Decreased GC levels in aged mice. (a) Total serum corticosteroid levels were quantified in sera of young (n = 24) and aged (n = 15) mice by ELISA. (b, c) Corticosterone (b) and 11‐dehydrocorticosterone (11‐DHC, c) concentrations were determined in sera of young (n = 15) and aged (n = 7) mice by LC‐HRMS/MS. (d, e) CRH (d) and ACTH (e) levels were determined in sera of young (n = 34) and aged (n = 33) mice by ELISA. (f) Expression levels of genes involved in cholesterol homeostasis and corticosteroid biosynthesis were measured in adrenal glands from young (n = 12) and aged (n = 8) mice by qPCR, normalized against the housekeeping gene (Csnk2a2), and expressed as x‐fold of samples from young mice. (g) Expression of GR mRNA (Nr3c1) in PMs and PBLs from young and aged mice (n = 14). Data were normalized against the housekeeping gene (Ppia) and are expressed as fold change of young. (h, i) GR protein levels were measured by Western blot in PMs obtained from young (n = 12) and aged (n = 16) mice. (h) Representative blot. (i) GR signal intensities were normalized to the loading control GAPDH and are expressed as x‐fold of young. Box plots show the 25–75th percentiles (box), mean (circle), median (line), and SD (whiskers). *p < .05, **p < .01, ***p < .001 by two‐tailed t test (a, d, e) or Mann–Whitney U test (b, c, f)