FRI511 Bexarotene: A Deceptive Cause Of Iatrogenic Hypothyroidism

Disclosure: S. Ghaith: None. A. Ramachandran: None. C. Alcorn: None. F. Amer: None. A. Thompson: None. A. Ramachandran: None. Background: Bexarotene, an antineoplastic agent used for the treatment of cutaneous T cell lymphoma, is associated with a rapid and profound suppression of thyrotropin, which may lead to central hypothyroidism. Case: A Seventy-four-year-old female with a recent diagnosis of Cutaneous T cell lymphoma presented with a 1-week history of progressive fatigue, lightheadedness, and night sweats. She was found to have hypotension and hypothermia. She was started on oral Bexarotene chemotherapy 225 mg daily five days prior to admission. The physical exam was unremarkable except for sinus bradycardia and hypothermia. Labs were significant for TSH 0.48 [0.4-4.2 uIU/mL] dropped from 3.3 uIU/mL one week prior, FT4 0.5 [0.8-1.7 ng/dL] and Total T3 111 ng/dL [87- 187 ng/dL] consistent with central hypothyroidism. Bexarotene was held during her hospitalization per hematology recommendations. The patient was started on Levothyroxine 50 mcg, which was later advanced to 75 mcg daily based on free T4 levels. At one month follow up, free T4 level normalized while on Levothyroxine and Bexarotene was restarted again by her hematologist. Discussion: Bexarotene, an antineoplastic agent approved for treating cutaneous T cell lymphoma, can rapidly suppress TSH levels by directly inhibiting its secretion and affect thyroid hormone metabolism, leading to central hypothyroidism. The degree of suppression of TSH increases with higher doses of Bexarotene while reduction in free T4 and TSH are reversible. Patients already receiving thyroid hormone therapy may require increased thyroid hormone doses to achieve therapeutic levels. If bexarotene is permanently discontinued, levothyroxine can either be stopped or can be resumed to doses used prior to bexarotene therapy. Recovery of thyroid function usually occurs within weeks of Bexarotene discontinuation. Conclusion: Oral Bexarotene can cause central hypothyroidism therefore, thyroid function tests should be measured at baseline and treatment. Thyroid hormone replacement should be considered if central hypothyroidism is suspected. Presentation: Friday, June 16, 2023