thyroid follicular adenoma

Definition: A benign, encapsulated tumour of the thyroid showing evidence of follicular differentiation.

Images

 Wikimedia : Follicular adenoma

Digital cases (Digital slides)

 HPC:395 : Thyroid follicular adenoma with papillary epithelial hyperplasia.
 JRC:2612 : Thyroid atypical follicular adenoma, thyroid Spindle cell and follicular neoplasm.
 JRC:14584 : Thyroid follicular adenoma, clear cell type.
 JRC:15430 : Thyroid follicular adenoma (with pseudoclear).

Follicular adenoma is a thickly encapsulated follicular patterned lesion with no capsular or vascular invasion.

Follicular adenomas are the most common of thyroid neoplasms. Autopsy series have shown an incidence of the order of 3% of the adult population.

They are slow-growing and show morphological and biochemical evidence of follicular cell differentiation.

Irrespective of their histological pattern they are benign and usually asymptomatic. Occasionally, haemorrhage occurs into an adenoma and the patient presents with a painful and tender mass.

Macroscopy

Common to these tumours, however, are the presence of a well circumscribed and complete fibrous capsule with compression of the surrounding thyroid gland and a relatively uniform appearance of the tumour within the capsule which differs from the surrounding thyroid.

Adenomas are usually solitary masses in contrast to colloid nodules in a multinodular goitre. Larger tumours may show degenerative changes with areas of fibrosis and calcification. They may undergo cystic degeneration.

A small minority of adenomas behave autonomously and cause hyperthyroidism. The majority of adenomas are hypofunctional and appear as cold nodules on radioisotope scanning.

Microscopy

There is a range of appearances with microscopic features recapitulating the embryology and functional states of the thyroid. Hence, there are embryonal, fetal, normofollicular and macrofollicular histological subtypes.

Adenomas may also demonstrate various metaplasias and degenerative changes and hence there are Hürthle cell, clear cell, signet-ring cell adenomas, the adenolipoma showing adipose metaplasia in its stroma and the adenochondroma with cartilaginous metaplasia. Atypical adenomas show nuclear atypia.

 Growth patterns

• Follicular adenomas have been referred to as ’’normofollicular, "macrofollicular" and "microfollicular", reflecting the size of the follicles comprising the tumour.
• Since a single tumour may show more than one of these architectural patterns, it can be difficult to apply these descriptive designations.
• Rarely, papillae can be present focally.

Variants

 thyroid microfollicular adenoma
 thyroid oncocytic adenoma (follicular adenoma of oxyphilic cell type, thyroid Hurthle cell adenoma )
 follicular adenoma with papillary hyperplasia
 fetal thyroid adenoma
 signet-ring cell follicular adenoma
 mucinous thyroid adenoma
 thyroid lipoadenoma
 thyroid clear cell adenoma
 throid hyperplastic adenoma
 thyroid signet ring cell and mucin-producing adenoma
 thyroid adenoma with mature fat tissue
 toxic adenoma thyroid (hyperfunctioning thyroid adenoma )
 thyroid atypical adenoma
 thyroid follicualr adenoma with bizarre nuclei

Epidemiology

The epidemiology of follicular adenoma is difficult to analyse because of the lack of consistent criteria for distinguishing hyperplastic nodules and adenomas.

Many do not make the diagnosis of adenoma in a multinodular gland, preferring to regard all of the lesions as nodules providing there is no evidence of malignancy.

In a large, pooild analysis of the relationship between benign thyroid disease and the devolopment of thyroid cancer, nodules and adenomas were treated together.

The biological basis for separating nodules from adenomas is dependent on their clonality and up to 60% of lesions in multinodular goiter have been shown to be monoclonal.

Solitary cold thyroid nodules occur in 4-7% of adults in iodine sufficient area. In iodine deficient areas, the incidence of nodules, usually multiple, is related to the level of dietary iodine intake, and can rise to 50% of the population.

Females are more commonly affected than males. The rate of progression ta malignancy is similar for both solitary cold nodules and multinodular goiter, but although males are less frequently affected, the risk of progression to malignancy in males is relatively greater.

Changing incidence of the occurrence of adenoma is related to changes in iodine intake, and in radiation exposure of differing populations.

Etiology

The etiological factors involved in follicular adenomas are largely shared with follicular carcinome.

The association with radiation is botter documented than that for follicular carcinome, and radiation induced adenomas show a very long mean latent period.

Adenomas are common in iodine deficient areas, usually as part of a nodular goiter. They can also occur as part of nodular goiters in Cowden syndrome, and in dyshormonogenesis.

Localization

Follicular adenoma can arise in the normal thyroid gland and in ectopie thyroid tissues (e.g. struma ovarii).

Clinical features

While small follicular adenomas are usually asymptomatic. they may be detected by the patient or may be palpable on careful physical examination. Spontaneous haemorrhage into an adenoma occasionally results in an acute episode of pain and enlargement.

Immunohistochemistry

Follicular adenomas are immunoreactive for cytokeratins, thyroglobulin and TTF1, but negative for CK19, calcitonin or pan-neuroendocine markers.

Differential diagnosis

The distinction between follicular adenoma and adenomatoid nodule (cellular colloid nodule) is sometimes rather arbitrary.

In general, adenomatoid nodules are multiple, lack a well-defined fibrous capsule, and are composed of follicres morphologically similar to those in the surrounding thyroid tissue.

The only histological features which reliably distinguish a follicular carcinoma from a follicular adenoma are the presence of vascular or capsular invasion, underscoring the importance of adequate sampling of any suspicious tumour to search for such features.

Capsule invasion should be distinguished from sites of prior fine needle aspiration biopsies which most commonly appear as linear tracts at right angles.

Vasoular endothelial proliferatron should be distingurshed from sites of vascular invasion, and this can be facilitated by the use of endothelial markers.

As a group, the fibrous capsule is often thicker in follicular carcinoma compared with follicular adenoma.

Medullary carcinoma should be excluded when a tumour presumed to represent follicular adenoma shows unusual histological features, such as prominent fibrovascular septa, solid growth, or spindle cells.

Parathyroid adenoma arising within the thyroid gland can mimic follicular adenoma of the microfollicular, clear tell or oncocytic type.

The cytology (such as presence of water-clear cells) should provide an important cluue to the correct diagnosis, which can be further confirmed by positive immunostaining for parathyroid hormone and chromogranin.

Predisposition (Exemples)

 dyshormonogenetic goiter

See also

 thyroid gland

• thyroid tumors
  • thyroid adenomas
  • thyroid carcinomas

• thyroid follicular tumors