Variations in intrathyroidal lithium content and their effect on the iodide uptake in mouse thyroid

Lithium (Li) is accumulated in the thyroid but the mechanism of Li accumulation is not known. In the present study, the causes of variation in Li concentration in the thyroid and the relation between cellular Li and iodide were examined. This was done by using mice treated with Li (0.01% as Li2CO3) for 4 weeks, co-administered with propylthiouracil (PTU, 0.5 mg/ml daily p.o.) or thyroxine (T4, 0.5 micrograms per day i.p.) for last 10 days. The total content of Li in a whole thyroid (ng/thyroid) was not changed through treatment with PTU or T4. But the Li concentration in terms of mg/kg of the gland was reduced with PTU and was unchanged or slightly increased with T4, due to the change in the mass of thyroid. Furthermore, short-term (3 h) Li uptake in the thyroid was not affected by pre-treatment with PTU or T4. These results indicate that the variation in thyroidal Li concentration was not due to a direct effect of PTU or T4 on Li transport, but to a thyroidal condition caused by the secondary influence of drugs. A measurement of the thyroid: serum iodide concentration ratio (T/S) of 125I showed that the iodide uptake was reduced when intracellular Li concentration was high, and that PTU alone elevated the T/S, but Li + PTU brought it back to a normal level; whereas, T4 alone diminished the T/S, Li + T4 made it rise significantly more than did T4 alone.(ABSTRACT TRUNCATED AT 250 WORDS)