Thyroid Adenoma

Epidemiology

Thyroid adenomas are classified based on their histology as papillary, follicular, and Hürthle cell. The most common benign thyroid neoplasm is the follicular adenoma. Thyroid adenomas also tend to be divided based on their hormonal activity. Most adenomas (90–95%) do not produce significant quantities of thyroid hormones. Solitary tumors are estimated to occur in 1 to 3% of the adult population. The incidence is more common in females (2:1). The incidence of thyroid nodules is approximately 0.1%/yr. In general, it is estimated that between 3 and 4% of patients who develop a solitary nodule have thyroid carcinoma.

Clinical Findings

Patients with nonhormonally active adenomas usually present with asymptomatic masses that are initially discovered on routine physical exam. Tumors typically become palpable when they reach 1 cm, but on occasion may reach 5 to 10 cm without being noticed.

Most functioning adenomas are quiescent and are incidentally found on clinical examination. Functioning adenomas that are > 3 cm tend to result in thyrotoxicity.

Pathology

True adenomas are encapsulated and compress adjacent tissue. Follicular adenoma varies in size from microscopic to 8 to 10 cm. These are composed of normal-appearing thyroid epithelium arranged in a follicular structure. The follicles may be very small with little colloid (fetal adenoma or microfollicular adenoma) to large distended structures (macrofollicular adenoma). An embryonal adenoma is a more primitive-appearing structure possessing very little colloid. Hürthle cell adenomas (oxyphil adenomas) are tumors composed of oxyphils. Hypercellular adenomas are difficult to differentiate from follicular carcinomas on needle biopsy.

Treatment