo-cAMP stimulated-[3H]thymidine incorporatioi} into FRTL-5 cells, indicating tkat it acts on steps both before and after the formation of cAMP. The data demonstrate(l an autoregulatory roie of iodide on the growth of FRTL-5 cells. Key words; FR IrL-5, iodicle metabo}ism, thymidine incorporation, thyroid, cAMP It is generally accepted that most thyroid functions, including thyroid cell growth, are controlled by TSHi,i3). It is also well known that excess iodide appears to depress the stimulatory action of TSH on cAMP accumulation iR thyreid tissues9,i8,i9). Even more intriguing is the effect of iodide on thyroid growth. In dog thyroid cells of primary cul£ure, the effect of iodide on thyreid growth is mediated, adeast in part, by regulatien of the thyroid amino acid transport system7). In this study, we examined the inhibitory effect of iodide on TSH-stimulated growth of FRTL-5 cells, which preserve many thyroid functioRs. MATERIALS AND METHODS