Why It Matters
Cortisol is the body's primary glucocorticoid stress hormone, secreted by the adrenal cortex in response to physical and psychological stressors. It regulates glucose mobilization, inflammation suppression, and the acute stress response. In athletes, cortisol is a double-edged marker: transiently elevated cortisol supports adaptation, but chronically elevated cortisol signals maladaptation, overtraining, or systemic stress (source, source).
Serum cortisol follows a strong diurnal rhythm, peaking within 30–60 minutes of waking and declining through the day. Sampling time must always be noted; morning values (7–10 a.m.) are the clinical standard. Post-exercise cortisol elevations are expected, but failure to return to baseline between sessions is a meaningful red flag (source).
Low Cortisol: Adrenal Fatigue and Suppression
Chronically low morning cortisol in athletes can reflect adrenal suppression from prolonged overtraining, severe caloric restriction, or a blunted HPA axis response. While the term "adrenal fatigue" is not a recognized clinical diagnosis, functional HPA axis suppression in overtrained athletes is well-documented and presents as persistent fatigue, impaired recovery, poor stress tolerance, and low motivation (source).
Iatrogenic causes include prolonged corticosteroid use (inhaled or systemic), which can suppress endogenous cortisol synthesis. Low cortisol with normal ACTH may indicate primary adrenal insufficiency and warrants referral.
High Cortisol: Training Load, Sleep, and Catabolic State
Persistently elevated morning cortisol in athletes most commonly reflects high cumulative training load, inadequate sleep, psychological stress, or caloric deficit. High cortisol promotes muscle protein catabolism, suppresses testosterone, impairs immune function, and reduces bone formation, all of which degrade athletic capacity over time (source, source).
The testosterone-to-cortisol (T:C) ratio is the most operationally useful derived metric from these two markers. A declining T:C ratio across a training block indicates catabolic dominance; values below 0.35 (using free T in nmol/L and cortisol in µmol/L) are associated with overreaching and performance decrements (source).
Coaching Application
Cortisol is most informative when tracked serially over training blocks and interpreted alongside testosterone, IGF-1, and subjective wellness scores. A single elevated value is not actionable; a trend of elevated morning cortisol over two or more consecutive weeks during a loading phase warrants a training load review and likely a deload week.
Prioritize sleep quality and caloric adequacy as first-line interventions for chronically elevated cortisol. If cortisol remains elevated after two weeks of load reduction and sleep optimization, refer for HPA axis evaluation.
