Organic iodine inhibits deoxyribonucleic acid synthesis and growth in FRTL-5 thyroid cells.

FRTL-5 thyroid epithelial cells in culture were used to study the possible inhibitory effects of iodine on thyroid growth. NaI exerted a dose-dependent, thyroid epithelial cell-specific inhibitory effect on [methyl-3H]thymidine incorporation into DNA, reduced the DNA content in the cell layer, and limited the increase in cell number mediated by TSH. The inhibitory effects of sodium iodide applied to growth stimulated by TSH-, cAMP-, and non-cAMP-dependent mechanisms and were prevented by 1-methylimidazole-2-thiol (methimazole) and 2-ethylthioisonicotinamide (ethionamide). The latter findings indicate that the inhibitory effects of NaI are mediated by some iodine-containing organic compound. The inhibitory effects of organic iodine on growth subsided 24-48 h after removal of excess NaI from the culture medium. In contrast, NaI had no effect on normal rat kidney fibroblast or thyroid fibroblast [methyl-3H]thymidine incorporation stimulated by epidermal growth factor or serum. These data demonstrate a specific inhibitory effect of organic iodine on thyroid epithelial cell growth.