Introduction- Some literature describes reduced serum cortisol levels in obesity however this is not a well recognized phenomenon and the number of subjects studied to date has been small. Low cortisol levels generate concern over adrenal reserve and necessitate further investigations such as Synacthen stimulation test which carries its own risk.
Aim -This study assessed relationship between serum cortisol levels, weight and body mass index (BMI).
Method – Subjects attending community pathology having assessment of serum cortisol at their GP request were offered participation in this study (n=71) Participants provided information for a questionnaire including details of medications, height and weight. Exclusions include those with  history of adrenal/pituitary disease or medications altering cortisol. BMI was calculated from participant provided data. Cortisol was assayed (Siemens Centaur XP) and results converted to Multiple of Medians (MoM) (to control for time of collection) using previously derived data (n=13,953). BMI and weight were correlated with cortisol and cortisol MoMs.
Results – A total 67 subjects, 41 females (mean age 39.7), 26 males (mean age 50.6), with BMI levels ranging from 16.6 - 46.4 were included. A left hand shift (to lower levels) in the distribution of Cortisol and Cortisol MoM was found in those with higher BMI. Cortisol results (p <0.01) and Cortisol MoM (p <0.01) were lower in both those with BMI > 25 compared with those with BMI < 25.

Conclusion – This study demonstrated a left hand shift (to lower levels) in the distribution of Cortisol and Cortisol MoM in obese subjects with a BMI > 25. This study suggests those with increased BMI (> 25) have a lower distribution of serum Cortisol and Cortisol MoM compared to lower BMI subjects and raises issues about interpretation of serum cortisol in obese patients.