Thyroid nodule classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mahshid Mir, M.D. [2]

Bethesda System for Reporting Thyroid Cytopathology

• 19888858

classification	FNA cytology	Predicted risk of malignancy
Benign	Macrofollicular Adenomatoid/hyperplastic nodules Colloid adenomas (most common) Nodular goiter Lymphocytic thyroiditis Granulomatous thyroiditis	0–3 %
Follicular lesion of undetermined significance	Mixed macro- and microfollicular nodules	5–15 %
Atypia of undetermined significance	Atypical cells
Follicular neoplasm	Microfollicular nodules Hurthle cell lesions	15–30 %
Suspicious for a follicular neoplasm	Suspicious for Hurthle cell neoplasm
Malignant	PTC (most common) MTC Anaplastic carcinoma High-grade metastatic cancers	97–99 %

The risk of malignancy development based on the FNA result in case of indefinite diagnosis are as below:

• Nondiagnostic or Unsatisfactory: 1–4 % predicted risk of malignancy.
• Suspicious for malignancy: 60–75 % predicted risk of malignancy

Thyroid carcinomas are classified according to the cell type from which they develop

Classification of neoplastic thyroid nodules based on their origin:

		Origin	histologic subtypes
Nonmedullary thyroid cancers (NMTCs)	95% of tumors	thyroid epithelial cells	papillary (85%)	95% are sporadic tumors 5% may be related to inherited genetics due to familial origin
			follicular (11%)
			Hürthle cell (3%)
			anaplastic (1%)
Medullary thyroid cancers (MTCs)	5% of all thyroid malignancies	calcitonin-producing parafollicular cells		20% they are familial and occur as part of the multiple endocrine neoplasia (MEN) syndromes

Follicular thyroid lesions	Benign follicular adenoma
	Minimally invasive follicular carcinoma
	Widely invasive follicular carcinoma
	Encapsulated follicular variant of papillary thyroid cancer
	Infiltrative variant of papillary thyroid cancer