We report a rare case of simultaneous medullary thyroid carcinoma on the left thyroid lobe with lymph node metastasis and papillary thyroid cancer on the right thyroid lobe. The 55-year-old woman was diagnosed with medullary thyroid carcinoma after left hemithyroidectomy for goitre. Completion thyroidectomy, central neck dissection and left modified neck dissection revealed the presence of papillary carcinoma on the right side. The extreme rarity and interesting pathological features are discussed and we raise the question of whether the finding of another thyroid cancer on the opposite side was coincidental or from possible activation of a common tumorigenic pathway for both follicular and parafollicular thyroid cells.