The tyrosine plasma concentration of 38 endogenous depressives was compared with that of neurotic depressives, schizophrenics, and healthy controls at 8 AM, 11 AM, 2 PM, and 5 PM. The 8 AM tyrosine concentration was about equal in all groups, but there was a significant decrease of the tyrosine level at 11 AM in endogenous depressives, as compared with the other three groups. The possible causes and the consequences of this tyrosine decrease are discussed.
Benkert O, Renz A, Marano C, Matussek N. Altered Tyrosine Daytime Plasma Levels in Endogenous Depressive Patients. Arch Gen Psychiatry. 1971;25(4):359-363. doi:10.1001/archpsyc.1971.01750160071013