TDO2 and Alzheimer disease: Furthermore, in a study of thirty-four individuals with AD (20 females and 14 males), a positive Spearman rank correlation between the Global Deterioration Scale (GDS) scores and blood levels of cortisol, an inducer of TDO, an enzyme that catalyzes tryptophan conversion into Kyn, an immediate AA precursor, was observed only in female individuals with AD after the administration of 0.5 mg of dexamethasone [97].