Endocrine Abstracts

Levothyroxine, the standard therapy for hypothyroidism, is usually well tolerated and very few cases of true allergy to levothyroxine have been reported to date. In these cases mechanisms have not been conclusively identified but have been hypothesized to be allergies to excipients. Here we report a rare occurrence of levothyroxine allergy and the effectiveness of desensitization. A 56-year-old lady with no previous history of allergies required a total thyroidectomy for Graves’ Disease. Post-operative recovery was initially unremarkable, however on the second day post-operation she developed facial flushing, urticaria and a rash on her chest and abdomen within 15 minutes of receiving a 125 micrograms dose of levothyroxine composed of 2 different brands. She remained hemodynamically stable with no airway compromise and was treated with antihistamine and steroid. The following day, she was re-administered levothyroxine and developed similar but more pronounced and rapid symptoms. Tryptase levels confirmed true allergy. It was felt that excipients in levothyroxine were the most likely cause. She was therefore tried on levothyroxine elixir (containing no shared excipients with the previous two formulations), but again developed urticaria and a rash, albeit more modest, despite being treated concurrently with antihistamine and steroid. However, she tolerated liothyronine well, with no allergic symptoms. We then undertook desensitization to levothyroxine with incremental doses every 30 minutes starting from 0.05microgram levothyroxine built up to 75 micrograms levothyroxine over the course of a day. A rash appeared at higher doses but treatment was continued and the rash settled within 24 h. She is currently tolerating 100 micrograms a day of levothyroxine with no adverse effects. Given her lack of other allergies and as the levothyroxine excipients are common in other drugs and food stuffs, we are currently investigating an allergy to dextrothyroxine (an enantiomer of levothyroxine, present in small amounts in levothyroxine formulations).