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.
Keywords: adrenal insufficiency; computer-assisted; cosyntropin; hydrocortisone; metabolic clearance rate; multivariable analysis; numerical analysis.