Thyroid cancer, in its most common forms (Papillary and Follicular), does not typically cause digestive issues like diarrhea. These cancers originate from follicular cells and do not produce hormones that directly affect the gastrointestinal tract. However, diarrhea can occur, but only in the context of a specific and rare subtype. For many patients, diarrhea is an indirect consequence related to cancer treatment.
The Specific Link to Medullary Thyroid Carcinoma
The direct cause of diarrhea from thyroid cancer is almost exclusively attributed to Medullary Thyroid Carcinoma (MTC). MTC is a rare neuroendocrine tumor, accounting for only about 1% to 5% of all thyroid cancer cases. This specific cancer type arises from the parafollicular C cells of the thyroid gland, distinct from the follicular cells that cause more common thyroid malignancies.
These C cells naturally produce the hormone calcitonin, and the MTC tumor retains this neuroendocrine ability. Diarrhea is observed in nearly one-third of patients with MTC, particularly when the disease has advanced or metastasized. The symptom is often severe, chronic, and watery, sometimes preceding the discovery of the thyroid mass itself. MTC-related diarrhea results from the tumor’s overproduction and secretion of various bioactive substances into the bloodstream.
How Hormonal Overproduction Causes Diarrhea
Medullary Thyroid Carcinoma tumors secrete high levels of substances that act on the digestive system, leading to secretory diarrhea. The primary substance involved is calcitonin, which the parafollicular C cells produce in excess. Elevated calcitonin levels circulate and stimulate the intestinal lining, reversing the normal process of fluid absorption into active secretion.
This hormonal effect causes the small intestine to pump water and electrolytes, such as sodium, into the gut lumen instead of absorbing them. The resulting diarrhea is typically large in volume and watery, a hallmark of secretory diarrhea. This type of diarrhea does not improve with fasting because it is driven by internal hormonal signals rather than the presence of food.
Beyond calcitonin, MTC cells can sometimes secrete other potent bioactive peptides, including vasoactive intestinal peptide (VIP) and prostaglandins. VIP is a powerful stimulator of intestinal fluid and electrolyte secretion, further contributing to the secretory diarrhea. These substances can also increase the motility and tone of the small intestine and colon, causing a reduced transit time for stool. The combination of increased secretion and faster movement results in persistent diarrhea for some patients with advanced MTC.
Diarrhea Related to Cancer Treatments
Diarrhea is a frequent side effect of treatments used for all types of thyroid cancer, including the common Papillary and Follicular types. It is a common adverse event associated with systemic therapies used for advanced or recurrent thyroid cancer. This is particularly true for targeted therapies, such as tyrosine kinase inhibitors (TKIs), often prescribed for patients with metastatic disease.
These targeted drugs interfere with signaling pathways in cancer cells, but they can also disrupt normal cell function in the gastrointestinal tract, leading to diarrhea. Chemotherapy agents, though less commonly used, can also damage the rapidly dividing cells of the gut lining, causing inflammation and fluid imbalance. Additionally, radioactive iodine therapy (RAI), a standard treatment for most differentiated thyroid cancers, can sometimes cause short-term gastrointestinal side effects, including diarrhea.
When to Consult Your Healthcare Provider
Diarrhea should always be reported to your medical team, as it can quickly lead to complications like dehydration and electrolyte imbalance. Seek immediate medical attention if you experience signs of severe dehydration, such as dizziness, lightheadedness, or significantly reduced urination. The presence of blood in the stool, a persistent fever, or severe abdominal pain accompanying the diarrhea are warning signs that require urgent evaluation.
It is helpful to track the frequency, volume, and consistency of the diarrhea, along with any other symptoms, before contacting your provider. Management strategies differ significantly based on the underlying cause, whether it is tumor-driven (MTC) or a side effect of a specific therapy. Timely communication ensures your care team can accurately determine the cause and implement the correct supportive measures, such as anti-diarrheal medications, dietary changes, or adjusting treatment dosages.