Incidental thyroid nodules, sometimes called thyroid incidentalomas, are discrete lesions in the thyroid gland found on cross-sectional imaging performed for indications other than thyroid evaluation. They are common but occasionally represent thyroid cancer 1. This article discusses the epidemiology of incidental thyroid nodules and recommendations for further evaluation.
About 5% of the population has a palpable thyroid nodule, while autopsy rates approach 50% (often multiple nodules) 2. As imaging techniques improve, the detection of incidental thyroid nodules has increased, moving closer to pathologists' pickup rate rather than the bedside clinicians' 3.
Incidental thyroid nodules are identified at different rates depending on the modality:
- extrathyroidal ultrasound (e.g. of the carotid arteries): 67% 5,6
- CT (e.g. of the neck, cervical spine, or chest): 25% 7,8
- MRI: 18% 8
- 18FDG-PET: <2% 9
Risk of malignancy
Despite the lower detection rate, around one-third of 18FDG-PET-positive nodules will be malignant 9. Rates of malignancy from other cross-sectional modalities (CT/MRI) only reach 12% in some studies 8,11. Ultrasound is lower still at <2% malignancy in a population-based study, illustrating the high proportion of benign nodules 12.
Multinodular goitres can have similar malignancy rates as solitary thyroid nodules 1.
Thyroid nodules in younger patients have a higher risk of malignancy.
The incidence of thyroid cancer has increased in recent decades without a change in thyroid cancer mortality rates, suggesting an "epidemic of diagnosis" 4.
There is variable practice in radiologists' reporting of incidental thyroid nodules 8,10. Professional organizations have developed recommendations in recent years to aid radiologists and other clinicians in their management.
Incidental thyroid nodules on CT/MRI/PET
CT, MRI, PET, and other nuclear medicine scans lack the spatial resolution to characterize suspicious features and do not perfectly agree with the size and number of nodules found on US 16. Size on CT or MRI, however, can be used to stratify need for evaluation.
American Thyroid Association recommendations
The American Thyroid Association published its guidelines on thyroid nodule management in 2015, recommending thyroid US for all patients in whom another imaging study suggested the presence of a thyroid nodule 13. The guidelines also state separately that in general, only nodules >1 cm merit evaluation.
With the finding of a thyroid nodule >1 cm, a serum thyroid-stimulating hormone (TSH, thyrotropin) level should also be obtained. If the level is low, a radionuclide thyroid scan should be obtained.
The findings of these tests then help guide the need for fine needle aspiration, the most accurate method of evaluating thyroid nodules.
American College of Radiology recommendations
The American College of Radiology Incidental Thyroid Findings Committee published a white paper in 2015 with recommendations based on the so-called 3-tiered system initially developed at Duke University 14. A nodule meeting any of the following criteria is recommended for further evaluation by thyroid US:
- focal thyroid uptake on FDG-PET or other nuclear medicine scans (e.g. octreotide, sestamibi)
- thyroid nodule with local tissue invasion
- thyroid nodule with suspicious lymph nodes (enlarged, cystic, calcified, or hyperenhancing)
- thyroid nodule ≥1 cm (in axial plane) in patients <35 years old
- thyroid nodule ≥1.5 cm (in axial plane) in patients ≥35 years old
When multiple thyroid nodules are present, the criteria apply to the largest nodule.
When the initial finding is a FDG-avid focus on PET, fine needle aspiration is recommended regardless of the ultrasound findings.
Patients who have limited life expectancy or serious comorbidities (that increase the risk of treatment or pose more risk to the patient than possible thyroid cancer) should not undergo further evaluation of incidental nodules lacking suspicion for local invasion or nodal metastases unless the patient or referring physician specifically requests so.
The Duke 3-tiered system has been found to reduce the number of incidental thyroid nodules requiring biopsy by one-third to nearly one-half compared with standard practices, with a false negative rate of 13% 17-19.
Incidental thyroid nodules on extrathyroidal ultrasound
Per the American College of Radiology White Paper recommendations, the same size cutoffs used for nonsonographic imaging also apply for extrathyroidal ultrasound:
- ≥1.0 cm for age <35 years
- ≥1.5 cm for age ≥35 years.
Dedicated thyroid ultrasound would also be indicated if suspicious features happen to be captured on the nonthyroidal ultrasound, such as
- marked hypoechogenicity
- lobulated/irregular margins
- taller-than-wide shape
The list of suspicious sonographic features been updated in the ACR TI-RADS 2017 publication 15.
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