• Richard I Dorin, Zhi Qiao, Clifford R Qualls, and Frank K Urban.
• Medical Service 111/Endocrinology, New Mexico Veterans Affairs Healthcare System, 1501 San Pedro Boulevard SE, Albuquerque, New Mexico 87108, USA. rdorin@salud.unm.edu
• J. Clin. Endocrinol. Metab. 2012 Apr 1;97(4):1285-93.

ContextCortisol secretion is related to ACTH concentration by a sigmoidal dose-response curve, in which high ACTH concentrations drive maximal cortisol secretion rates (CSR(max)).ObjectiveWe sought to estimate CSR(max) and free cortisol half-life in healthy humans (n = 21) using numerical methods applied to data acquired during cosyntropin (250 μg) stimulation. We also evaluated the effect of overnight dexamethasone (DEX; 1 mg) vs. placebo on estimates of CSR(max) and free cortisol half-life.DesignThis study was a double-blind, placebo-controlled, randomized order of overnight DEX vs. placebo, cosyntropin (250 μg) stimulation with frequent serum cortisol sampling and computer-assisted numerical analysis.SettingThe study was conducted at a single academic medical center.ParticipantsTwenty-one healthy adult subjects (15 females and six males), mean aged 46 yr, participated in the study.InterventionIntervention in the study included DEX vs. placebo pretreatment, cosyntropin (250 μg) iv with frequent cortisol sampling.Main Outcome MeasuresCSR(max) and free cortisol half-life estimates, R² for goodness of fit, were measured.ResultsMean ± sd CSR(max) was 0.44 ± 0.13 nm/second, with free cortisol half-life of 2.2 ± 1.1 min. DEX did not significantly affect estimates of CSR(max) or free cortisol half-life. Our model accounts for most of the variability of measured cortisol concentrations (overall R² = 90.9 ±11.0%) and was more accurate (P = 0.004) during DEX suppression (R² = 94.6 ± 4.6%) compared with placebo (R² = 87.2 ± 8.7%).ConclusionsApplication of a mass-action model under conditions of cosyntropin stimulation provides a relatively simple method for estimation CSR(max) that accurately predicts measured cortisol concentrations. DEX administration did not significantly affect estimates of CSR(max) or free cortisol half-life.

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