Insights into the Incidence, Course, and Management of Lithium-Induced Hypothyroidism in Real-World Psychiatric Practice in Italy

<b>Background</b>: Lithium is a cornerstone in the treatment of bipolar disorder (BD). However, lithium use requires careful monitoring of thyroid function due to associated dysfunctions. The aim of our real-world study is to retrospectively evaluate the impact of lithium on thyroid function and how these thyroid alterations can be measured and managed. <b>Methods</b>: A retrospective observational study was performed on 150 patients with BD who started lithium treatment at the University Hospital of Siena. Thyroid function was assessed at baseline and after the introduction of lithium by measuring TSH, T3, and T4 levels at baseline and after 3, 6, 9, and 12 months, during which changes in psychiatric symptoms were also evaluated using specific psychometric scales. <b>Results</b>: Significant increases in TSH levels were observed at 3 and 6 months, while T3 and T4 levels decreased significantly at 3 months. Transient thyroid dysfunction occurred in 36.7% of patients, but normalized without the discontinuation of lithium or need for thyroid replacement therapy in most cases; however, replacement therapy was initiated in 8.7% of patients. There were no significant differences in treatment response between patients with and without thyroid abnormalities, as the abnormalities were transient or resolved. <b>Conclusions:</b> In our sample, lithium induced some cases of hypothyroidism, which, being transient or corrected with replacement therapy, did not interfere with symptomatic improvement. These findings underscore the necessity for continuous thyroid function monitoring during lithium therapy. Clinicians should be prepared to initiate thyroid replacement therapy, when necessary, as timely management can prevent the interruption of lithium treatment and ensure ongoing symptomatic improvement in BD patients. Future studies could include larger and more diverse populations to validate these findings further, extending the follow-up period beyond 12 months to better observe long-term thyroid function trends and management outcomes.