Endocrine Abstracts

Introduction: Lithium is a common pharmacological intervention for the treatment of bipolar affective disorder and is advocated by clinical practice guidelines (NICE 2021). Blood test monitoring is essential for management of lithium treatment and UK NICE guidance recommends 6-monthly serum testing of thyroid function. We here examine conformity to guidance and the consequences of monitoring outside these intervals.

Methods: We extracted serum lithium/thyroid hormone results at one centre Jan 2009-Dec 2020. We identified 266 patients who started lithium during this period with no history of thyroid abnormality within the previous 2 years and were at risk of developing thyroid abnormalities. We determined interval between tests, time between onset of lithium testing and first TSH outside laboratory reference range and assessed the impact of testing outside recommended 6-monthly intervals.

Results: The most common testing frequency was 3-monthly (±1month), accounting for 37.3% of test intervals. Kaplan-Meier analysis showed that most thyroid dysfunction manifests within 3 years (proportion with abnormal TSH at 3 years=91.4%, 19.9% of total patients). In the first 3 months from commencing lithium therapy, 8 patients developed subclinical hypothyroidism and had clinical follow-up data available. Of these, half spontaneously normalised without clinical intervention. In the remaining patients, thyroxine replacement was only initiated after multiple episodes of subclinical hypothyroidism (median2 years after initiating lithium, range 6 months-3 years).

Conclusions: Our data suggest that thyroid function tests are typically requested too frequently in patients on lithium, potentially resulting in over-diagnosis of thyroid abnormalities. 90.4% of TSH tests were outside the recommended 6-monthly (±1 month) interval. There was no evidence that early detection of abnormal thyroid results leads to earlier treatment. The initial thyroid dysfunction spontaneously resolved in half of cases without intervention. Importantly more than 90% of patients who developed thyroid dysfunction did so within three years of commencing lithium.