Medullary thyroid cancer (MTC) is driven primarily by mutations in a single gene called RET, which controls cell growth and division. About 75% of cases arise spontaneously from mutations that develop during a person’s lifetime, while the remaining 25% are inherited. Unlike more common thyroid cancers, MTC starts in a completely different type of thyroid cell, which is why it behaves differently and requires a distinct approach to detection and management.
Where MTC Starts: C Cells
Your thyroid gland contains two main types of cells. The follicular cells, which most people associate with the thyroid, produce thyroid hormone. But tucked between them are parafollicular C cells, which produce a hormone called calcitonin. Calcitonin helps regulate calcium levels in your blood and has nothing to do with thyroid hormone production.
MTC originates in these C cells. Because the cancer comes from a fundamentally different cell type, it doesn’t respond to the same treatments used for the more common papillary or follicular thyroid cancers. It also produces excess calcitonin, which becomes a useful marker for detecting the disease. Calcitonin levels above 100 pg/mL are essentially diagnostic, carrying a 100% positive predictive value for MTC.
The RET Gene: The Central Cause
The RET proto-oncogene sits at the center of nearly all medullary thyroid cancer. This gene provides instructions for building a receptor on the surface of cells that controls when cells grow, divide, and mature. When RET is functioning normally, it only sends growth signals when triggered by the right chemical messenger. When it’s mutated, the receptor gets stuck in the “on” position, continuously telling cells to multiply. Over time, this unchecked growth leads to cancer.
More than 100 different RET mutations have been identified in MTC patients. These mutations fall into two categories based on where they occur:
- Germline mutations are inherited. They exist in every cell of a person’s body from birth. These account for roughly 25% of MTC cases and are passed from parent to child with a 50% chance of inheritance.
- Somatic mutations develop spontaneously in thyroid C cells during a person’s lifetime. They are not inherited and are found only within the tumor itself. Somatic RET mutations show up in 40% to 65% of sporadic MTC cases.
In sporadic cases where no RET mutation is found in the tumor, other genetic changes are likely responsible, though RET remains by far the most common driver.
Sporadic MTC: The Most Common Form
Three out of four MTC cases are sporadic, meaning they occur in people with no family history of the disease. These cases typically appear in adults and usually affect only one lobe of the thyroid. The somatic RET mutations behind sporadic MTC accumulate through the normal process of DNA copying errors over a lifetime. No specific environmental trigger, dietary factor, or radiation exposure has been definitively linked to causing these mutations, which makes MTC unusual among cancers. While radiation is a well-known risk factor for other thyroid cancers, it does not appear to play the same role in MTC.
Because sporadic MTC lacks a clear external cause, there’s no known way to prevent it. It’s typically discovered when a person or their doctor notices a lump in the neck, or it’s found incidentally during imaging done for another reason.
Inherited MTC: The MEN2 Syndromes
The remaining 25% of cases are hereditary, caused by germline RET mutations passed down through families. These inherited forms fall under a group of conditions called Multiple Endocrine Neoplasia type 2 (MEN2), which has two main subtypes.
MEN2A
MEN2A is the more common inherited form. People with MEN2A develop MTC along with a risk of tumors in other endocrine glands, specifically adrenal gland tumors (pheochromocytomas) and overactive parathyroid glands. Not everyone with MEN2A develops all three, but MTC appears in virtually all carriers eventually. A further subtype, familial medullary thyroid carcinoma (FMTC), is now considered a variant of MEN2A in which the thyroid cancer appears alone, without the adrenal or parathyroid problems, or those problems develop much later.
The most common MEN2A mutation occurs at a specific location on the RET gene called codon 634. This mutation is associated with an early progression from precancerous C cell overgrowth to full MTC, which is why guidelines recommend preventive thyroid removal by age 5 for children carrying this mutation.
MEN2B
MEN2B is rarer and more aggressive. About 95% of MEN2B cases are caused by a single specific mutation called M918T. Children with MEN2B develop MTC at very young ages, sometimes in infancy. Beyond thyroid cancer, MEN2B causes distinctive physical features: bumpy growths on the lips and tongue (mucosal neuromas), abnormal nerve tissue in the digestive tract, and a characteristically tall, thin body shape. Because MTC in MEN2B is so aggressive, preventive thyroid removal may be recommended in the first year of life.
How Inherited Risk Is Identified
Genetic testing for RET mutations has transformed how hereditary MTC is managed. When someone is diagnosed with MTC, they are typically offered RET testing. If a germline mutation is found, their close relatives can be tested as well. A positive result doesn’t mean cancer is already present, but it does mean MTC will almost certainly develop over time without intervention.
The specific location of the RET mutation matters enormously. Different mutations carry different levels of aggressiveness, and the timing of preventive surgery is matched to the mutation’s risk level. Some mutations call for thyroid removal in early childhood, while others allow monitoring into adolescence. This is one of the clearest examples in cancer medicine of a genetic test directly guiding a life-saving decision.
Calcitonin as a Disease Marker
Because MTC arises from calcitonin-producing C cells, the tumors pump out excess calcitonin that can be measured with a simple blood test. This makes calcitonin an unusually direct window into the disease. Elevated levels can signal MTC before a lump is even detectable, and levels above 100 pg/mL are considered virtually certain evidence of the cancer. After treatment, calcitonin tracking helps gauge whether the cancer has been fully removed or has returned.
A second blood marker, carcinoembryonic antigen (CEA), is sometimes monitored alongside calcitonin. CEA is not specific enough to catch MTC early, but rising CEA levels in someone already diagnosed can signal disease progression. When calcitonin exceeds 400 pg/mL, or lymph node spread is confirmed, imaging scans of the chest, neck, and liver are typically used to check for distant spread.