FNAC is the least diagnostic in which thyroid carcinoma-
FNAC is the best investigation for the diagnosis of all thyroid carcinomas except follicular carcinoma.
It cannot differentiate b/w follicular carcinoma and follicular adenoma. The definitive diagnosis of carcinoma is made when capsular and vascular invasion by the carcinoma is revealed.

Important features of follicular carcinoma

Second most common type of thyroid cancer.
Hurthle cells are seen.
Differentiated from follicular adenoma by capsular and/or vascular invasion.
Unlike in papillary carcinoma, lymphatic spread is rare, and vascular invasion is common in follicular cancers.
Anaplastic
Papillary
Follicular
Thyroiditis