Transdermal Absorption of Methimazole- a Cat’s Tale

Introduction: In modern civilization different kinds of animals live with us as pets. These pets have different diseases and are on medications. Close contact with animals can cause medication adverse reactions or may worsen pre-existing conditions in humans. Here we present an interesting case of hypothyroidism worsened by transdermal absorption of methimazole administered for feline hyperthyroidism. Case History: 66-year-old Caucasian female with past medical history of postoperative hypothyroidism s/p total thyroidectomy secondary to multinodular goiter due to Hashimoto’s thyroiditis with a 0.2 cm papillofollicular microcarcinoma and 0.7 cm follicular adenoma presented for follow up. She complained of weight gain, lethargy and dry skin for the past 4 months. She was on a stable dose of levothyroxine 112 mcg daily for the past year. She took her pill correctly and did not miss any doses. Her other medical problems were impaired fasting glucose, osteopenia and B12 deficiency. Her repeat thyroid function tests showed TSH 11.2 mc IU/ l (0.4 -4) (TSH - 0.538 mc IU/ L 6 months back), T4 - 6.8 mcg/ dl (4.5 - 12) (T4 8.9 6 months back). She had a measurable serum thyroglobulin of 0.4 ng/ml with antithyroglobulin antibody 11 IU/ml (<115) consistent with some residual thyroid tissue despite her history of a “total thyroidectomy”. Due to recent worsening of her symptoms with elevated TSH on background of previous stable levothyroxine requirement, further detailed history was taken. She reported that her cat was suffering from hyperthyroidism, treated with methimazole 10 mg daily. The patient used to cut the pill in half with bare hands and fed it to her pet. She also used to handle wet methimazole that her cat coughed up. Her levothyroxine dose was continued at 112 mcg daily. The patient was advised to use gloves before feeding and wash her hands after feeding her cat. Her symptoms resolved after she took precautions and TSH normalized to 1.170 mc IU/ l with T4 7.6 two months later. Conclusion: Absorption of methimazole by transdermal administration has been shown in cats(1). A study by Kasraee et al showed safety of a 5% topical methimazole application for treatment of post inflammatory hyperpigmentation in humans with no change in thyroid function tests(2). Our case contradicts this study and indicates that methimazole might be absorbed transdermally in humans. To conclude, more studies are needed to study the effect of transdermal administration of methimazole in humans. References: 1. Hill KE, Mills PC, Jones BR et.al. Percutaneous absorption of methimazole: an in vitro study of the absorption pharmacokinetics for two different vehicles. J Vet Pharmacol Ther. 2015;38(6):581-589. PMID: 25683868 2. Kasraee B et al. Safety of topical methimazole for the treatment of melasma. Transdermal absorption, the effect on thyroid function and cutaneous adverse effects. Skin Pharmacol Physiol. 2008;21(6):300-305. PMID: 18667842