Normal cortisol levels, diurnal variation and it's significance in respiratory system

Serum cortisol levels

Normal levels

Morning -8am- 5-23 mcg/dL

Evening -4pm- 3-13 mcg/dl

Diurnal variation is maintained by suprachiasmatic nucleus of the hypothalamus.

Cortisol levels peak during early morning and gradually decline throughout the day reaching lowest point around midnight 

Effects on the respiratory system

Increased Inflammatory response when the levels of cortisol reach lowest at around midnight. 

Low cortisol levels at midnight lead to airway hyper responsiveness leading to morning dip in Peak expiratory flow rate.

During cortisol trough at around late night, eosinophil counts rise leading to influx of inflammatory cells into the airway mucosa, exacerbating conditions like Eosinophilic Bronchitis or severe asthma.

Cortisol is essential for surfactant production in fetus. It stimulates the maturation of Type II pneumocytes, which is why exogenous glucocorticoids are administered in cases of threatened preterm labor to prevent Respiratory Distress Syndrome (RDS).