Follicular thyroid cancer is the second most common type of thyroid cancer, accounting for roughly 10 to 15 percent of all cases. Despite being a malignancy, this form of cancer is highly treatable and often curable, especially when detected early. The thyroid gland, located at the base of your neck, produces vital hormones that control how your body uses energy and manages important functions like heart rate and temperature regulation.
Epidemiology
Follicular thyroid cancer represents between 10 and 15 percent of all thyroid cancers diagnosed in the United States. In comparison, papillary thyroid cancer, the most prevalent type, makes up approximately 70 to 80 percent of thyroid cancer cases. This places follicular thyroid cancer firmly in second place among thyroid malignancies.[1][2]
The disease shows a clear demographic pattern. Women are affected significantly more often than men, with a ratio estimated at 3 to 1. The cancer typically occurs in older individuals compared to papillary thyroid cancer, with peak onset between the ages of 40 and 60 years. While anyone can develop follicular thyroid cancer, it appears less frequently in children and is only rarely linked to radiation exposure, which is a known risk factor for papillary thyroid cancer.[2][3]
Globally, the incidence of thyroid cancer has been rising since the 1970s, largely due to improved detection methods such as ultrasound, CT scans, and MRI technology. Thyroid cancer has become the fifth most common cancer in women, though overall survival rates remain exceptionally high, with five-year survival reaching 98 percent across all types.[15]
Causes
The exact cause of follicular thyroid cancer is not fully understood. The disease occurs when cells in the thyroid gland begin to grow abnormally and uncontrollably. These cells, which normally produce thyroid hormones in tiny sac-like structures called follicles, develop changes in their DNA that cause them to multiply and form tumors.[1]
Researchers have identified several genetic mutations that appear in follicular thyroid cancer cells. About half of all follicular carcinoma cases show mutations in what are called RAS genes, which include HRAS, NRAS, and KRAS. Another one-third of cases show a specific genetic rearrangement involving two genes called PAX8 and PPAR-gamma. Only about three percent of cases show both types of genetic changes.[3][7]
Unlike papillary thyroid cancer, follicular thyroid cancer occurs only rarely after exposure to radiation. This is an important distinction because radiation exposure has historically been a significant risk factor for other thyroid cancers, particularly following events like the Chornobyl nuclear disaster in 1986.[2][3]
Follicular thyroid cancer is not contagious and cannot be spread from one person to another through any form of contact.[1]
Risk Factors
Several factors may increase your chances of developing follicular thyroid cancer, though having one or more of these risk factors does not mean you will definitely get the disease.
Iodine deficiency in your diet is considered a risk factor. Iodine is a mineral that the thyroid gland needs to produce hormones. In areas where iodine intake is low, the risk of follicular thyroid cancer may be higher.[3][15]
Exposure to radiation, particularly to the head, neck, or chest area, can increase risk. This might occur through radiation therapy used to treat other cancers such as lymphoma or breast cancer, or through exposure during nuclear disasters. However, it’s important to note that this connection is much weaker for follicular thyroid cancer than for papillary thyroid cancer.[2][15]
Other potential risk factors include obesity, diabetes, and certain thyroid conditions like Hashimoto thyroiditis. A strong family history of thyroid cancer or inherited genetic syndromes associated with thyroid cancer may also increase risk. Some research suggests that exogenous estrogen use and certain dietary choices may play a role, though more research is needed to understand these connections fully.[3][15]
Symptoms
Many people with follicular thyroid cancer do not experience any symptoms, especially in the early stages of the disease. The cancer may be discovered during a routine physical examination when a doctor feels the neck area, or it may be detected incidentally during imaging tests performed for other reasons.[1]
When symptoms do occur, the most common sign is a lump in the neck, known as a thyroid nodule. This lump forms in the thyroid gland and may be felt during self-examination or by a healthcare provider. It’s important to know that thyroid nodules are quite common, and most are not cancerous. Only a small number of thyroid nodules turn out to be cancer.[1][4]
Other potential symptoms include pain in the ear, jaw, or neck. Some people experience hoarseness, which is a change in voice quality that makes it sound breathy or strained. This medical term is called dysphonia. You might also notice swollen lymph nodes in your neck, which are small bean-shaped structures that are part of your immune system.[1][4]
As the cancer grows, it may cause difficulty breathing or trouble swallowing, a condition called dysphagia. In some cases, the thyroid gland may become visibly enlarged, creating a noticeable swelling at the base of the neck called a goiter.[1][4]
Prevention
There is no guaranteed way to prevent follicular thyroid cancer, as the exact causes are not fully understood. However, certain measures may help reduce your risk or lead to earlier detection when the cancer is most treatable.
Ensuring adequate iodine intake through your diet or iodized salt may help, particularly if you live in an area where iodine deficiency is common. Iodine is essential for proper thyroid function, and deficiency has been linked to an increased risk of follicular thyroid cancer.[3]
If you have had radiation exposure to your head, neck, or chest area in the past, particularly from medical treatments, it’s important to inform your healthcare provider. They may recommend more frequent monitoring of your thyroid gland. Avoiding unnecessary radiation exposure, especially during childhood, is also prudent.[3]
If you have a family history of thyroid cancer or certain genetic syndromes associated with thyroid disease, genetic counseling and testing may be beneficial. This can help identify whether you carry genetic mutations that increase your risk and allow for early surveillance strategies.[3]
Regular medical check-ups can help with early detection. During routine physical examinations, your doctor can check your neck for lumps or enlargement of the thyroid gland. If you notice any changes in your neck, such as a lump or swelling, or if you experience symptoms like hoarseness or difficulty swallowing that persist, seek medical attention promptly.[1]
Pathophysiology
Follicular thyroid cancer develops from the follicular cells of the thyroid gland. These are specialized epithelial cells that line the follicles, which are the functional and structural units of the thyroid. Under normal circumstances, these cells can be cuboidal or columnar in shape, depending on how active they are in producing thyroid hormones.[3]
In follicular thyroid cancer, genetic mutations cause these cells to grow abnormally and form tumors. The cancer cells resemble normal thyroid follicles when examined under a microscope, which is why this cancer is classified as “well-differentiated.” Well-differentiated means the cancer cells still look somewhat like the normal cells they came from, which generally indicates a better prognosis and response to treatment.[1][3]
A key characteristic that distinguishes follicular thyroid cancer from benign follicular adenomas is the presence of capsular invasion or vascular invasion. Capsular invasion means the cancer cells have broken through the fibrous capsule that normally surrounds a thyroid nodule. Vascular invasion means cancer cells have invaded blood vessels within or around the thyroid. These features can only be determined by examining tissue under a microscope after it has been removed during surgery.[6][7]
Follicular thyroid cancer has a characteristic pattern of spread that differs from papillary thyroid cancer. While papillary cancer commonly spreads through the lymphatic system to lymph nodes in the neck, follicular cancer is more likely to spread through the bloodstream to distant organs. This process, called hematogenous spread, means cancer cells travel through blood vessels to reach other parts of the body.[2][6]
The most common sites for distant spread include the lungs and bones, though follicular thyroid cancer can also metastasize to the brain, liver, bladder, and skin. When cancer spreads to bones, it creates what are called osteolytic lesions, which means the cancer destroys bone tissue. This distant spread can occur even when the original tumor in the thyroid is small. Lymph node involvement in follicular thyroid cancer is far less common than in papillary cancer, occurring in only about 8 to 12 percent of cases.[2][6]
The prognosis of follicular thyroid cancer is directly related to several factors, including tumor size, the degree of vascular invasion, and the patient’s age. Tumors less than 1 centimeter in diameter generally have an excellent prognosis. Age is also an important factor. Patients over 55 years of age often have more aggressive disease than younger patients, and their tumors typically do not concentrate iodine as well, which affects treatment options.[2][6]
