Role of Fine Needle Aspiration Cytology (FNAC) in Hyperthyroidism
FNAC is not a primary diagnostic tool for hyperthyroidism but is essential for evaluating thyroid nodules that may be present in hyperthyroid patients to rule out malignancy. 1
Diagnostic Approach for Thyroid Nodules in Hyperthyroid Patients
- FNAC is recommended as the first diagnostic test for evaluating suspicious thyroid nodules in patients who are clinically euthyroid or hyperthyroid, ideally performed before other imaging studies 1
- Ultrasound of the thyroid and central neck should accompany FNAC for comprehensive evaluation 1
- Ideally, serum TSH results should be known before FNAC is performed, though this may not always be practical in clinical settings 1
Indications for FNAC in Hyperthyroid Patients with Nodules
- FNAC should be performed in any thyroid nodule >1 cm 1
- For nodules <1 cm, FNAC is indicated if there are suspicious clinical features:
- History of head and neck irradiation 1
- Family history of thyroid cancer 1
- Suspicious features on palpation (firm, fixed to adjacent structures, rapidly growing) 1
- Presence of cervical lymphadenopathy 1
- Suspicious ultrasound features (hypoechogenicity, microcalcifications, absence of peripheral halo, irregular borders, solid aspect, intranodular blood flow, taller-than-wide shape) 1
Diagnostic Value of FNAC
- FNAC is a very sensitive tool for differentiating between benign and malignant nodules, though it has limitations 1, 2
- The overall sensitivity of FNAC for detecting thyroid neoplasia ranges from 55-98%, with specificity between 30.9-84% depending on how cytologic positivity is defined 2, 3
- Ultrasound-guided FNAC has a significantly lower non-diagnostic rate compared to freehand FNAC 2
Limitations of FNAC in Hyperthyroid Context
- Two major limitations of FNAC are inadequate samples and follicular neoplasia 1, 4
- In cases of inadequate samples, FNAC should be repeated 1
- For follicular neoplasia with normal TSH and "cold" appearance on thyroid scan, surgery should be considered 1
- False-negative results can occur (primarily due to sampling errors), so a reassuring FNAC should not override concerns when clinical findings are worrisome 1, 3
Cytologic Examination Categories
FNAC results are typically categorized as:
- Carcinoma (papillary, medullary, or anaplastic) or suspicious for malignancy 1
- Follicular or Hürthle cell neoplasm 1
- Follicular lesion of undetermined significance 1
- Thyroid lymphoma 1
- Benign (nodular goiter, colloid goiter, hyperplastic/adenomatoid nodule, Hashimoto's thyroiditis) 1
- Insufficient biopsy (nondiagnostic) 1
Special Considerations
- Concurrent Hashimoto thyroiditis does not appear to influence the accuracy of cytopathological diagnosis of nodules or predispose FNAC to be non-diagnostic or indeterminate 5
- Multiple passes should be performed in various parts of a large nodule or from different nodules to reduce the risk of false-negative findings 3
- Pathology and cytopathology slides should be reviewed at the treating institution by a pathologist with expertise in diagnosing thyroid disorders 1
- For indeterminate thyroid nodules, repeating FNAC may not be useful in most cases, particularly for older patients with nodules lacking suspicious characteristics 6
Pitfalls to Avoid
- Relying solely on FNAC without considering clinical context and ultrasound findings 1
- Overlooking that certain conditions can mimic other pathologies (Hürthle cell neoplasms can mimic medullary carcinoma; anaplastic thyroid cancer can be difficult to distinguish from other primary thyroid malignancies) 1
- Failure to perform multiple passes in different areas of large nodules, which can lead to sampling errors and false-negative results 3
- Over-treatment based on indeterminate cytology without considering patient factors and nodule characteristics 4, 6