Cortisol (PM)

In a healthy body, cortisol is high in the morning and falls through the day, hitting its low point in the evening. An afternoon cortisol that is still elevated when it should be coming down is one of the earliest signs that this rhythm has broken, a pattern linked to higher cardiovascular mortality, worse cancer survival, and disorders of the adrenal glands.

A single morning cortisol can look perfectly normal in someone with subtle hormone problems. The afternoon value is where the rhythm tends to fail first, which is why physicians use PM (afternoon) cortisol to investigate suspected Cushing syndrome, adrenal insufficiency, and the chronic stress patterns now linked to long-term health risk.

What Cortisol (PM) Actually Measures

Cortisol is the main glucocorticoid hormone, a chemical messenger your adrenal glands release in response to signals from your brain and pituitary. The full feedback loop is called the HPA axis (hypothalamic-pituitary-adrenal axis), the system that turns stress signals into hormone output. Cortisol mobilizes energy, modulates immunity, and helps regulate blood pressure.

Cortisol follows a daily rhythm. Levels peak shortly after waking and fall steadily through the day. The PM (afternoon) blood test, typically drawn around 4 PM, captures cortisol at a point where it should be well below morning values. When the afternoon number stays high, it suggests the daily rhythm is flat, and a flat rhythm is what the strongest outcome studies have linked to disease.

Cardiovascular Risk and Mortality

The most consistent finding across large cohorts is that a flatter cortisol rhythm, where afternoon and evening levels stay too high relative to morning, predicts cardiovascular death. In the Whitehall II study of 4,047 adults, a flatter daily slope of salivary cortisol was associated with higher all-cause mortality and especially cardiovascular mortality. The KORA-F3 study of 1,090 adults found the same pattern, with greater diurnal variation appearing protective.

In a cohort study of 2,305 hypertensive patients, steeper diurnal cortisol slopes were associated with reduced cardiovascular disease risk, while flatter slopes and higher midnight cortisol raised the risk. In 250 coronary artery bypass patients, a steeper presurgical diurnal slope predicted fewer adverse cardiac events and lower mortality. These studies measured cortisol at multiple times of day rather than PM alone, but they all converge on the same conclusion: when the afternoon and evening values fail to drop, the heart pays for it.

Cancer Survival

Higher evening cortisol has been linked to shorter survival in several cancer types. A pilot study in 40 head and neck cancer patients found that elevated evening cortisol predicted shorter progression-free survival. In 99 women with metastatic breast cancer, evening salivary cortisol was a sensitive marker of the flattened rhythm associated with poorer prognosis. Similar associations were reported in 62 lung cancer patients, 154 ovarian cancer patients, and an earlier landmark study of 104 metastatic breast cancer patients where abnormal diurnal cortisol predicted survival.

Most of these studies used salivary cortisol, which captures the unbound (free) fraction. Your PM blood test measures total cortisol, which behaves similarly across the day but uses different reference values. The directional signal, that afternoon and evening cortisol staying high carries prognostic weight, is consistent across both specimen types.

Cushing Syndrome and Hypercortisolism

Cushing syndrome is the disease of cortisol excess, caused by tumors of the pituitary, adrenal glands, or other tissues that drive chronic overproduction. It carries severe multisystem consequences and increased mortality. Because cortisol normally falls toward the end of the day, an inappropriately high afternoon or late-night value is one of the earliest signals.

In a meta-analysis of 139 studies including 14,140 participants, midnight serum cortisol had pooled sensitivity of 96.1% and specificity of 93.2% for detecting Cushing syndrome. Late-night salivary cortisol performed similarly. PM serum cortisol drawn earlier in the afternoon is less sensitive than midnight values but contributes to the diagnostic picture, especially when paired with morning cortisol to establish whether the daily rhythm is intact.

Adrenal Insufficiency

The opposite problem, an underactive adrenal gland, is also detected through cortisol testing. Morning cortisol is the primary screening test, but a low afternoon value alongside a low morning value strengthens the case. In a study of 416 outpatients, basal cortisol drawn between 9 AM and 1 PM had an area under the curve of 0.82 for diagnosing adrenal insufficiency, with values below 85 nmol/L showing 99.7% specificity. An ACTH stimulation test confirms the diagnosis.

Mental Health and Cognitive Risk

Elevated evening cortisol shows up repeatedly in research on depression, anxiety, and dementia risk. In a study of 90 full-time workers in Cartagena, Colombia, state anxiety was most strongly associated with elevated evening cortisol and triglycerides. A meta-analysis on adolescent cortisol and depression found that elevated nocturnal cortisol was a risk factor for major depressive disorder. A long-term cohort study using repeated 24-hour urinary cortisol found that higher within-person cortisol variability independently predicted increased Alzheimer's risk.

These studies used salivary, urinary, or nighttime serum cortisol rather than the standard PM blood draw. The biological signal of an afternoon cortisol that fails to drop is consistent with the broader pattern of HPA axis dysregulation that these studies describe.

Reference Ranges

These ranges come from a Swedish laboratory using radioimmunoassay in 197 healthy adults. They are illustrative orientation, not a target. Your lab will likely report different numbers and may use different units. Compare your results within the same lab over time for the most meaningful trend.

Many US labs report cortisol in µg/dL. To convert nmol/L to µg/dL, divide by approximately 27.6. The ratio between your morning and afternoon value matters more than the afternoon number alone. A healthy pattern shows the afternoon value at roughly half the morning value or lower.

Why a Single Reading Is Not Enough

Cortisol is one of the most variable hormones in the body. Within-person variation can account for roughly 50% to 73% of total variance in diurnal cortisol measures. A reliability analysis of diurnal salivary cortisol concluded that detecting between-person differences requires at least 3 days of sampling for the mean, 4 to 8 days for area-under-the-curve measures, and up to 10 days for slope. Detecting within-person change over time requires similar repetition.

Practically, this means a single PM cortisol can mislead in either direction. Get a baseline, retest in 3 to 6 months if you are working on sleep, stress, or any intervention that might affect the HPA axis, and then at least annually to track your trend. Always sample at the same time of day to make values comparable. If results are abnormal, the next step is rarely to act on one number; it is to repeat the measurement and pair it with a morning cortisol drawn the same day.

When Results Can Be Misleading

• Acute stress before the draw: physical or emotional stress, including the venipuncture itself, can spike cortisol within minutes. Sit quietly for 10 to 15 minutes before the blood draw.
• Recent intense exercise: moderate to vigorous exercise transiently raises cortisol. A study in 12 trained men showed cortisol rises with moderate to high intensity exercise and falls with very low intensity. Avoid hard workouts in the hours before the test.
• Oral contraceptives: women on oral contraceptives showed roughly double the morning and evening cortisol of women not on them, because the pill raises cortisol-binding globulin. Total cortisol rises even though free, biologically active cortisol may be unchanged.
• Pregnancy, kidney disease, or systemic illness: these conditions alter cortisol binding proteins and clearance. In chronic kidney disease, worsening kidney function is linked to altered cortisol dynamics that can mimic hypercortisolism.

What to Do With an Abnormal Result

An isolated abnormal PM cortisol is rarely diagnostic on its own. If your afternoon value is high relative to a morning draw, the standard next step is a 1 mg overnight dexamethasone suppression test, which checks whether your body can shut off cortisol production when given a synthetic steroid. A late-night salivary cortisol or 24-hour urinary free cortisol can confirm. If your afternoon value is low alongside a low morning cortisol, an ACTH stimulation test rules in or out adrenal insufficiency.

Repeated abnormal results, especially when paired with symptoms (weight gain, easy bruising, hypertension, mood changes, fatigue, or muscle weakness), warrant referral to an endocrinologist. If your numbers are borderline and you have no symptoms but a flat daily rhythm, the workup shifts toward the lifestyle factors driving chronic HPA activation: sleep, stress load, alcohol, and exercise patterns.