Cytological presentation of various subtypes of follicular tumors
Follicular adenoma - Case 20: a microfollicular adenoma
3-8 cells form a follicle which lacks colloid.

Follicular adenomas - Case 7: a  normofollicular adenoma
8-20 cells form a follicle which frequently contains colloid.

Follicular adenoma - Case 8: a dominantly macrofollicular adenoma
More than 20 cells form a follicle which contains colloid. Because of the size of the follicle cystic degeneration is a frequent finding in such adenomas.

Most adenomas contain all 3 types of follicles and the dominant subtype decides whether an adenoma is microfollicular, normofollicular or macrofollicular. It means that on a cytological sample we cannot decide with enough safety from which type of adenoma we gained the smear. Moreover, follicles are the basic structure of the thyroid, therefore in every thyroid sample we can found follicles of one or more subtypes.
In the everyday practice microfollicular proliferation has relevance. The basic rules are as follows:

  • The greater the proportion of cells forming folliculi, the greater the chance of a follicular tumor.
  • The greater the proportion of well-preserved microfolliculi, the greater the chance of microfollicular adenoma and minimally invasive follicular carcinoma.
  • The greater the proportion of small, irregular folliculi, the greater the chance of an atypical follicular tumor.

Other rare tumors - Case 13: a hyalinizing trabecular adenoma
The basic unit of a trabecular adenoma is also a follicle but these are arranged in trabecular cords. These are characteristically very cellular.

Oxyphilic adenomas - Case 2

Oxyphilic adenomas are a subgroup of follicular adenomas, therefore the basic structure in these tumors is also a follicle. Because of abundant cytoplasm it is less conspicious the presence of the follicular pattern in a cytological sample, dispersed cells predominate most smears. This property and the presence of abundant cytoplasm makes the pattern of an oxyphilic tumor similar to medullary carcinoma.