Plasma free and total tryptophan, blood serotonin, and the hyperactivity syndrome: no evidence for the serotonin deficiency hypothesis.

Total and free plasma tryptophan and plasma cortisol levels were determined for children diagnosed as hyperactive or learning disabled and in normal siblings of these subjects. No differences were observed among the groups on any of the dependent variables related to a number of parameters measured for the hyperactive children such as degree of hyperactivity, presence of food allergies, favorable response to methylphenidate, and brain dysfunction. Moreover, in further comparisons no differences were found between hyperactive and age-matched controls on these variables as well as blood serotonin, nor were any differences found for a group of hyperactive children between placebo and methylphenidate trials. These data do not support earlier suggestions of a serotonin deficiency in hyperactive children.