This is a fetal adenoma which is a type of microfollicular adenoma. There are 4 types of adenoma. They all have the same clinical significance. There is an additional type, the Hurthle cell adenoma which is histologically distinct. Notice the large number of small follicles embedded in an abundant stroma. There is little if any colloid in these follicles. Arrow 1 indicates a follicle with some colloid. Arrow 2 indicates a follicle without colloid.