The loop you are about to override
Testosterone production runs on a feedback loop between three glands: the hypothalamus releases GnRH in pulses, which tells the pituitary to release LH and FSH, which tell the testes to make testosterone and sperm. When enough testosterone is circulating, it feeds back and suppresses the top of the loop. The body cannot tell the difference between testosterone you made and testosterone you injected — both feed back, both shut down LH and FSH. On any exogenous androgen, LH and FSH suppress to near zero. That is the loop working, not a disease — and it is the mechanism behind everything else in this chapter.
Diagnose before you dose: primary versus secondary
Low testosterone is not one condition. It splits into two, with opposite lab signatures and different correct responses:
- Primary hypogonadism (testicular failure): the brain is signaling fine — LH and FSH are high — but the testes can't produce. The rarer presentation.
- Secondary hypogonadism (signaling failure): the testes are capable, but the signal is low — LH and FSH are low or inappropriately normal despite low testosterone.
In the clinical TRT population — men presenting with low testosterone and symptoms but no organic disease — secondary/functional is the dominant pattern, often driven by sleep debt, chronic stress, opioids, or visceral fat (adipose tissue aromatizes testosterone to estradiol, which suppresses the axis harder). This matters because a man whose low testosterone is functional may not need lifelong replacement — he may need the upstream cause fixed. Same number on the lab, completely different path. That is why bloodwork (including LH and FSH) comes before injections.
Fertility: the consequence people underestimate
Exogenous testosterone is, mechanistically, a male contraceptive — but be precise about how, because getting this wrong has real consequences. In the controlled data (the WHO multicenter study on 200 mg/week), the average time to azoospermia was about four months, and only roughly two-thirds of men reached full azoospermia — the rest dropped to severe oligospermia but not zero. Two takeaways for an honest course: first, suppression is slower and less complete than the "you'll be sterile in two months" shorthand suggests, so TRT is not a reliable contraceptive; second, men who want to preserve fertility should address it before and during a protocol (the restart and fertility-preserving tools are covered in the PCT chapter), not assume it will simply return.
Restore or replace
The honest fork: some men are better served restoring their own production than replacing it. A younger man with functional secondary hypogonadism — driven by lifestyle, recoverable — may restore the axis by fixing the upstream cause or with a restart protocol, preserving fertility and avoiding a lifelong dependency. A man with genuine primary testicular failure has no such option; replacement is the path. The maximalist culture skips this question entirely and goes straight to replacement. The disciplined approach asks it first: can this be restored, or does it need replacing? — because the answer changes the whole protocol.
What you monitor from day one
TRT is not a set-and-forget prescription. From the start you track the markers the next chapters detail: testosterone (at trough, on injectables) and free testosterone against symptoms, sensitive estradiol, SHBG, and — as the hard safety line — hematocrit. The goal is symptom resolution at the lowest effective dose with the safety markers held in range, not the highest number you can chase.
Educational content, not individual medical advice.