Where and Why: Locations, Subtypes, and Symptoms of NENs

Where a neuroendocrine neoplasm (NEN) starts in the body and whether it produces hormones significantly shapes your experience with the disease. Because these tumors grow from cells that can produce chemicals, they can create symptoms that feel like many other common conditions, often leading to a long and confusing path to a correct diagnosis.

Functional vs. Non-Functional Tumors

The first major distinction your care team will make is whether your tumor is functional or non-functional:

• Non-functional tumors (the most common type) do not produce extra hormones ^[1]. They are often found incidentally during a scan for something else or cause symptoms by growing large enough to press on nearby organs ^[2].
• Functional tumors produce and release hormones or other “vasoactive” substances into your bloodstream ^[3]. These chemicals cause specific physical symptoms, often called “syndromes.”

Common Locations and Their Impact

NENs can appear almost anywhere, but they are most frequently found in two areas:

1. Gastroenteropancreatic (GEP-NENs)

These occur in the digestive system, including the stomach, intestines, and pancreas ^[4].

• Small Intestine: Often the source of Carcinoid Syndrome.
• Pancreas: Can produce specialized functional tumors like insulinomas ^[5].

2. Lung NENs

Tumors in the lung account for about 20–25% of all NENs ^[6]. While they can sometimes cause hormonal symptoms, they often present with a persistent cough or wheezing that may be mistaken for asthma or bronchitis ^[7][8].

Understanding Carcinoid Syndrome

Carcinoid Syndrome occurs when a tumor (usually in the small intestine or lung) releases excess serotonin and other chemicals into the bloodstream ^[3].

• The “First-Pass” Effect: Normally, the liver breaks down these chemicals before they reach the rest of the body ^[9]. Because of this, Carcinoid Syndrome most often occurs when the tumor has spread to the liver, allowing the chemicals to bypass this “filter” and enter the general circulation ^[10].
• Classic Symptoms: The hallmark signs are flushing (sudden redness and warmth in the face and neck) and diarrhea ^[11]. Other signs include wheezing and abdominal pain.
• Common Triggers: Many patients find that their flushing and diarrhea are triggered by specific daily factors, including alcohol, spicy foods, foods high in amines (like aged cheeses), and physical or emotional stress.
• Heart Health Impact: Because serotonin can damage heart valves over time, it is highly recommended that patients with functional tumors request a baseline and routine echocardiogram (ECHO) to screen for Carcinoid Heart Disease ^[12].

Rare Functional Syndromes: Insulinoma

A primary example of a functional pancreatic tumor is the insulinoma. These tumors produce excess insulin, causing your blood sugar to drop dangerously low ^[5]. This presents as Whipple’s Triad:

1. Symptoms of low blood sugar (tremors, sweating, confusion, or dizziness) ^[13].
2. A documented low blood glucose level.
3. Immediate relief of symptoms after eating sugar ^[14].

Why the Delay? The Great Mimic

NEN symptoms are frequently misdiagnosed because they look like common, non-cancerous conditions:

• IBS: The diarrhea and cramping of a NEN are often mistaken for Irritable Bowel Syndrome ^[15].
• Menopause: NEN-related flushing is frequently dismissed as menopausal “hot flashes” ^[2].
• Asthma: Wheezing caused by serotonin can lead to a diagnosis of asthma that doesn’t respond to standard inhalers ^[8].

If you have been struggling with “refractory” (hard-to-treat) symptoms like these, it is essential to discuss the possibility of a functional NEN with your doctor. Correctly identifying these symptoms is a major step toward effective management ^[15].