Thyroid carcinoma presenting as a hyperfunctioning thyroid nodule is rare. A further complexity is added when interpretation of the histopathology itself is not straightforward. We describe a case of a 16-year-old girl presenting with clinical and biochemical evidence of thyrotoxicosis, and a 4 cm thyroid mass. An ultrasound and thyroid uptake scan demonstrated a toxic adenoma. Owing to the nodule size, fine needle aspiration of the thyroid adenoma was performed, which showed findings consistent with toxic adenoma. However, in view of the size of the nodule, a hemithyroidectomy was performed. Histological examination of the thyroid revealed a follicular variant of papillary carcinoma of the thyroid, and the patient underwent completion thyroidectomy. We report on the case and briefly review the available literature relating to the diagnostic challenge of this presentation.