The HPT and HPG Axes —
The Stories Nobody Tells About Thyroid and Sex Hormones

Part One of this series established that the HPA axis — the stress hormone system — is the command centre of the hormonal hierarchy. That cortisol, through the mechanism of pregnenolone steal, directly depletes the precursors from which sex hormones are made. And that a morning blood cortisol draw tells you almost nothing about the diurnal pattern that actually determines your hormonal health.

This post covers the two axes downstream of the HPA: the hypothalamic-pituitary-thyroid axis (HPT) and the hypothalamic-pituitary-gonadal axis (HPG). Both are less well understood than the HPA. Both are more commonly mistreated. And both have clinical stories that are significantly more interesting — and more consequential — than the standard conversation suggests.

The HPT Axis — Why Your Thyroid Test Is Probably Incomplete

The hypothalamic-pituitary-thyroid axis operates on the same architecture as the HPA: the hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary to release thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to produce thyroid hormones — primarily thyroxine, known as T4.

The standard thyroid test measures TSH. In isolation. And then reports "your thyroid is normal."

The problem is that T4 is not the active thyroid hormone. It is a precursor — a storage form that the body converts into the biologically active form, triiodothyronine (T3), in the peripheral tissues. Primarily in the liver, kidneys, gut wall, and muscles. The conversion requires a functional enzyme called deiodinase, adequate selenium as a cofactor, and a biological environment that is not chronically inflamed or stressed.

A normal TSH tells you that the pituitary is appropriately stimulating the thyroid gland. It tells you nothing about whether the T4 being produced is being converted efficiently to T3. It tells you nothing about whether the T3 reaching the cells is actually being used — or whether reverse T3, the inactive mirror image of T3 that competes for the same receptor sites, is blocking it.

A client with normal TSH, normal T4, low free T3, and elevated reverse T3 has significant thyroid dysfunction by any functional measure. Their cells are not receiving adequate thyroid hormone signal. Their metabolic rate is suppressed, their body temperature is low, their gut motility is slow, their cognitive function is impaired, their hair is thinning, their mood is flat. And they have been told their thyroid is normal.

What Blocks T4 to T3 Conversion

The clinical implication is that treating thyroid dysfunction without identifying which of these conversion blockers is operating — and addressing it — produces partial and temporary results. Supplementing with T4 (levothyroxine) when the conversion to T3 is blocked by cortisol or inflammation does not resolve the problem. It increases the substrate for a conversion that is still impaired.

"A TSH result tells you what the pituitary is asking the thyroid to do. It tells you nothing about whether the thyroid hormone produced is reaching the cells in its active form. These are completely different questions — and standard medicine routinely answers only the first while believing it has answered both."

The HPG Axis — Sex Hormones as a Downstream Luxury

The hypothalamic-pituitary-gonadal axis governs reproductive hormone production in both sexes. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary to release luteinising hormone (LH) and follicle-stimulating hormone (FSH), which stimulate the gonads — ovaries in women, testes in men — to produce oestrogen, progesterone, and testosterone respectively.

The word "luxury" in the section heading is deliberate and precise. From the perspective of biological survival, reproductive function is expendable in a way that metabolic function and immune function are not. An organism that cannot reproduce in the current season can reproduce in the next one, if it survives. An organism whose stress response fails does not get a next season. The HPG axis is therefore the first hormonal system the body down-regulates under sustained stress — and the last to recover when the stress resolves.

Oestrogen — The Three Pathways That Determine Everything

Oestrogen is not a single hormone. It is a family of hormones — primarily oestrone (E1), oestradiol (E2), and oestriol (E3) — that are produced, metabolised, and eliminated through a series of enzymatic steps. The pathway through which oestrogen is metabolised is as clinically important as the level of oestrogen produced.

This is the oestrogen metabolism story that is almost never told in standard medical consultations and rarely addressed in conventional hormone replacement prescribing. A woman who is prescribed oestrogen — bioidentical or synthetic — without her oestrogen metabolism pathways having been mapped is taking a hormone intervention whose safety profile depends entirely on which pathway her body uses to clear it. If the 4-OH pathway is dominant, adding more oestrogen substrate into that system is adding fuel to a fire whose presence nobody has checked for.

The Oestrobolome — Where the Gut Comes Back In

The oestrobolome is the collection of gut bacteria responsible for metabolising oestrogen in the large intestine. Conjugated oestrogens arriving in the gut from the liver are processed by these bacteria — ideally eliminated in the stool. When the oestrobolome is disrupted — from dysbiosis, antibiotic use, or low-fibre diet — the enzyme beta-glucuronidase produced by dysbiotic bacteria cleaves the conjugated oestrogens, releasing them back into circulation for reabsorption.

This is one of the primary mechanisms of oestrogen dominance — not from overproduction, but from recirculation. The GI-MAP measures beta-glucuronidase directly. A client with oestrogen dominant symptoms and elevated beta-glucuronidase on GI-MAP has a gut dysbiosis problem driving a hormonal problem. The intervention is not oestrogen management. It is gut restoration.

Progesterone — The Most Commonly Deficient and Least Understood

Progesterone is the calming counterbalance to oestrogen's stimulating effects. It is produced primarily in the second half of the menstrual cycle by the corpus luteum after ovulation. Low progesterone — the most common hormonal finding in women presenting with mood instability, sleep disruption, anxiety, heavy periods, and PMS — is almost never caused by the ovaries failing to respond. It is almost always caused by one of three upstream problems:

• Inadequate ovulation — no corpus luteum, no progesterone. Chronic stress suppresses GnRH pulsatility, disrupting the LH surge required for ovulation. The HPA axis stealing pregnenolone is the mechanism. You cannot have adequate progesterone production without adequate ovulation, and you cannot have reliable ovulation in a chronically stressed HPA axis.
• Pregnenolone steal — as covered in Part One, chronic cortisol demand diverts progesterone precursor toward the cortisol pathway.
• Thyroid insufficiency — this is the connection between the HPT and HPG axes. Low T3 impairs the sensitivity of the ovarian cells to LH stimulation, reducing progesterone output from the corpus luteum even when ovulation occurs. Fix the thyroid conversion problem and progesterone frequently improves without any direct progesterone intervention.

Testosterone — The Lifestyle Hormone

Testosterone is produced in the testes in men and in the ovaries and adrenal glands in women. It is universally described as the hormone of drive, confidence, libido, and muscle mass — and universally managed, in conventional medicine, by either prescribing it (if deficient) or suppressing it (if elevated in women with PCOS). Neither approach addresses why testosterone is at the level it is.

The Hormone Replacement Question — Addressed Directly

There is a growing and largely well-intentioned conversation about bioidentical hormones — progesterone, oestrogen, testosterone, and DHEA — presented as safer, more natural alternatives to synthetic pharmaceutical hormone preparations. Some of this conversation is clinically sound. Some of it is not. And the distinction matters because exogenous hormone administration, bioidentical or otherwise, carries real biological consequences that are not neutralised by the word "natural."

The case for investigating before prescribing is not complicated:

• If oestrogen dominance is driven by beta-glucuronidase and oestrobolome dysbiosis, the intervention is gut restoration — not oestrogen management
• If low progesterone is driven by anovulation secondary to HPA axis stress, the intervention is stress physiology support and cortisol normalisation — not exogenous progesterone
• If low testosterone is driven by sleep deprivation and insulin resistance, the intervention is sleep hygiene and metabolic correction — not testosterone replacement
• If hypothyroid symptoms exist with normal TSH and low free T3, the intervention may be selenium, gut restoration, cortisol normalisation, and liver support — not levothyroxine

In each of these cases, the hormonal finding is real and the symptoms are real. The question is whether the hormonal finding is the cause or the consequence. Exogenous hormones administered when the finding is a consequence of upstream dysfunction treat the signal rather than the source — and carry the biological risks of hormone administration without necessarily producing the outcomes the client is seeking.

The Three Axes Together — Why Integration Is the Only Approach That Works

The HPA, HPT, and HPG axes are not three separate systems that happen to exist in the same body. They are three components of a single integrated regulatory network, continuously communicating and continuously influencing each other.

Chronic HPA activation suppresses HPT conversion and HPG production simultaneously. Hypothyroidism reduces progesterone production and impairs the clearance of cortisol — creating a feedback loop that worsens HPA dysregulation. Sex hormone deficiency — particularly oestrogen loss in perimenopause — sensitises the HPA axis, increasing the cortisol response to stressors that a more hormonally supported nervous system would manage without difficulty.

The DUTCH Plus is the single test that maps all three axes in one collection. Cortisol diurnal pattern and CAR. DHEA and its metabolites. Oestrone, oestradiol, oestriol, and their metabolites through the 2-OH, 4-OH, and 16-OH pathways. Progesterone metabolites. Testosterone and its metabolites. Melatonin. Neurotransmitter metabolites including serotonin and dopamine precursors. One test. The complete hormonal picture that makes sense of the presenting symptoms rather than treating each individually.