Follicular thyroid cancer (FTC) differs from other thyroid gland malignancies in that the diagnosis cannot be established by fine-needle aspiration (FNA). Patients typically present with a painless thyroid nodule (a small knotlike protuberance) that FNA categorizes as a follicular neoplasm (tumor).
Follicular neoplasms account for 15% to 20% of thyroid nodules; of these, 80% to 85% are adenomas (benign) and 15% to 20% are carcinomas (malignant).
FTC occurs more commonly in middle-aged women (a female-to-male ratio of approximately 3:1). Unlike other thyroid cancers, FTC infrequently spreads to the lymph nodes (5%).
But at the time of their initial diagnosis, up to 33% of patients may have distant metastasis (spread of the cancer) to the bones, lung, liver, or brain.
- Ultrasound of the neck (thyroid and lymph node basins)
- FNA of the thyroid nodule and suspicious lymph nodes (even though the risk of lymph node metastasis is only 5%)
- Thyroid function test and determination of thyroglobulin (an iodine-containing protein) level
Surgery is the only means to establish a definitive diagnosis. Since most (80% to 85%) of these tumors are benign), a thyroid lobectomy (surgical removal of a lobe) on the affected side can be performed instead of a total thyroidectomy (surgical removal of the entire thyroid gland), depending on the associated risk factors and on patient preference:
Total thyroidectomy is the preferred operation when patients have high risk factors for having thyroid cancer, including evidence of local tumor extension or distant metastasis, history of head and neck radiation, and family history of thyroid cancer.
Nodules on the contralateral (opposite side) thyroid lobe and patient preference are also considered for this option.
Thyroid lobectomy is performed in patients without the above risk factors and who have a single dominant thyroid nodule or prefer this option.
NOTE: If final pathologic examination shows carcinoma, then a completion thyroidectomy is necessary.
After surgery, final pathologic examination of the tumor will classify the tumor into a follicular adenoma (benign) or carcinoma (malignant). FTC is further classified into 2 main groups:
(1) minimally invasive FTC with microscopic features of capsular or vascular invasion or
(2) widely invasive FTC. Patients with minimally invasive FTC have a better prognosis.
Pathologic examination might reveal that the patient has the follicular variant of papillary thyroid cancer (PTC). The behavior of this follicular variant is similar to that of PTC and should be treated in a similar fashion.
Another pathologic finding might be that the patient has an insular type of FTC that behaves more aggressively than FTC usually does.
These 2 postoperative treatments are recommended for most patients with FTC:
Thyroid-stimulating hormone (TSH) is secreted by the pituitary gland and normally controls thyroid growth and hormone secretion. However, TSH also acts as a tumor growth factor. For this reason, patients with FTC need to suppress TSH by taking thyroxine (levothyroxine or liothyronine).
The target TSH level is typically 0.1 to 0.4 mU/mL. But in patients with aggressive tumors or distant metastases, the target TSH level is < 0.1.
Radioactive Iodine (131)
Radioactive iodine is typically recommended for all patients. However, patients younger than 30 with small, minimally invasive tumors may not require 131I.
Before beginning treatment with 131I, any remnant thyroid tissue should be ablated (destroyed) with radioactive iodine.
Characteristics of Follicular Thyroid Cancer
- Peak onset of follicular thyroid cancer is between ages 40 and 60 years old.
- Follicular thyroid cancer is more common in femails than males by 3:1 ratio.
- The prognosis directly related to tumor size (less than 1.0 cm [3/8 inch] is a good prognosis).
- This cancer is rarely associated with radiation exposure.
- Cancer that spreads to lymph nodes is uncommon (~10%) in follicular thyroid cancer.
- Invasion into vascular structures (veins and arteries) within the thyroid gland is common.
- Distant spread (to lungs or bones) is uncommon, but it is more common than with papillary cancer.
- Overall cure rate is high (near 95% for small lesions in young patients), but this decreases with age.
Management of Follicular Thyroid Cancer
Thyroid gland has 2 lobes connected by an isthmus.Considerable controversy exists when discussing the management of well-differentiated thyroid carcinomas (papillary and even follicular).
Some experts contend than if these tumors are small and not invading other tissues (the usual case), then simply removing the lobe of the thyroid that harbors the tumor (and the small central portion called the isthmus) will provide as good a chance of cure as removing the entire thyroid.
These proponents of conservative surgical therapy relate the low rate of clinical tumor recurrence despite the fact that small amounts of tumor cells can be found in up to 88% of the opposite lobe thyroid tissues.
The other side of the controversy is a total thyroidectomy. This is a more aggressive surgery.
But what are some common follicular thyroid cancer treatments? The following is a typical plan for treating follicular thyroid cancer: Follicular carcinomas that are well circumscribed, isolated, minimally invasive, and less than 1 cm in a young patient (younger than 40 years old) may be treated with hemi-thyroidectomy and isthmusthectomy.
All others should probably be treated with total thyroidectomy and removal of any enlarged lymph nodes in the central or lateral neck areas.
More detailed information on the different thyroid operations are included on our surgical options article.
Radioactive Iodine for Follicular Thyroid Cancer
Thyroid cells are unique in that they have the cellular mechanism to absorb iodine. The iodine is used by thyroid cells to make thyroid hormone.
No other cell in the body can absorb or concentrate iodine. Physicians can take advantage of this fact and give radioactive iodine to patients with thyroid cancer.
There are several types of radioactive iodine, with one type being toxic to cells. Follicular cancer cells absorb iodine (although to a lesser degree in older patients) and therefore, they can be targeted by giving the toxic isotope (I-131).
Once again, not everybody with follicular thyroid cancer needs this therapy, but those with larger tumors, spread to lymph nodes or other areas, tumors that appear aggressive microscopically, tumors that invade blood vessels within the thyroid, and older patients may benefit from this therapy.
This is extremely individualized, and your doctor will make the best recommedation for your case. But this is an extremely effective type of “chemotherapy” with few potential down sides (no hair loss, nausea, weight loss, etc).
Uptake is enhanced by high thyroid-stimulating hormone (TSH) levels; thus patients should be off of thyroid replacement and on a low iodine diet for at least 1 to 2 weeks prior to therapy.
It is usually given 6 weeks post-surgery (although this can vary) and can be repeated every 6 months if necessary (within certain dose limits).
Thyroid Hormone Replacement for Follicular Thyroid Cancer
Regardless of whether a patient has just one thyroid lobe and the isthmus removed, or the entire thyroid gland removed with a total thyroidectomy, most experts agree these patients should be placed on thyroid hormone for the rest of their lives.
This is to replace the missing hormone in those who have had the thyroid gland removed and to suppress further growth of the gland in those with some tissue left in the neck.
There is good evidence that follicular carcinoma (such as papillary cancer) responds to TSH secreted by the pituitary, therefore, exogenous thyroid hormone is given. This results in decreased TSH levels and a lower impetus for any remaining cancer cells to grow.
What Kind of Long-term Follow-up Is Necessary?
In addition to the usual cancer follow-up, patients should receive a yearly chest x-ray, as well as a check of thyroglobulin levels.
Thyroglobulin is not useful as a screen for initial diagnosis of thyroid cancer, but it is useful in follow up of well-differentiated carcinoma (if a total thyroidectomy has been performed).
A high serum thyroglobulin level that had previously been low following total thyroidectomy, especially if gradually increased with TSH stimulation, is indicative of recurrence.
A value of greater than 10 ng/ml is often associated with recurrence even if an iodine scan is negative.
All patients require strict and consistent follow-up. Initially, they should undergo an ultrasound of the neck and a serum thyroglobulin level check every 6 months, then yearly if there is no evidence of recurrence.
FTC confers a slightly worse prognosis than PTC. For FTC patients, the average overall 10-year survival rate is 85% (range, 43% to 95%).