Effect of serum lithium levels on thyrotropin levels

Previous research has demonstrated laboratory-detected hypothyroidism as a consequence of lithium usage in 3% to 30% of all recipients and abnormal TSH secretion in up to 15%. This has generally been considered a non-dose-related consequence of lithium therapy. This study examined whether serum levels of lithium are related directly to changes in thyroid function. In a retrospective review, data from 39 cases were analyzed for correlations between serum lithium level, TSH, T3 RU, and T4 by RIA. A positive correlation was found between changes in serum lithium concentration and changes in TSH level. None of the other variables measured showed any correlation. Results of this study suggest that lithium-induced changes in TSH are serum level dependent. Possible mechanisms and clinical implications have been discussed.