The 5-Hydroxytryptamine (5-HT), or serotonin, serum test measures serotonin levels in blood serum to evaluate serotonin production and metabolism. It is primarily used to diagnose and monitor conditions related to serotonin imbalance, such as carcinoid syndrome or neuroendocrine tumors. Purpose: Detects elevated serotonin levels indicative of carcinoid tumors, which may cause flushing, diarrhea, or wheezing. It also aids in assessing serotonin-related disorders, though it’s less commonly used than 5-HIAA (its metabolite) for this purpose. Sample Type: Serum (blood drawn from a vein, processed within 1–2 hours; stored at -20°C if delayed). Procedure: No special preparation is typically required, but avoiding serotonin-affecting foods (e.g., bananas, walnuts) and medications (e.g., antidepressants) for 24–48 hours is recommended. Normal serum levels range from 50–200 ng/mL, varying by lab. Limitations: Serum levels reflect circulating serotonin, not brain levels (regulated differently). False results can occur due to diet, stress, or drug interference. Interpretation requires clinical correlation with symptoms and other tests like 5-HIAA.