7699 Unanticipated Harmony: PD-L1 Inhibitor Treatment for Urothelial Cancer Resolving Graves' Disease in a Patient - A Case of Immune Modulation Surprises

Abstract

Abstract Disclosure: K. Tiwari: None. C.A. Resta: None. Introduction: Programmed death ligand 1(PD-L1) inhibitors constitute a class of immune checkpoint inhibitors (ICI) employed in treating various cancers. Thyroid-related immune-related adverse events (irAEs), such as hypothyroidism, are found to be in 3.9% of patients with PD L1 treatments. This case explores a unique scenario where a patient with pre-existing Graves' disease, undergoing PD-L1 inhibitor therapy for cancer, experienced an unexpected sequence of events leading to the default treatment of Graves' disease and subsequent progression to hypothyroidism. Case Presentation: A 59-year-old male, who had been effectively managed on methimazole for Graves' disease over two years and had chosen not to pursue definitive therapy for the condition, presented with tremors and a bilaterally enlarged thyroid gland during treatment for urothelial carcinoma. Laboratory findings included a TSH of 0.03 mIU/L (nl 0.40-4.50 mIU/L), fT4 of 3.4 ng/dL (nl 0.8-1.8 ng/dl), and tT3 of 284ng/dl (nl 76-181 ng/dl). The patient had recently started treatment with Avelumab, a PD-L1 inhibitor, as maintenance therapy following 4 cycles of gemcitabine/cisplatin cycles. Concerned about worsening Graves' disease, the methimazole dose was increased, and follow-up labs were performed after three weeks. These results showed low-normal fT4 of 0.8 ng/dL, TSH of 1.01 mIU/L(N), and low T3 of 58 ng/dL. TSI was mildly elevated at 168%. Due to rapidly changing thyroid function tests and suspicion of thyroiditis, methimazole was stopped. A PET scan a week later indicated diffuse thyroid uptake consistent with thyroiditis. Repeat labs 10 days later revealed a TSH of 67.36 mIU/L(H) and fT4 of 0.4 ng/dL(L), indicative of overt primary hypothyroidism. The patient exhibited symptoms including fatigue, constipation, and cold intolerance. The patient was initiated on levothyroxine therapy to address overt primary hypothyroidism, likely secondary to destructive thyroiditis from ICI. Discussion: Thyroid irAEs are typically incidentally discovered during routine monitoring of ICI treatment. It includes thyrotoxicosis, subclinical hypothyroidism, and overt hypothyroidism. These events commonly occur in patients lacking pre-existing thyroid conditions. In contrast, our patient, with a history of Graves' disease, experienced resolution of Graves' disease following treatment with PD-L1 inhibitor use with subsequent progression to hypothyroidism. Conclusion: This case emphasizes the importance of recognizing atypical presentations of thyroid irAEs in individuals with pre-existing thyroid disorders undergoing ICI treatment. Presentation: 6/3/2024