Follicular thyroid adenoma

Last revised by Ammar Ashraf on 5 Oct 2022

Follicular thyroid adenoma is a commonly found benign neoplasm of the thyroid consisting of differentiated follicular cells. It cannot be differentiated from follicular carcinoma on cytologic, sonographic or clinical features alone 1.

Follicular thyroid adenoma is more commonly found in women, increases in incidence with increasing age and in regions in which the diet is iodine deficient 3. Follicular adenomas are encountered approximately 5 times more frequently than follicular carcinomas 2.

Macroscopically follicular adenomas are round to oval, with a surrounding fibrous capsule that is usually regular and thin. They typically range in size between 1 and 3 cm, and changes including cystic degeneration, hemorrhage, ossification, calcification and fibrosis can be seen 2.

Functioning follicular adenomas occur as a result of a monoclonal expansion of thyroid follicular cells with a high prevalence of activating mutations in the gene for the TSH receptor 1.  N-RAS and K-RAS mutations may be present in patients with follicular adenoma and have been implicated in the evolution of follicular adenoma to follicular carcinoma 4.

Ultrasound features of follicular adenomas share many features with follicular carcinomas. In general follicular thyroid adenomas:

  • thin peripheral halo 6
  • predominantly cystic or mixed cystic and solid lesions 6
  • isoechoic or predominantly anechoic 6
  • can be homogenous or heterogeneous 6
  • absence of internal flow or predominantly peripheral flow indicates is associated with reduced probability of thyroid follicular malignancy 5

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