Increased levels of serotonin indicate metastasis from which tumor?

Increased levels of serotonin are primarily associated with tumors like small bowel carcinoids, particularly when they metastasize to the liver. Carcinoid tumors, which are neuroendocrine tumors often arising from the enterochromaffin cells in the gastrointestinal tract, can produce serotonin and other neuroamines. When they metastasize, especially to the liver, the excess serotonin they produce enters systemic circulation resulting in elevated levels.

The syndrome of well-described carcinoid, which involves symptoms like flushing, diarrhea, and wheezing, is typically seen when these tumors metastasize, particularly to the liver, where they escape the first-pass metabolism that normally mitigates the effects of serotonin in the bloodstream. This is why high levels of serotonin strongly indicate the presence of a small bowel carcinoid that has metastasized to the liver.

Other tumors listed, such as neuroblastoma, renal cell carcinoma, and colon adenocarcinoma, do not usually lead to significant serotonin production. Thus, the elevation of serotonin levels is a key marker for the carcinoid syndrome associated with metastatic small bowel carcinoid tumors.