A benign encapsulated tumour of the thyroid gland that is surrounded by a thin fibrous capsule

Aetiology

• More commonly found in women
• Increases in incidence with increasing age
• Increased incidence in regions of iodine deficiency

Genetic factors

• <20% have a mutant ras or PIK3CA
• Functional adenomas have an activating mutation in the TSHR signalling pathway → increased cAMP
• N-ras and K-ras mutations have been implicated in the evolution of follicular adenoma to follicular carcinoma

Pathophysiology

• Usually non-functional
• Around 1% develop into a toxic adenoma - produces thyroid hormone autonomously
• FA histology shows neoplastic thyroid follicles encapsulated by a surrounding collagen cuff

Clinical features

• Discrete solitary mass in anotherwise normal thyroid gland
• May be incidental finding
• Patients with larger tumours may present with local symptoms e.g. dysphagia

Investigations

• Ultrasound scan
• FNA: cannot distinguish between follicular adenoma and follicular carcinoma