[en] In order to elucidate the mechanism of action of lithium on the thyroid gland, we studied the effect of lithium on the ¹²⁵I up-take in thyroid cells in vitro. After culturing FRTL-5 cells 7 days in a medium lacking TSH, the cells were incubated with various concentrations of lithium (0.5∼10 mmol/L) and TSH (50, 100 mU/L) for 24 h. Iodide uptake was initiated by adding 500 μl buffered HBSS containing 0.1 μCi carrier-free Na ¹²⁵I and 10 nmol/L NaI. After incubation, the cells were dissolved and their radioactivities counted. The ¹²⁵I uptake in FRTL-5 cells rose to peak level in 30 minutes. Prolonging the incubation period did not change the iodide uptake. During the peak phase sodium perchloride suppressed the ¹²⁵I uptake more than 90%. The iodide uptake induced by TSH was inhibited at 1 mmol/L lithium (P<0.05). More than 50% inhibition was observed at 2∼10 mmol/L lithium. There was no effect of lithium on the iodide uptake in the basal condition of FRTL-5 cells (lacking TSH). Lithium inhibits iodide uptake in thyroid cells in the presence of TSH