A 29-year-old woman presented to the emergency department with dyspnea and palpitations, as well as a pruritic and painful rash that had developed over the previous 5 days. Her temperature was 38.6[degrees]C, heart rate was 174 beats/min and respiratory rate was 24 breaths/min. We observed a diffusely enlarged thyroid gland as the patient swallowed and annular, erythematous-violaceous plaques on her face, trunk and extremities (Figure 1). Laboratory investigations showed a free tetraiodothyronine level of 2.37 (normal 0.70-1.48) ng/dL, a thyroid-stimulating hormone level of less than 0.008 (normal 0.35-4.94) [micro]IU/mL, an antithyroid peroxidase antibody level of 67.8 (normal < 5.6) lU/mL, a thyroid-stimulating hormone receptor antibody level of 7.5 (normal < 1.75) IU/L and a complement C3 level of 75.6 (normal 90-180) mg/dL. Skin biopsy from a lesion on the patient's abdomen showed perivascular infiltrates with numerous neutrophils, nuclear dust, scattered eosinophils and erythrocyte extravasations, consistent with leukocytoclastic vasculitis (Appendix 1, available at www.cmaj.ca/lookup/doi/10.1503/ cmaj.211926/tab-related-content).
We diagnosed hypocomplementemic...