HPA-G Basic Profile

Serotonin is a monoamine neurotransmitter synthesised from tryptophan, predominantly in enterochromaffin cells of the gastrointestinal tract and within central serotonergic neurones. It modulates mood, sleep architecture, appetite, and gastrointestinal motility. Peripheral serotonin does not cross the blood-brain barrier, so urinary excretion reflects a composite of gut-derived and metabolic turnover rather than synaptic central nervous system activity.

In functional neurotransmitter assessment, serotonin is interpreted as a marker of mood regulation and sleep-wake balance. Low urinary serotonin is hypothesised to correlate with low mood, sleep disturbance, and carbohydrate craving, while elevations may reflect increased synthesis or turnover. These interpretations remain exploratory and are not validated as diagnostic thresholds.

Dopamine is a catecholamine neurotransmitter and precursor to noradrenaline and adrenaline, synthesised from tyrosine via L-DOPA. It governs reward, motivation, motor control, and executive function. Urinary dopamine originates from both central and peripheral sources and undergoes extensive metabolism; urinary excretion does not directly quantify synaptic dopamine activity.

Functional neurotransmitter testing interprets urinary dopamine as a surrogate for motivation, focus, and reward-drive. Practitioners use patterns of dopamine alongside other catecholamines to inform nutritional or botanical support strategies, though the correlation with central dopaminergic tone has not been established in controlled studies.

Norepinephrine is a catecholamine synthesised from dopamine in sympathetic neurones and the adrenal medulla, serving as both a neurotransmitter and a hormone. It mediates arousal, attention, and the sympathetic 'fight or flight' response. Urinary norepinephrine reflects total-body sympathetic output and adrenal secretion rather than localised synaptic release.

In the HPA-G framework, norepinephrine is interpreted as a marker of stress resilience and alertness. Elevations are associated with chronic sympathetic activation or acute stressors, while low values may suggest adrenal insufficiency, chronic fatigue, or depleted catecholamine reserves. These patterns guide functional interventions but do not replace endocrine diagnostic pathways.

Epinephrine is synthesised from norepinephrine in the adrenal medulla via phenylethanolamine-N-methyltransferase and is the principal 'stress hormone' released during acute sympathetic activation. It increases heart rate, mobilises glucose, and redirects blood flow to vital organs. Urinary epinephrine excretion reflects adrenal medullary secretion and whole-body sympathetic tone.

Functional neurotransmitter profiles interpret epinephrine as a marker of acute stress response and adrenal reserve. Elevated values may indicate chronic stress, anxiety, or sympathetic overdrive, while low values are associated with adrenal fatigue or blunted stress reactivity. These interpretations remain within the functional-medicine paradigm and are not diagnostic of endocrine pathology.

GABA is the principal inhibitory neurotransmitter in the mammalian central nervous system, synthesised from glutamate by glutamic acid decarboxylase. It modulates neuronal excitability, anxiety, and sleep. Peripheral GABA is synthesised in multiple tissues, including the gut, and urinary GABA predominantly reflects peripheral metabolism rather than synaptic central GABAergic activity.

Functional neurotransmitter testing interprets urinary GABA as a marker of calm, relaxation, and inhibitory tone. Low values are associated with anxiety, sleep disturbance, and muscle tension, while elevations may reflect increased peripheral synthesis or gut microbial activity. The clinical validity of urinary GABA as a surrogate for central GABAergic function is contested and not supported by pharmacokinetic data.

Glutamate is the major excitatory neurotransmitter in the central nervous system and a key intermediate in amino-acid metabolism. It is synthesised from glutamine and alpha-ketoglutarate and participates in protein synthesis, energy metabolism, and the urea cycle. Peripheral glutamate is abundant, and urinary excretion does not correlate with central glutamate neurotransmission, as glutamate does not cross the blood-brain barrier in physiological conditions.

In functional neurotransmitter panels, glutamate is interpreted as a marker of excitatory tone and metabolic activity. Elevations are hypothesised to correlate with excitotoxicity, anxiety, or poor sleep, while low values may reflect impaired synthesis or metabolic depletion. These interpretations are speculative and lack validation in controlled neurochemical studies.

Cortisol is a glucocorticoid hormone synthesised in the adrenal cortex under ACTH control, following a circadian rhythm with peak levels in the early morning and a nadir at night. It regulates glucose metabolism, immune function, and the stress response. Urinary cortisol excretion integrates total cortisol production over the collection window and is influenced by timing, hydration, and renal function.

In the HPA-G profile, cortisol is interpreted as the central marker of hypothalamic-pituitary-adrenal axis activity and stress adaptation. Random urinary cortisol provides a snapshot rather than the integrated assessment of a 24-hour collection or the dynamic information from a salivary diurnal curve. Elevated cortisol suggests chronic stress or HPA activation, while low values raise the possibility of adrenal insufficiency or HPA axis suppression, though single random samples lack the diagnostic power of formal endocrine testing.

This derived marker expresses the relative balance between downstream catecholamines (norepinephrine and epinephrine) and their precursor dopamine. It is intended to reflect the efficiency of catecholamine synthesis and the balance between dopaminergic reward-drive and adrenergic stress response.

A high ratio suggests preferential conversion of dopamine to norepinephrine and epinephrine, interpreted as a stress-dominant or hyperadrenergic state. A low ratio may indicate dopamine accumulation or impaired hydroxylation, hypothesised to correlate with low motivation or 'dopamine-dominant' patterns. These interpretations are framework-specific and not validated outside functional-medicine practice.