FSH and LH: why a testosterone number alone tells you almost nothing
FSH and LH are the two pituitary hormones that decide whether a low testosterone reading is a testicular problem or a brain-signal problem. Reading them together is the difference between a guess and a diagnosis.
What to remember before reading on.
- 1FSH and LH are pituitary hormones. They are the signal from your brain to your testes.
- 2Low testosterone with high FSH/LH is a testicular problem (primary hypogonadism). Low testosterone with low FSH/LH is a pituitary or hypothalamic problem (secondary hypogonadism).
- 3FSH is the single most sensitive marker for impaired sperm production — it rises before total testosterone falls.
- 4Without FSH and LH, you cannot decide whether TRT, hCG, lifestyle change, or further work-up is the right next step.
What FSH and LH are, in 90 seconds
The pituitary gland sits at the base of the brain. It releases two hormones that talk to the testes: luteinising hormone (LH) and follicle-stimulating hormone (FSH).
- LH acts on Leydig cells in the testes and tells them to produce testosterone.
- FSH acts on Sertoli cells, which physically support and direct spermatogenesis — the production of sperm.
Both are released in pulses, regulated by GnRH from the hypothalamus, and are governed by negative feedback from circulating testosterone, oestradiol, and inhibin B (a Sertoli-cell hormone). It is a closed loop, and the loop is what makes the system diagnosable.
Why a single testosterone number is rarely enough
Imagine three men who all walk into a clinic with the same total testosterone — say, 230 ng/dL. That's clearly below the typical adult male range. Without LH, you don't know what to do.
- Man A has LH of 14 IU/L (well above the upper limit). His brain is shouting; his testes aren't answering. This is primary hypogonadism: a testicular problem. Causes include past chemotherapy, varicocele, undescended testes in childhood, Klinefelter syndrome, post-orchitis. The intervention is testosterone replacement if symptoms warrant — the testes themselves can't be "fixed."
- Man B has LH of 0.4 IU/L (suppressed). His brain isn't signalling. This is secondary hypogonadism — pituitary or hypothalamic. Causes include exogenous androgen use (often un-disclosed), opioids, severe under-eating, prolactinoma, traumatic brain injury, rare genetic syndromes. The intervention is finding and addressing the cause; for some men, GnRH or hCG-based therapy preserves fertility better than direct testosterone replacement would.
- Man C has LH of 5 IU/L (mid-range). His axis is partially functional but compensating poorly. He needs the rest of the panel — SHBG, prolactin, sometimes a repeat — to make sense of it.
Three men. Same total testosterone. Three completely different decisions. That is the entire reason a hormone panel exists.
FSH is the most sensitive marker for fertility
If LH is the spotlight on the testes, FSH is the spotlight on spermatogenesis specifically.
When Sertoli-cell function deteriorates — from heat exposure, varicocele, post-mumps damage, idiopathic causes, or simply age — the cells make less inhibin B. Inhibin B normally restrains FSH. Less inhibin B means more FSH. So FSH rises early, often before total testosterone is visibly affected.
A man with a low-normal sperm count and an elevated FSH is heading toward a fertility problem even if his total testosterone is unremarkable. A man with a low sperm count and a low or low-normal FSH is far more likely to have a pituitary or hypothalamic cause that is sometimes reversible.
This is the diagnostic information you simply cannot get from a testosterone-only panel.
Reference ranges, with caveats
Adult male reference ranges from major labs and assay manufacturers cluster around:
- FSH: 1.5–12.4 IU/L
- LH: 1.7–8.6 IU/L
Two important caveats. First, both hormones are pulsatile — single readings can vary by 20–30% between samples taken hours apart. A single odd value warrants a repeat before any decision. Second, reference ranges are population-wide; what matters clinically is your number in the context of your testosterone, your symptoms, and ideally a previous baseline.
What different patterns suggest
| Total testosterone | LH | FSH | Most likely picture |
|---|---|---|---|
| Low | High | High | Primary hypogonadism (testicular) |
| Low | Low | Low | Secondary hypogonadism (pituitary/hypothalamic) |
| Low | Mid | Mid | Compensated or early pituitary insufficiency |
| Normal | High | High | Sub-clinical testicular compromise |
| High | Suppressed | Suppressed | Exogenous androgen use |
Each row leads to a different next step. Each row is invisible to a panel that only measures testosterone.
What this means for your decisions
Three practical implications.
Before TRT, insist on FSH and LH. A clinician who is willing to start replacement therapy on the basis of a single total testosterone reading without the pituitary context is making a decision with one eye closed. You only get one chance to have a real, untreated baseline.
During TRT, FSH and LH should be near zero. That is the expected pattern. Anything else needs explaining.
For fertility questions, FSH is the headline. If you are trying to conceive and your partner has been investigated, the male side starts with hormones — and FSH is the highest-value single number on that panel.
A panel that includes FSH, LH, total + free testosterone, SHBG, prolactin, and oestradiol gives you all six perspectives at once. That is what the FutureKit Hormone Panel 01 is.
Sources cited: ESHRE guideline group (workup of male and couple infertility) — full entry on /science.