squamous metaplasia in the thyroid gland

Squamous metaplasia (SM) occurs in a variety of thyroid conditions, both neoplastic and non-neoplastic.

In a small subset of benign thyroid lesions, SM can create a diagnostic pitfall by mimicking a malignant process.

SM of the thyroid represents an important diagnostic pitfall for the cytopathologist, and should be included in the differential diagnosis of thyroid nodules, particularly those with potentially benign cystic changes.

Squamous epithelium in the thyroid, which may be basaloid, epidermoid, or truly squamous, may be derived from several sources.

In the normal thyroid, usually rests of one type or another are responsible for these cell nests. Thus, thymic or ultimobranchial remanants may be found in the lateral lobes of normal thyroid glands.

Maldevelopment or excessive descent of thyro-glossal duct remnants may be responsible for squamous epithelial cysts noted in the central portions of the thymus, especially in the isthmus.

In diseased thyroids, the most likely derivation of squamous or epidermoid epithelium is through metaplasia of follicular epithelial cells.

Thus, in adenomatous goiter and various types of thyroiditis as well as follicular derived neoplasms, such a derivation is most likely.

In tumors containing malignant squamous epithelium (adenoacanthoma, adenosquamous carcinoma, or pure squamous carcinoma), there is a range of such lesions, and since many of them appear to be derived from follicular epithelium, it is most likely that squamous metaplasia which has undergone malignant transformation has occurred in these neoplasms.

Associations

 Hashimoto thyroiditis (2915541)

 thyroid carcinomas (2481935)

• Squamous metaplasia in the thyroid gland is usually seen in association with a pathological lesion such as mucoepidermoid carcinoma, squamous cell carcinoma and diffuse sclerosing variant of papillary thyroid carcinoma.
• Squamous differentiation can arise in thyroid carcinomas.
• Tumor cell nests with squamous differentiation (CNSD) are most probably due to extensive squamous differentiation (squamous metaplasia) in undifferentiated carcinoma and papillary carcinoma.
• Cases diagnosed solely as squamous cell carcinoma sometimes show a prognosis similar to that of undifferentiated carcinoma.
• They may well represent extensive squamous differentiation in such tumors rather than true squamous cell carcinoma of the thyroid.

Paywall references

 Atypical squamous metaplasia in a benign cystic thyroid nodule mimicking high-grade carcinoma.
Pellicer DL, Sadow PM, Stephen A, Faquin WC.
Diagn Cytopathol. 2013 Aug;41(8):706-9. doi : 10.1002/dc.22803
PMID: 22144088

 Squamous differentiation in thyroid carcinoma. With special reference to histogenesis of squamous cell carcinoma of the thyroid.
Katoh R, Sakamoto A, Kasai N, Yagawa K.
Acta Pathol Jpn. 1989 May;39(5):306-12.
PMID: 2481935

 Squamous cells in the human thyroid gland.
LiVolsi VA, Merino MJ.
Am J Surg Pathol. 1978 Jun;2(2):133-40.
PMID: 655339