CHGA and Cowden syndrome 1: Considering the usability of serum 5-HIAA in the follow-up of patients with CS, our study shows a strong correlation between serum 5-HIAA and the presence of CS-related symptoms of flushing, diarrhea, and CHD, as well as tumor burden as measured by chromogranin A. The correlation of serum 5-HIAA with tumor burden aligns with the findings of previous studies showing a correlation between chromogranin A and serum 5-HIAA in 112 patients with a NET (18).