Bimodal distribution of serum trypotphan level.

Serum levels of total L-tryptophan (L-TP) were determined before and during treatment in patients suffering from endogenous depression, who were treated with L-TP, 6 g daily, and unilateral electroconvulsive therapy (ECT). The gaussian curve of initial L-TP concentrations changed into a bimodal distribution during treatment. The patients with initially lower L-TP levels also had lower L-TP levels during the treatment, thus forming the low-concentration group within the bimodal distribution. In the patients with higher initial concentrations, the L-TP levels increased to a much greater degree, forming the high-concentration group. Patients in this group needed a larger number of ECT's, probably owing to L-TP's property of shortening the seizure duration. No other background data or clinical variables showed significant differences between the groups. It is suggested that the bimodality of L-TP levels is genetically determined.