Immunotherapy-Associated Hypothyroidism: Comparison of the Pre-Existing With Hypothyroidism.

Background: Immunotherapy has revolutionized the treatment of solid malignancies, but is associated with endocrine-related adverse events. This study aims to dissect the natural course of immunotherapy-induced hypothyroidism and provide guidance regarding diagnosis and management in patients with and without pre-existing hypothyroidism.

Methods: A retrospective analysis was conducted using patients who received immunotherapy between 2010-2019 within a multicenter hospital system. Participants were separated in three groups-those with pre-existing hypothyroidism, those who developed primary hypothyroidism and those with hypophysitis within a year of their first immunotherapy. Serial effects of immunotherapy on thyroid function tests (TFTs) and levothyroxine dosing were evaluated.

Results: 822 patients were screened, with 85 determined to have pre-existing hypothyroidism, 48 primary hypothyroidism and 12 hypophysitis. All groups displayed fluctuations in TFTs around weeks 6-8 of treatment. In the pre-existing hypothyroidism group, the levothyroxine dose was higher at 54 weeks than at baseline with the difference showing a trend towards statistical significance (p=0.06). The observed mean levothyroxine dose was significantly lower than the mean calculated weight-based dose for all groups. This finding was most clinically significant for the hypophysitis group (mean difference: -58.3 mcg, p<0.0001). The mean 0.9 mcg/kg levothyroxine dose at week 54 for the hypophysitis group was statistically lower than the other groups (p=0.009).

Conclusion: It is reasonable to screen with TFTs every 4 weeks, and space out TFTs surveillance to every 12 weeks after week 20. Our findings suggest a more conservative approach for levothyroxine dosing in those developing hypothyroidism, especially hypophysitis, such as initiating at 0.9-1.2 mcg/kg.