Effect of chronic lithium treatment on hypothalamic-pituitary regulation of thyroid function

We have previously observed that chronic treatment with Li causes a decrease in the goitrogenic response to iodine deficiency in rats. Since this is likely to be mediated by an alteration of thyrotropin (TSH) secretion, we have investigated the hypothalamic and pituitary function of rats fed an iodine deficient Li supplemented diet for 16 weeks. The Li group had lower thyroid and pituitary weights than the corresponding control group. Plasma thyroxine (T4) and 3,5,3'-triiodothyronine (T3) were low and comparable in both groups but plasma TSH was significantly lower in the Li group. There was no difference in the TSH content of pituitaries in the two groups. TSH release from the pituitaries in vitro was similar in both groups in the basal state as well as upon stimulation by thyrotropin releasing hormone (TRH). We conclude that chronic treatment with Li blunts the TSH response to iodine deficiency. The data also provide indirect evidence that the effect of Li is exerted at the hypothalamic rather than at the pituitary level.