Are all types of thyroid nodules / lumps cancerous?

No: The majority of lumps in the thyroid when noted to be multiple on scan represent the condition "multinodular goiter" These are almost always benign. When a solitary lesion is present that is "cold" on isotope scan, this is suspicious for possible malignancy and should always be removed. Bx has a certain degree of error especially if a follicular lesion looking benign has invaded capsule.