Medullary thyroid cancer (MTC) is a rare form of thyroid cancer that develops in cells called parafollicular C cells, which produce a hormone called calcitonin. It accounts for about 3% to 4% of all thyroid cancers. Unlike the more common types of thyroid cancer, MTC behaves differently, responds to different treatments, and has a strong genetic component that makes testing family members an important part of managing the disease.
Where MTC Comes From
Your thyroid gland contains two main types of cells. Follicular cells produce thyroid hormones that regulate metabolism. Parafollicular C cells produce calcitonin, a hormone involved in calcium regulation. MTC starts in these C cells, which is why it behaves so differently from papillary or follicular thyroid cancers that originate in follicular cells.
Because MTC arises from calcitonin-producing cells, the tumors themselves pump out excess calcitonin. This overproduction becomes both a hallmark of the disease and a useful tool for detecting and tracking it. The tumors can also secrete other substances like prostaglandins and serotonin, which explains some of the unusual symptoms MTC can cause.
The Genetic Connection
About 75% of MTC cases are sporadic, meaning they develop without any family history. The remaining 20% to 25% are hereditary, caused by inherited mutations in a gene called RET. These hereditary cases are linked to a condition called multiple endocrine neoplasia type 2 (MEN2), which can also cause tumors in the adrenal glands and parathyroid glands.
Here’s the important part: between 1% and 10% of people who appear to have sporadic MTC actually carry a germline RET mutation when tested. That means they have a hereditary form of the disease that could affect their children and siblings. For this reason, genetic testing is recommended for everyone diagnosed with MTC, regardless of family history. If a RET mutation is found, family members can be tested too, and in some cases preventive surgery can be performed before cancer ever develops.
Symptoms and Warning Signs
MTC often first shows up as a lump or nodule in the thyroid that you or your doctor can feel. Many people have no other symptoms at that point. As the tumor grows or spreads to nearby lymph nodes, you might notice swelling in the neck, difficulty swallowing, or hoarseness.
What sets MTC apart from other thyroid cancers is its tendency to cause systemic symptoms. Persistent, watery diarrhea is one of the more distinctive signs, thought to result from the tumor secreting calcitonin, prostaglandins, and serotonin. Facial flushing can also occur. These symptoms are more common when the disease is advanced or has spread to the liver, but they can sometimes appear early enough to prompt a diagnosis.
How MTC Is Diagnosed
A fine-needle biopsy of a thyroid nodule can identify MTC under the microscope. But blood tests play an unusually important role in this particular cancer. Calcitonin levels in the blood serve as a highly reliable marker. Basal calcitonin levels above 100 pg/mL have a 100% positive predictive value for MTC, meaning if your level is that high, the diagnosis is essentially confirmed.
Another blood marker called CEA (carcinoembryonic antigen) isn’t useful for initial detection, since it can be elevated in many other conditions. It becomes more helpful after diagnosis, as a way to track whether the disease is progressing or responding to treatment. If calcitonin levels exceed 400 pg/mL or lymph nodes are involved, imaging scans of the chest, neck, and liver are typically used to check for spread.
Treatment Starts With Surgery
The primary treatment for MTC is surgical removal of the entire thyroid gland (total thyroidectomy) along with the central lymph nodes in the neck. This has historically been an aggressive upfront approach because MTC tends to spread to lymph nodes relatively early compared to other thyroid cancers.
Whether surgeons should also remove the lymph nodes along the sides of the neck when they appear cancer-free on imaging remains an active debate. Some surgeons recommend it based on tumor size and calcitonin levels, since there’s a documented correlation between these factors and hidden spread to lateral neck nodes. Others argue the added surgery carries risks, including nerve damage, without clear survival benefit in every patient. Current guidelines don’t give a definitive recommendation either way, so this decision is often individualized.
Targeted Therapy for Advanced Disease
When MTC can’t be fully removed surgically or has spread to distant sites, targeted medications that block tumor growth signals become the main treatment option. Three drugs are currently approved for advanced MTC. Cabozantinib and vandetanib were the first options, both targeting the pathways that drive tumor growth. More recently, selpercatinib was approved specifically for MTC driven by RET mutations, offering a more precisely targeted approach with a different side effect profile.
Unlike common thyroid cancers, MTC does not respond to radioactive iodine therapy. C cells don’t absorb iodine the way follicular cells do, so that standard thyroid cancer treatment simply doesn’t work here. This is one of the key reasons MTC requires a distinct treatment approach.
Survival Rates by Stage
The overall five-year survival rate for MTC is 93%, but outcomes vary significantly depending on how far the cancer has spread at diagnosis.
- Localized (confined to the thyroid): greater than 99% five-year survival
- Regional (spread to nearby lymph nodes): 94% five-year survival
- Distant (spread to lungs, liver, or bones): 50% five-year survival
These numbers, based on people diagnosed between 2015 and 2021, underscore how much early detection matters. MTC caught while still in the thyroid is highly curable. Once it reaches distant organs, the outlook changes considerably.
Long-Term Monitoring
After surgery, calcitonin levels become the primary way doctors track your status. If the surgery removed all the cancer, calcitonin should drop to undetectable levels. Rising calcitonin over time signals that disease may be recurring or progressing.
The speed at which calcitonin levels double, known as the calcitonin doubling time, is one of the most useful prognostic tools. A doubling time longer than two years generally indicates a slower-growing disease with a better long-term outlook. A doubling time under six months suggests more aggressive behavior that may need earlier intervention. Doctors typically track at least four calcitonin measurements spread over a two-year period to calculate this accurately. If calcitonin levels are declining over time, that’s usually a good sign. CEA levels are tracked alongside calcitonin to give a more complete picture of disease behavior.
This ongoing monitoring is a lifelong commitment for MTC patients. Even when the initial treatment is successful, the cancer can recur years or even decades later, making regular blood work an essential part of follow-up care.