Low and very low lithium levels: Thyroid effects are small but still require monitoring

Abstract Aim Low doses of lithium, as might be used for mood or dementia prevention, do not carry the same renal, toxicity, and tolerability problems of doses used for prophylaxis or treatment of mania. However, thyroid effects of low doses have not been investigated. Our goal in this study was to assess the changes in thyroid‐stimulating hormone (TSH) associated with a broad range of lithium levels, including those well below the therapeutic range for bipolar disorders. Methods This study was conducted in a small healthcare system with 19 associated primary care clinics served by a Collaborative Care program of psychiatric consultation. In this retrospective review of electronic records, we searched for patients who had received a lithium prescription and both pre‐ and post‐lithium thyroid‐stimulating hormone (TSH) levels. Results Patients with low lithium levels (<0.5 mEq/L, N = 197) had a mean thyroid‐stimulating hormone (TSH) increase of 0.52 mIU/L. Patients with maintenance lithium levels (0.5–0.8 mEq/L; N = 123) had a mean TSH increase of 1.01 mIU/L; and patients with antimanic lithium levels (>0.8 mEq/L; N = 79) had a mean TSH increase of 2.16 mIU/L. The probability of TSH exceeding the upper limit of normal in our laboratory (>4.2 mIU/L) was positively associated with pre‐lithium TSH. Conclusion These results suggest that the risk of lithium‐induced hypothyroidism is dose‐related, and relatively small with very low doses, but thyroid monitoring, including a pre‐lithium TSH, is still warranted.