To the Editor: A 65-year-old woman was admitted to Peking Union Medical College Hospital because of rash with pruritus for 1 month after taking Euthyrox. Rash attacked without other symptoms 20 days after she took Euthyrox 50 μg once a day and aggravated when Euthyrox was continued for the next 2 days. The past history included partial thyroidectomy for hyperthyroidism in 1982; tuberculosis in 1990; hypertension in 2009, and cerebral hemorrhage in 2010. It was noted that the patient occurred fever and vomiting 2–3 days after antituberculosis therapy, and the symptoms alleviated 3 days after rifampicin withdrawal. Penicillin skin test was positive. The laboratory examination showed thyroid-stimulating hormone 8.78 μU/ml (normal range: 0.38–4.34 μU/ml), T3, T4, FT3, and FT4 were normal, which suggested subclinical hypothyroidism. Patch test of Euthyrox was probable positive to 25–50 μg and positive to 100–150 μg at 48–72 h, while only probable positive to letrox 150 μg. Moreover, patch test of health control showed negative. Prick test was negative of two drugs. Hence, drug rash induced by Euthyrox was diagnosed according to the laboratory test results. The condition was stable 1 week after discontinuation of Euthyrox and rash disappeared completely after 1 month. Thyroid function returned to normal 4 months after taking Letrox and no rash reappeared.
Levothyroxine substitutive therapy is widely used for hypothyroidism. Hypothyroidism prevalence is approximately 1:1000 in males and 4:1000 in females, and up to 10% of the population may have subclinical hypothyroidism. Trade name of levothyroxine includes Euthyrox (Merck KGaA, Darmstadt, Germany) and Letrox (Berlin Chemie, Berlin, Germany), and another substitution in China. Rash induced by levothyroxine is rare and may be caused by drug ingredient or adjuvant. Hence, the differentiation of real cause for allergic reaction is important for further treatment.
Rash attacked after taking Euthyrox, aggravated during drug continued, and alleviated after drug withdraws, so Euthyrox induced drug rash was diagnosed according to Naranjo scoring method. Type IV allergic reaction was indicated for the medical history. Challenge test was avoided because of complex basic diseases.
Broadly speaking, in vivo allergen tests include patch test and skin test. Type IV allergic reaction is usually checked by patch test. Skin test includes prick test and intracutaneous test, which shows Type I allergic reaction. Prick test is more secure than intracutaneous test for damaging epidermis and trace allergen.
According to patch test and prick test of Euthyrox and Letrox, allergic to Euthyrox and not to Letrox was confirmed. The patient received the treatment of Letrox, no rash appeared and thyroid function returned to normal 4 months later, thereby avoid desensitization.
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Edited by: Xin Chen