The role of iodine and thyroid cell growth

The autoregulatory effects of iodide on thyroid growth and function are discussed to be mediated by iodinated derivatives of essential fatty acids (EFA), esp. iodolactones. We now reevaluated the effect of iodide on proliferation of isolated porcine thyroid follicles by determination of cell counts and investigated the effects of pretreatment of the follicles with arachidonic acid (AA, C 20:4 n6) in comparison to docosahexaenoic acid (DHA, C22:6 n3). Growth experiments were performed in multi-well culture plates and cell counts were determined after 6 d of incubation. EGF (5 ng/ml) significantly stimulated thyroid cell proliferation (151 +/- 6%; Mean +/- SD vs. basal control, 100 +/- 8%). 2.5 microM of iodide, added 24 h before EGF, had a weak stimulatory effect (168 +/- 9%) whereas higher concentrations of iodide (5-80 microM) exerted significantly dose-dependent inhibitory effects (117 +/- 1% at 80 microM of KI) which could be abolished with 500 microM of methimazole (155 +/- 11% at 80 microM of KI). Isolated porcine follicles showed a rapid uptake of EFA (25 microM) measured by specific tracer activity in the ethanol/acetic acid extracts of follicles (1.60 +/- 0.48 mumol EFA/ml follicle/24 h). Treatment with DHA (100 and 300 microM) significantly enhanced the inhibitory effect of 10 microM of iodide on thyroid follicle proliferation (84 +/- 2% and 45 +/- 4%) in contrast to follicles pretreated with AA (100 +/- 8% and 60 +/- 8%). These results demonstrate the biphasic effect of iodide on thyroid growth which can be abolished by inhibition of iodide organification with methimazole.(ABSTRACT TRUNCATED AT 250 WORDS)