Endocrine · Year 2 · from Endocrine

Case 3: Follicular Thyroid Carcinoma

Patient Demographics

  • Age: 55 years
  • Sex: Female
  • Occupation: Retired teacher

Chief Complaint

"My doctor found a thyroid nodule on a CT scan done for something else."

History of Present Illness

A 55-year-old woman had a CT scan of her chest performed for evaluation of chronic cough, which incidentally revealed a thyroid nodule. She is asymptomatic with no neck mass, dysphagia, hoarseness, or symptoms of thyroid dysfunction. She has no history of head/neck radiation and no family history of thyroid cancer. She has had the cough for 3 months, attributed to post-nasal drip.

Physical Examination

  • Vital Signs: Normal
  • General: Well-appearing woman in no distress
  • Neck:
  • 2.5 cm smooth, firm nodule in left thyroid lobe
  • Mobile with swallowing
  • No cervical lymphadenopathy
  • Pulmonary: Clear to auscultation

Workup

  • Laboratory Studies:
  • TSH: 3.4 mIU/L (normal)
  • Ultrasound Thyroid:
  • Left lobe: 2.8 cm isoechoic to slightly hypoechoic nodule
  • Smooth margins, no calcifications
  • Increased peripheral vascularity ("halo" sign)
  • TI-RADS 4 (moderately suspicious)
  • FNA Cytology:
  • Bethesda IV - Follicular neoplasm/suspicious for follicular neoplasm
  • "Microfollicular pattern, scant colloid"
  • Molecular Testing (Afirma, ThyroSeq): Suspicious - cannot rule out malignancy

Diagnosis

Follicular neoplasm - cannot distinguish benign follicular adenoma from follicular thyroid carcinoma on cytology; requires surgical pathology

Treatment

  1. Diagnostic surgery:
  • Thyroid lobectomy (hemithyroidectomy) initially
  • Final pathology determines if benign adenoma vs. carcinoma
  1. If follicular carcinoma confirmed on final pathology:
  • Completion thyroidectomy (removal of remaining lobe)
  • Radioactive iodine ablation for intermediate/high-risk features
  • TSH suppression with levothyroxine
  1. If benign follicular adenoma:
  • No further surgery needed
  • Levothyroxine only if hypothyroid post-lobectomy

Post-operative pathology result: Follicular thyroid carcinoma with capsular and vascular invasion (minimally invasive)

Final Treatment

  • Completion thyroidectomy performed
  • Low-dose radioactive iodine ablation
  • Levothyroxine with TSH suppression to 0.5-2.0 mIU/L (low-risk after complete resection)
  • Surveillance with thyroglobulin, neck ultrasound

Clinical Pearl

Follicular thyroid carcinoma is the second most common thyroid malignancy (10-15% of cases). Unlike papillary carcinoma, it spreads hematogenously rather than via lymphatics, with common metastases to bone and lung. The critical diagnostic challenge is that FNA cannot distinguish follicular adenoma from carcinoma - this distinction requires demonstrating capsular or vascular invasion on surgical pathology. The Bethesda System category IV ("Follicular neoplasm") has a 15-30% malignancy rate, making diagnostic surgery necessary. Molecular testing can help stratify risk but cannot definitively rule out malignancy in most cases.

Clinical Image

Histopathology of follicular thyroid carcinoma demonstrating capsular invasion, the key feature distinguishing it from benign follicular adenoma.

Image Source: Wikimedia Commons - "Follicular thyroid carcinoma" License: CC BY-SA 3.0 URL: https://commons.wikimedia.org/wiki/File:Follicular_thyroid_carcinoma_-_capsular_invasion.jpg

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