Understanding Cortisol

Patient Information Sheet 

Summary: Stress, Cortisol, and Adrenal Fatigue

Cortisol is one of the body’s main stress hormones. In the short term, it helps us cope with physical, emotional, and environmental demands. However, when stress is ongoing, the stress-response system can become overworked.

Initially, chronic stress often leads to higher cortisol levels, keeping the body in a constant “fight or flight” state. Over time, this constant demand may overwhelm the system, leading to reduced cortisol output and poor stress regulation. This stage is often referred to as adrenal fatigue or HPA axis dysfunction.

When this happens, the body’s ability to cope with stress, maintain energy, regulate weight, and recover properly becomes impaired.

What is Cortisol?

Cortisol is a hormone produced by the adrenal glands, which sit on top of the kidneys. It plays a vital role in helping the body:

  • Respond to stress

  • Maintain energy and stamina

  • Regulate blood sugar

  • Support blood pressure

  • Control inflammation

  • Maintain a healthy sleep–wake cycle

Cortisol is best thought of as a gauge of the body’s ability to cope with stress — both physical and emotional.

Why Do We Test Cortisol?

Cortisol testing helps us understand how well your stress-response system is functioning. It can provide insight into symptoms such as:

  • Ongoing fatigue or burnout

  • Poor stress tolerance

  • Sleep disturbance

  • Brain fog or poor concentration

  • Mood changes (anxiety, low mood)

  • Blood sugar instability

  • Difficulty recovering from illness, injury, or exercise

Both high cortisol and low cortisol can cause symptoms.

Cortisol Follows a Daily Rhythm

Cortisol naturally follows a circadian rhythm:

  • Highest in the morning to help you wake up and function

  • Gradually declines throughout the day

  • Lowest at night, allowing rest and sleep

Morning Cortisol Target 

In Australia, cortisol is measured in nmol/L.

  • We aim for an 8:00 am cortisol level of around 450 nmol/L

  • Typical laboratory reference range: 100–535 nmol/L

While values within the reference range are considered “normal,” optimal function is usually closer to the upper-middle range in the morning.

What Does High Cortisol Mean?

Acute Stress Response

Higher cortisol levels are commonly seen during short-term or acute stress, such as:

  • Emotional or work stress

  • Illness or infection

  • Poor sleep

  • Excessive exercise or overtraining

  • High caffeine intake

In the short term, elevated cortisol is adaptive — it helps the body cope with immediate demands.

Problems with Prolonged High Cortisol

If cortisol remains elevated for long periods, it may contribute to:

  • Anxiety or feeling “wired but tired”

  • Poor sleep quality

  • Blood sugar fluctuations

  • Abdominal weight gain

  • Increased inflammation

  • Difficulty relaxing or switching off

What Does Low Cortisol Mean?

Chronic Stress and Reduced Resilience

After prolonged or repeated stress, the body may struggle to maintain adequate cortisol production. Over time, this can lead to lower cortisol levels, particularly in the morning.

This reflects stress-system dysregulation (often described as adrenal or HPA axis fatigue), rather than permanent adrenal damage.

Possible Effects of Low Cortisol

  • Persistent fatigue

  • Poor ability to cope with stress

  • Feeling overwhelmed easily

  • Brain fog or poor focus

  • Low motivation

  • Dizziness or low blood pressure

  • Slow recovery from illness or exertion

Low cortisol reduces the body’s capacity to respond to stress, making even small challenges feel exhausting.

Why Balance is Important

Cortisol works best when it is balanced and rhythmic:

  • Too high → the body remains in “fight or flight”

  • Too low → insufficient energy and resilience

The goal is a healthy morning rise (around 450 nmol/L at 8:00 am), followed by a gradual decline throughout the day.

How Cortisol Affects Weight

Cortisol plays a major role in metabolism, appetite regulation, and fat storage, so changes in cortisol levels can significantly affect body weight.

Early / Acute Stress – Weight Loss

During early or acute stress, higher cortisol levels can:

  • Suppress appetite

  • Increase energy expenditure

  • Increase breakdown of muscle and fat

This can lead to unintentional weight loss, especially when stress is intense or prolonged without adequate recovery.

Chronic Stress and Adrenal Fatigue – Weight Gain

Over time, chronic stress can disrupt cortisol regulation and blood sugar balance. This may lead to:

  • Increased cravings, particularly for sugar and refined carbohydrates

  • Blood sugar instability

  • Increased fat storage, especially around the abdomen

  • Reduced metabolic flexibility

  • Fatigue that limits physical activity

In this later stage, weight gain becomes more common, even when food intake has not increased. Many people find weight becomes harder to lose despite best efforts.

Interpreting Your Results

Cortisol results always need to be interpreted in relation to the time of day.

As a general guide, we aim for a cortisol level of around 450 nmol/L at 8:00 am, as this reflects a healthy morning rise that supports energy, alertness, and stress resilience. However, each person has their own normal, so numbers should always be interpreted alongside symptoms, medical history, and overall health.

Cortisol levels naturally decline throughout the day. For example, a result of around 200 nmol/L at 4:00 pm may be appropriate as part of a normal daily rhythm.

Key Takeaways

  • Cortisol reflects how well your body manages stress

  • Chronic stress can progress from high cortisol to adrenal fatigue

  • Cortisol strongly influences metabolism and body weight

  • Early stress may cause weight loss; long-term stress often leads to weight gain

  • Morning cortisol should be strongest around 8:00 am

  • Balanced cortisol supports energy, mood, focus, recovery, and healthy weight

Rachael Reed, balancing hormones, Naturally.