Thyroid parenchymal expansion can result from diffuse enlargement or infiltration of the thyroid gland or from the presence of one or more thyroid nodules. A thyroid nodule is a discrete lesion distinct from the surrounding thyroid parenchyma. Enlargement of other nearby anatomical structures, such as the parathyroid glands or regional lymph nodes, as well as branchial cleft and thyroglossal duct cysts, may sometimes be confused with thyroid nodules. Thyroid nodules may be palpable at presentation or may be incidentally detected during an imaging procedure (40% are self-identified, 30% are physician-identified, and 30% are incidentally discovered on imaging  ). Incidental detection, in addition to the use of thyroid ultrasonography for surveillance, is thought to be behind an increased reported incidence of thyroid cancers throughout the industrialised world. Thyroid ultrasonography and autopsy studies detect nodules in over half of the general population by the fifth or sixth decades of life. 
Most nodules are benign and only 5% to 12% of the nodules detected on ultrasonography are malignant.   Most thyroid nodules, including thyroid cancers, are asymptomatic. The clinical conundrum in the evaluation of the thyroid nodule is differentiating a benign from a malignant lesion. Management of thyroid nodules is based on sonographic characteristics and cytology from fine needle aspiration biopsy (FNA). Molecular biomarkers are being increasingly used to enhance the diagnostic clarity from the FNA in cases of follicular neoplasms and follicular lesions of unknown significance (FLUS).
The concept of risk stratification of patients presenting with thyroid nodules that end in a malignant cytological diagnosis is key to understanding the management approach for thyroid cancer.  It serves as an objective way of prescribing the extent of surgical resection to thyroid cancer patients, and has been illustrated based upon an escalating scale of known risks.
Based upon experience in Japan, there is a movement to offer observation of thyroid nodules (biopsy-proven papillary cancers or ultrasonographically suspicious nodules prior to biopsy) to selected patients pending tumour/neck ultrasound characteristics (e.g., primary tumour size and location within the thyroid), individual circumstance (e.g., willingness to accept surveillance, age, comorbid conditions), and the experience of the multidisciplinary clinical team.  
Surgical Affairs Committee
American Thyroid Association
Department of Otolaryngology-Head and Neck Surgery
University of Arkansas for Medical Sciences (UAMS)
UAMS Thyroid Center
BCS is an author of a number of references cited in this monograph.
Dr Brendan C. Stack would like to gratefully acknowledge Dr Steven P. Hodak and Dr Hussain Mahmud, the previous contributors to this monograph. SPH and HM declare that they have no competing interests.
Assistant Professor of Medicine
Division of Endocrinology
Drexel University College of Medicine
REA declares that she has no competing interests.
Guy’s & St. Thomas’ NHS Foundation Trust
PVC declares that he has no competing interests.
Consultant Endocrine Surgeon
Newcastle Upon Tyne Hospitals NHS Trust
RB declares that he has no competing interests.
Clinical Associate Professor
Thyroid Cancer Program
Stanford University Medical Center
HA declares that she has no competing interests.
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