The HPG axis — three glands, one loop
Testosterone production runs on a closed feedback loop between three endocrine glands: the hypothalamus, the anterior pituitary, and the testes (specifically, the Leydig cells embedded in the connective tissue surrounding the seminiferous tubules). The loop runs like this:
The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses. GnRH travels a short distance to the anterior pituitary, where it triggers the release of two gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH is the direct signal to the Leydig cells in the testes to manufacture testosterone. FSH acts on a different cell population — the Sertoli cells — and drives spermatogenesis. Once testosterone enters circulation in sufficient quantity, it feeds back to both the hypothalamus and the pituitary, suppressing GnRH secretion and making the pituitary less responsive to whatever GnRH does get released. The loop self-regulates.
That self-regulation is the entire problem with exogenous testosterone. The body cannot distinguish between testosterone you produced and testosterone you injected. Both bind the same receptors. Both feed back to the hypothalamus and pituitary. Both shut down GnRH, LH, and FSH. The practitioner corpus is consistent: within 2–3 months of 200 mg/week of injected testosterone, roughly 95% of men become azoospermatic or oligospermatic. Exogenous testosterone is, mechanistically, a male contraceptive. That's not a side effect — it's the unavoidable downstream consequence of how the feedback loop is wired.
Primary vs. secondary — diagnose before you dose
Low testosterone is not a single disease. It splits cleanly into two categories, and the protocol for each is different:
- Primary hypogonadism (testicular failure). Hypothalamus and pituitary are signaling correctly. LH and FSH are elevated — the brain is screaming for testosterone — but the testes can't produce. This is the rarer presentation.
- Secondary hypogonadism (signaling failure). The testes are mechanically capable. The brain isn't sending the signal. LH and FSH are low or low-normal despite low total testosterone. The practitioner corpus is direct: most men with low testosterone have secondary hypogonadism, not primary. The testicles can make testosterone — the brain isn't asking for it.
The triggers for secondary suppression that recur across the practitioner corpus: chronic stress (cortisol antagonizes GnRH pulsatility), head injury, poor sleep, excessive heat exposure (sauna, hot tubs), heavy alcohol use, and visceral adiposity. The last one matters because adipose tissue is the body's largest aromatase reservoir — it converts testosterone to estradiol, and elevated estradiol feeds back to the hypothalamus to suppress GnRH even harder. Fat begets low T begets more fat.
This is why labs matter before injections. A man with secondary hypogonadism driven by sleep debt and visceral fat may not need lifelong TRT — he may need the upstream signal restored. A man with primary testicular failure has no such option. Same total T number on the lab report, completely different protocols.
SHBG and the bioavailable fraction
Total testosterone is the headline number, but it's not what your tissues see. Testosterone circulates in three pools:
- Bound to sex hormone-binding globulin (SHBG) — biologically inert, locked away
- Loosely bound to albumin — bioavailable, can dissociate at the tissue level
- Free testosterone — unbound, immediately bioactive (~1–2% of total)
Bioavailable testosterone is the sum of free + albumin-bound — the fraction your androgen receptors actually access. SHBG-bound testosterone is, functionally, off the field. The practitioner corpus emphasizes this: a man with total T at 600 ng/dL and SHBG at 80 nmol/L can be symptomatically hypogonadal, while a man at 450 ng/dL with SHBG at 25 nmol/L feels fine. The total number lies. The free and bioavailable numbers don't.
SHBG itself is modulated by upstream factors — high-fat low-carb diets can artificially elevate it, chronic stress elevates it (cortisol-driven), and thyroid status moves it in both directions. This is why the practitioner consensus on TRT lab panels never stops at total testosterone. You need free T, bioavailable T, SHBG, LH, FSH, estradiol (sensitive assay), and ideally DHT to see the whole picture.
Aromatase — the conversion you can't shut off
Testosterone doesn't sit static in the bloodstream. Two enzymatic conversions happen continuously: 5-alpha reductase converts T to dihydrotestosterone (DHT), the more potent androgen — the practitioner corpus describes DHT as roughly five-fold more receptor-active than testosterone itself. Aromatase converts T to estradiol (E2). Both conversions are necessary. The practitioner corpus is explicit on this point: testosterone must be allowed to convert to estradiol for estrogen's protective effects on bone, brain, cardiovascular tissue, and lipid metabolism. It must be allowed to convert to DHT for full androgenic effect on libido, mood, and tissue.
The mistake men make on TRT is reflexively crushing aromatase with an inhibitor (anastrozole, letrozole) at the first sign of elevated E2. The practitioner consensus pushes back hard: blocking aromatization is a fundamental misunderstanding of the biology. Elevated E2 on TRT is often a signal that the testosterone dose is too high or that visceral adiposity is driving excess conversion — not that aromatase needs to be pharmacologically shut down. Fix the upstream cause, not the downstream enzyme.
What's NOT happening yet
- Your testes are not "broken" until labs prove it. Don't assume primary hypogonadism. Most low-T cases are secondary signaling failures.
- Exogenous testosterone is not a "boost" — it's a replacement. It substitutes for your endogenous production by shutting that production off. There is no version of TRT that preserves natural function without additional compounds in the stack.
- Your fertility is not preserved by default. Within 2–3 months of standard TRT dosing, spermatogenesis collapses in the overwhelming majority of men. If fertility matters, the protocol changes before you start, not after.
- Estradiol is not the enemy. Aromatization is a feature, not a bug. The protocol target is balanced ratios, not crushed E2.
- Total testosterone alone tells you nothing actionable. Without LH, FSH, free T, SHBG, and sensitive E2, you're flying blind.
This chapter is diagnostic, not prescriptive — no stack yet. Chapter 2 maps the upstream lever: which break in the loop you're actually working with, and which compound class addresses it.
The practitioner corpus is consistent on the mechanism. Track your own labs against it. Adjust.