Purpose: The use of morning basal serum cortisol levels as an alternative to dynamic testing for assessment of hypothalamic-pituitary-adrenal (HPA) axis has previously been reported. The purpose of this study was to determine the lower and upper cutoff values that would obviate subsequent HPA axis testing.
Methods: A single-centre, retrospective study from a tertiary care endocrinology clinic was conducted, analyzing data from 106 adult individuals referred for HPA axis testing who had undergone a 0800-0900 morning basal serum cortisol test followed by a standard dose (250 μg) adrenocorticotropin (ACTH) stimulation test. The ability of morning basal serum cortisol values to predict post-ACTH 30 or 60 minute peak cortisol value of >500 or >550 nmol/L was investigated.
Results: A morning basal cutoff of <128 nmol/L is sufficient for predicting a post-ACTH value<550 nmol/L, and morning basal cutoff levels of >243 nmol/L and >266 nmol/L predict peak post-ACTH values of >500 and >550 nmol/L respectively, obviating the need for dynamic testing. Regression analysis further demonstrated the log-linear relationship between morning basal and peak levels, while also finding a significant decrease in peak post-ACTH levels for patients diagnosed with secondary hypothyroidism (76 nmol/L lower, p=0.003) or secondary hypogonadism (61 nmol/L lower, p=0.02). These data suggest that the risk of cortisol deficiency is significantly higher in individuals with additional pituitary insufficiencies. The odds ratios for cortisol deficiency in patients with history of isolated secondary hypothyroidism was 3.41 (p=0.015), with isolated secondary hypogonadism was 4.77 (p=0.002) and with both was 7.45 (p=0.0002).
Conclusion: Morning basal serum cortisol levels show promise as an effective screening test for HPA insufficiency for most patients. Clinicians should consider the high probability of HPA insufficiency in patients with one or more pituitary insufficiencies.