Follicular Thyroid Carcinoma – Symptoms and Treatment

What is Follicular Thyroid Carcinoma?

Follicular thyroid carcinoma (FTC) is a malignant tumor arising from the follicular epithelial cells of the thyroid gland – the same cells responsible for producing thyroid hormones. It is the second most common type of thyroid cancer after papillary thyroid carcinoma, accounting for approximately 10–15% of all thyroid malignancies. As a differentiated thyroid cancer, its cells still resemble normal thyroid tissue, which allows for targeted and effective treatment strategies.

Causes and Risk Factors

The exact causes of follicular thyroid carcinoma are not yet fully understood. However, several established risk factors may contribute to its development:

• Iodine deficiency: Chronic iodine deficiency is considered a significant risk factor, as it leads to prolonged stimulation of the thyroid gland.
• Radiation exposure: Exposure to ionizing radiation, particularly during childhood or adolescence, increases the risk of developing thyroid cancer.
• Genetic mutations: Specific genetic alterations, including RAS gene mutations and PAX8/PPARgamma rearrangements, are commonly associated with follicular thyroid carcinoma.
• Pre-existing thyroid conditions: Benign thyroid nodules (adenomas) or goiter may, in rare cases, undergo malignant transformation.
• Sex and age: Women are more frequently affected than men, with a peak incidence between the ages of 40 and 60.

Symptoms

In its early stages, follicular thyroid carcinoma is often asymptomatic and may be discovered incidentally during a neck ultrasound. As the tumor progresses, the following symptoms may appear:

• A palpable or visible lump in the neck
• Difficulty swallowing or a sensation of pressure in the throat
• Hoarseness caused by pressure on the recurrent laryngeal nerve
• Breathing difficulties in cases of larger tumors
• Bone pain or pathological fractures due to distant metastases (most commonly to bone or lung)

A characteristic feature of follicular thyroid carcinoma is its tendency for hematogenous spread – meaning it metastasizes via the bloodstream to distant organs such as the lungs, bones, or liver, rather than through the lymphatic system.

Diagnosis

The diagnosis of follicular thyroid carcinoma typically involves several steps:

Imaging

• Thyroid ultrasound: The primary imaging modality for evaluating thyroid nodules; suspicious features are classified using the TIRADS system.
• Thyroid scintigraphy: Assesses the functional activity of a nodule; cold (non-functioning) nodules are considered more suspicious.
• CT or MRI: Used when metastases are suspected or to assess the local extent of the tumor.

Laboratory Tests

• TSH, fT3, fT4: To assess overall thyroid function
• Thyroglobulin (Tg): A tumor marker used for follow-up monitoring after thyroid surgery
• Calcitonin: To exclude medullary thyroid carcinoma

Fine Needle Aspiration (FNA)

Fine needle aspiration cytology is used to sample cells from suspicious nodules. Importantly, follicular carcinoma cannot be reliably distinguished from a benign follicular adenoma by cytology alone. A definitive diagnosis requires histological examination of the surgically removed specimen, specifically looking for evidence of capsular or vascular invasion.

Treatment

Surgery

Surgical removal of the thyroid gland (thyroidectomy) is the primary treatment. In most cases, a total thyroidectomy is performed. For small, low-risk tumors, a hemithyroidectomy (removal of one lobe) may be sufficient.

Radioiodine Therapy

Following surgery, many patients receive radioiodine therapy (RAI) using radioactive iodine (I-131). This treatment destroys residual thyroid tissue and potential metastases, as follicular carcinoma cells often retain the ability to absorb iodine.

Hormone Therapy (TSH Suppression)

Long-term treatment with levothyroxine (L-thyroxine) is required after surgery to replace missing thyroid hormones and to suppress TSH levels. Keeping TSH low reduces the stimulus for growth of any remaining tumor cells.

Systemic Therapy for Advanced Disease

For patients with metastatic or inoperable tumors that no longer respond to radioiodine, targeted therapies such as tyrosine kinase inhibitors (e.g., sorafenib or lenvatinib) are used to control tumor growth.

Prognosis

Follicular thyroid carcinoma generally carries a favorable prognosis. The 10-year survival rate for localized tumors exceeds 90%. When distant metastases are present, the prognosis is less favorable but remains relatively good compared to many other cancers. Regular follow-up care – including thyroglobulin measurements and imaging – is essential for long-term monitoring.

References

1. Haugen BR et al. – 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid. 2016;26(1):1–133.
2. Schlumberger M et al. – Lenvatinib versus Placebo in Radioiodine-Refractory Thyroid Cancer. New England Journal of Medicine. 2015;372(7):621–630.
3. World Health Organization (WHO) – Classification of Tumours: Endocrine and Neuroendocrine Tumours, 5th Edition. IARC Press, Lyon, 2022.