A 16-year-old female presented with thyrotoxicosis seven months after cessation of amiodarone treatment for paroxysmal supraventricular (ectopic atrial) tachycardia. Laboratory tests confirmed hyperthyroidism, thyroid ultrasound was unremarkable, however, scintigraphy showed rudimentary tracer uptake. Anti-thyroid drug (ATD) and beta-blocker were introduced. Later on, ultrasound showed a slightly enlarged hypoechogenic gland with slightly increased vascular flow. The results of subsequent control tests allowed for a reduction of ATD dose and for introduction of L-thyroxine (LT-4) therapy. After five months of treatment, the patient appeared clinically euthyroid with an unremarkable ultrasound. However, two months later, ultrasound revealed signs suggestive of autoimmune thyroid disease.