SLOT: Full Definition
What is the HPA axis?
The HPA axis — short for the hypothalamic-pituitary-adrenal axis — is the body's central stress response system. It is a coordinated communication loop between three glands:
- The hypothalamus in the brain, which senses stress and releases CRH
- The pituitary gland, which responds with ACTH
- The adrenal glands, which sit on top of the kidneys and release cortisol and other stress hormones
Cortisol is the main downstream output. It rises and falls in a daily rhythm — high in the morning to wake you up, tapering down through the day, low at night so you can sleep. This pattern is called the Cortisol Rhythm or diurnal cortisol curve.
Why the HPA axis matters
The HPA axis is designed for short-term stress: a deadline, a near-miss in traffic, a hard workout. Cortisol spikes, the body mobilizes energy, and then the system resets. Modern life rarely allows that reset. Chronic stress — work pressure, poor sleep, caloric restriction, infections, unprocessed emotions, relationship strain — keeps the axis activated for years on end.
When the HPA axis is dysregulated, it does not affect cortisol alone. It influences nearly every other hormone system, including:
- Thyroid function — chronic cortisol suppresses TSH, slows T4 To T3 Conversion, and raises Reverse T3 (see Thyroid Adrenal Connection)
- Sex hormones — progesterone is preferentially shunted into cortisol production ("pregnenolone steal"), worsening estrogen-progesterone balance
- Blood sugar and insulin sensitivity
- Immune function — chronic activation suppresses healthy immunity and can drive autoimmunity
- Sleep, mood, and digestion
The term Adrenal Fatigue is often used colloquially to describe HPA axis dysregulation, though the more accurate clinical framing is HPA axis dysfunction — the adrenals are not failing, the signaling is off.
How MTC applies the HPA axis
At Modern Thyroid Clinic, the HPA axis is treated as foundational. You can prescribe perfect thyroid medication and the most precise Hormone Replacement Therapy, but if cortisol is dysregulated, women rarely feel fully well.
Evaluation typically includes a four-point salivary cortisol test or a Dutch Test — both of which capture the daily cortisol rhythm rather than a single morning blood draw. Patterns reveal whether cortisol is too high, too low, flat, or reversed (low in the morning, high at night).
Treatment is layered:
- Lifestyle foundations — sleep, blood sugar stability, sufficient protein and calories, gentle movement
- Stress physiology work — breathwork, nervous system regulation, time outdoors, relational support
- Targeted nutrients — magnesium, B vitamins, vitamin C, adaptogens like ashwagandha when appropriate
- Addressing root drivers — chronic infection, gut dysfunction (see Leaky Gut), inflammation, trauma
The HPA axis cannot be supplemented past chronic stress. The work is to remove what is keeping it activated, then rebuild capacity. Done well, women feel a return of energy, sleep, mood, and resilience that thyroid medication alone could never produce.
Common symptoms
Common questions
Is HPA axis dysfunction the same as adrenal fatigue?
They describe the same experience but use different language. "[Adrenal-fatigue]" is the popular term and implies the adrenals are exhausted. The more accurate framing is "HPA axis dysfunction" — the adrenals themselves are usually fine; it is the brain-gland signaling that is dysregulated. Either way, the symptoms are real: persistent fatigue, poor stress tolerance, sleep disruption, brain fog, salt or sugar cravings, and feeling "tired but wired." At Modern Thyroid Clinic, we evaluate cortisol patterns objectively and treat the underlying drivers, not just the labels.
How is the HPA axis tested?
A single morning serum cortisol — the typical conventional test — captures one moment in time and misses the bigger picture. Better tools include a four-point salivary cortisol test (saliva collected at waking, midday, late afternoon, and bedtime) or a [dutch-test], which uses dried urine to map cortisol, cortisone, and their metabolites across the day. These tests reveal the rhythm — whether cortisol is high, low, flat, or reversed — which guides treatment far better than a single number ever could.
Can HPA axis dysfunction cause thyroid symptoms?
Yes — and it is one of the most common reasons women feel hypothyroid even with normal thyroid labs. Chronic cortisol elevation suppresses TSH, impairs [t4-to-t3-conversion], and raises [reverse-t3], producing fatigue, weight gain, brain fog, and cold intolerance that look exactly like hypothyroidism. The thyroid is downstream of stress physiology. Until the HPA axis is supported, thyroid medication adjustments often deliver only partial relief. This is why MTC evaluates both systems together — see [thyroid-adrenal-connection].
Think you might be dealing with this?
Talk to a Modern Thyroid Clinic specialist about your symptoms, labs, and next steps.
Book a Discovery CallThis content is for educational purposes only and is not medical advice. Consult a licensed clinician for diagnosis and treatment. Content on this page does not create a doctor-patient relationship with Modern Thyroid Clinic.