Characterization of Cortisol Secretion Rate in Secondary Adrenal Insufficiency

Context In secondary adrenal insufficiency (SAI), chronic deficiency of adrenocorticotropin (ACTH) is believed to result in secondary changes in adrenocortical function, causing an altered dose-response relationship between ACTH concentration and cortisol secretion rate (CSR). Objective We sought to characterize maximal cortisol secretion rate (CSRmax) and free cortisol half-life in patients with SAI, compare results with those of age-matched healthy controls, and examine the influence of predictor variables on ACTH-stimulated cortisol concentrations. Design CSRmax was estimated from ACTH1-24 (250 μg)-stimulated cortisol time-concentration data. Estimates for CSRmax and free cortisol half-life were obtained for both dexamethasone (DEX) and placebo pretreatment conditions for all subjects. Setting Single academic medical center. Patients Patients with SAI (n = 10) compared with age-matched healthy controls (n = 21). Interventions The order of DEX vs placebo pretreatment was randomized and double-blind. Cortisol concentrations were obtained at baseline and at intervals for 120 minutes after ACTH1-24. Main Outcome Measures CSRmax and free cortisol half-life were obtained by numerical modeling analysis. Predictors of stimulated cortisol concentrations were evaluated using a multivariate model. Results CSRmax was significantly (P < 0.001) reduced in patients with SAI compared with controls for both placebo (0.17 ± 0.09 vs 0.46 ± 0.14 nM/s) and DEX (0.18 ± 0.13 vs 0.43 ± 0.13 nM/s) conditions. Significant predictors of ACTH1-24-stimulated total cortisol concentrations included CSRmax, free cortisol half-life, and baseline total cortisol, corticosteroid-binding globulin, and albumin concentrations (all P < 0.05). Conclusions Our finding of significantly decreased CSRmax confirms that SAI is associated with alterations in the CSR-ACTH dose-response curve. Decreased CSRmax contributes importantly to the laboratory diagnosis of SAI.