7596 Thyroid Function Tests in Breast Cancer Patients Following Immune Checkpoint Inhibitor Treatment: What Happened in a Population with Preexisting Thyroid Dysfunction?

Abstract

Abstract Disclosure: M. Villanova: None. L. Min: None. Introduction: Thyroid dysfunction occurs commonly following immune checkpoint inhibition, but the etiology remains unclear. Immune checkpoint inhibitor (ICI) is not contraindicated in patients with a preexisting thyroid dysfunction, but little is known about the impact of ICI on thyroid function tests (TFTs) in this setting. This study sought to define TFTs abnormalities following ICI treatment in patients with previous thyroid dysfunction. Methods: We performed a retrospective cohort study of thyroid dysfunction in patients with breast cancer undergoing anti-programmed cell death protein-1/ligand-1 therapy from May 2016 to Dec 2023. Results: A total of 30 patients were included with a mean follow-up of 18 ± 12 months. Two patients (7%) had a history of Graves’ disease, 4 patients (13%) had a post ablative hypothyroidism, and 3 patients (10%) had a post-surgical hypothyroidism. The etiology of the hypothyroidism was not further specified in the remain cases (70%). Twenty-three patients (77%) were treated with levothyroxine at baseline at the mean dose of 1.31 ± 0.68 mcg/kg. Baseline TFTs were available in 27 patients (90%). Twenty-four patients (80%) had at least one TFTs abnormality after initiating ICI treatment. Subclinical hypothyroidism (n = 11) was the most common abnormality, followed by overt thyrotoxicosis (n = 10), subclinical thyrotoxicosis (n =7), and overt hypothyroidism (n = 7). Thirteen out of 16 patients with normal baseline TFTs developed at least one TFTs abnormality after initiating ICI treatment. Subclinical hypothyroidism (n = 6) and overt thyrotoxicosis (n =6) were the most common alterations, followed by subclinical thyrotoxicosis (n = 5), and overt hypothyroidism (n = 3). Three patients with post-ablative hypothyroidism had normal baseline TFTs. Among these, one patient developed an overt thyrotoxicosis, one patient developed a subclinical hypothyroidism followed by overt thyrotoxicosis and one patient had no TFTs alterations. One post-operative hypothyroid patient had a normal baseline TFT and developed a subclinical hypothyroidism followed by overt thyrotoxicosis. Twenty-five patients (83%) were on levothyroxine at the last follow-up at the mean dose of 1.41 ± 0.60 mcg/kg. The dose was higher than that at baseline but did not reach statistical difference (Student T test, P = 0.059). Levothyroxine dosage was adjusted in 16 patients (53%) and 25 patients (83%) achieved normal TFTs at the last follow-up. Conclusion: ICI treatment related TFTs abnormalities were common in patients with preexisting thyroid dysfunction. There were multiple distinct phenotypes. The mechanisms underlying the TFTs abnormalities is not clear. Destructive thyroiditis, medication adherence, or interference with TFT assay by ICI are among the possible etiologies. ICI is not contraindicated in this group of patients but their TFTs should be monitored closely. Presentation: 6/1/2024