Urinary Excretion of 5-Hydroxyindoleacetic Acid, Serotonin and 6-Sulphatoxymelatonin in Normoserotonemic and Hyperserotonemic Autistic Individuals

<i>Objective:</i> A substantial proportion of individuals with autism have elevated levels of platelet serotonin (5-HT). We examined whether platelet hyperserotonemia is associated with increased gut 5-HT synthesis, altered 5-HT catabolism or altered melatonin production. <i>Methods:</i> Urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA) and 5-HT was compared in 10 normoserotonemic and 10 hyperserotonemic age-matched autistic individuals. The relationship of urinary 6-sulfatoxymelatonin (6-SM) excretion to platelet 5-HT, and to urinary 5-HT and 5-HIAA excretion, was also examined. <i>Results:</i> In the hyperserotonemic group, significant increases at trend level in urinary excretion of 5-HIAA (p = 0.061) and 5-HT (p = 0.071) and a significant decrease for 6-SM were found (p = 0.027). The urinary 5-HIAA:5-HT ratio was similar in the normo- versus the hyperserotonemic groups. <i>Conclusions:</i> The catabolism of 5-HT does not differ in the groups, but greater exposure of the platelet to 5-HT cannot be ruled out as a cause of the platelet hyperserotonemia of autism. Although only trend level significant, the data point to a need for larger studies to examine more thoroughly the relationships between platelet hyperserotonemia, gut 5-HT synthesis and melatonin production.