Background: The most common neuroendocrine tumor (NET) of the gastroenteropancreatic axis is a midgut carcinoid tumor producing serotonin, which is characterized by slow growth but high malignancy potential. The tumor is commonly asymptomatic until it becomes large or has metastasized. Up to 10% of the patients develop carcinoid syndrome due to the release of tumor secretions into the systemic circulation, avoiding the portal circulation. Clinically, this is characterized by hot flushes, diarrhea, bronchospasm, and carcinoid heart disease at diagnosis in approximately 50% of the patients with carcinoid syndrome, but only in 10‑20% is this the first clinical symptom of the syndrome.
Case Report: The case of a 34-year-old woman with carcinoid heart disease in the course of carcinoid syndrome as the first clinical symptom of midgut carcinoid tumor is presented. This unusual course of the clinical picture significantly prolonged and hindered the diagnostic process. However, extensive surgical, cardiologic, and endocrinological treatment was successful. One-year, two-year, and five-year follow-ups showed improved heart function and no signs of tumor relapse.
Conclusions: Even though heart disease coexists with other symptoms typical of carcinoid syndrome, it should be borne in mind that right-sided heart failure could be the first sign of the disease. Echocardiography is the investigation of choice, revealing a unique valvular appearance at diagnosis, and should be performed at the time of diagnosing serotonin-producing NET. Multidisciplinary management (surgical and medical) could improve prognosis and the quality of life of the patients.

Keywords: midgut carcinoid tumor, Carcinoid Heart Disease, valvular dysfunction