A Rare Case of Asymmetric Periorbital Edema due to Delayed Hypersensitivity Reaction to Thyroid Hormone Replacement.

Background/objective: Hypersensitivity reactions to medications often exhibit identifiable symptoms, yet their diagnosis can be challenging due to individual variability. Although hypersensitivity reactions to levothyroxine are rare, the presence of excipients in different formulations can trigger allergic manifestations. We present an unusual case of delayed hypersensitivity reaction characterized by isolated asymmetric periorbital edema, occurring within 1 to 2 months of being on levothyroxine therapy.

Case Report: A 77-year-old woman, on thyroid hormone replacement for over 20 years, was taken off levothyroxine due to low thyroid-stimulating hormone. Within 2 months, her thyroid-stimulating hormone rose above 10 mIU/mL. Therefore, generic levothyroxine was restarted at the dose of 50 μg orally daily. After 1 month, she developed asymmetric bilateral periorbital edema without other symptoms such as rash, itching, redness, or fever. Discontinuing levothyroxine resolved the issue, but the reaction recurred with retrial of brand Synthroid. Despite the lack of typical allergy signs, the timing suggested a drug allergy. Switching to Tirosint capsules, with fewer inactive ingredients, successfully resolved the problem without further reactions.

Discussion: Levothyroxine allergy is rare since its structure is identical to natural thyroxine. Allergic reactions are often caused by excipients such as fillers or dyes. Diagnosis can be challenging in atypical cases, and treatment may include desensitization if no suitable formulation is found.

Conclusion: This case underscores the importance of remaining vigilant for medication-induced hypersensitivity reactions and highlights the potential benefits of using formulations with minimal excipients when drug allergy is suspected, which can help us avoid complicated desensitization process.