Start here: lower, not higher
Cortisol is the hormone that helps the body manage stress, energy, and inflammation, normally peaking in the morning. The intuitive guess for a chronic illness might be high cortisol. In long COVID, the reproducible finding is the opposite: morning cortisol tends to be low.
In a large study using machine learning to find what separates long COVID from recovery, low cortisol emerged as one of the strongest distinguishing features. That makes it one of the more solid biological associations in the field, and an intriguing one.
low cortisol a top distinguishing featureassociation, not a steroid indication
Why low cortisol could matter
Cortisol supports alertness, energy mobilisation, and blood pressure regulation, and it restrains inflammation. If it runs low, several long COVID symptoms become easier to understand: fatigue, poor stress tolerance, lightheadedness on standing, and unchecked inflammation could all follow from insufficient cortisol signalling.
It also links to other findings. Low cortisol would remove a brake on the persistent inflammation seen elsewhere in this section, and it overlaps with the orthostatic problems that trouble many patients. A single hormonal shift could touch many systems. The node also raises glucocorticoid resistance, the idea that cells respond poorly to cortisol even when it is present, which could produce similar effects without the level itself being low, and which would not be fixed by simply adding more hormone.
What the study found
The large immune-profiling study measured many hormones and immune signals across people with and without long COVID. Cortisol levels were lower in the long COVID group, and this difference was prominent enough that the machine learning models leaned on it to classify long COVID status.
That this emerged from an unbiased analysis, rather than from a study set up to look for it, strengthens the finding. The same study also found elevated antibodies against Epstein-Barr virus, tying the cortisol result into a broader immune picture. Finding the same hormonal signal through an approach not designed to look for it is part of why this result is taken seriously rather than treated as a one-off.
Why it is not a green light for steroids
The obvious but wrong inference is to take cortisol-like steroids to top up the low level. The evidence does not support that. The low reading is an association, its cause is unclear, and the body's stress-hormone system is a finely tuned loop that external steroids can suppress further, potentially making things worse.
Steroid replacement is appropriate only for diagnosed adrenal insufficiency, identified through proper endocrine testing, not inferred from a research association. Self-treating with steroids based on this finding risks real harm, including dependency and suppression of the body's own production.
do not self-treat with steroids based on this finding
What it means for you
The finding validates that long COVID involves a measurable hormonal disturbance, which is useful against dismissal. If you have symptoms suggesting an endocrine problem, it is reasonable to ask a clinician about proper assessment of your adrenal and pituitary function.
Formal testing can distinguish a research-level low average from genuine, treatable adrenal insufficiency, which is a different and specific diagnosis. That is the appropriate path, rather than acting on the population finding alone.
How to read claims about it
Low cortisol in long COVID is real and replicated, which makes it attractive to those selling adrenal support supplements or steroid protocols. Ask whether a treatment has been shown in a controlled trial to help long COVID, not merely that cortisol can be low.
The honest stance is that the cortisol finding is a strong clue to the biology and a weak basis for any specific hormone treatment. Take it seriously as science and cautiously as a prescription, and let proper endocrine evaluation, not marketing, guide any treatment.
What we don't know
Honest about the edges of the evidence. These are open questions, not settled answers.
- Whether low cortisol drives long COVID symptoms or is a downstream consequence.
- Why cortisol is low: adrenal, pituitary, or signalling-level causes.
- Whether correcting cortisol safely improves symptoms in trials.
- How low cortisol interacts with persistent inflammation and orthostatic symptoms.
- Which patients have a clinically meaningful deficit versus a statistical average.
- Whether glucocorticoid resistance, not just low levels, is part of the picture.
What this means for you
Low morning cortisol is one of the more reproducible biological findings in long COVID and validates that the illness involves a real hormonal disturbance. It could help explain fatigue, poor stress tolerance, and orthostatic symptoms.
It is not a reason to take steroids on your own, which can suppress your body's own production and cause harm. If an endocrine problem is suspected, ask for proper testing to distinguish a research average from genuine, treatable adrenal insufficiency.
References
Each reference links to the source on PubMed, PMC, or the publisher.