The feature that differentiate a follicular carcinoma from a follicular adenoma of thyroid is
Follicular adenomas and carcinomas both are composed of well - differentiated follicular epithelial cells;the latter are distinguished by evidence of capsular and/or vascular invasion. They both have fairly uniform cells forming small follicles containing colloid. So there is no reliable cytological difference between them. (ref:ROBBINS Text book of pathology) REf img :
Nuclear pleomorphism
Huhle cell change
Capsular invasion
Absence of colloid
Endocrinology