LH, FSH and Prolactin: The Pituitary Side of Low Testosterone

A low testosterone result answers one question — "is it low?" — but not the more useful one: why? That's where three pituitary hormones come in. LH, FSH and prolactin turn a bare testosterone number into an actual diagnosis: whether the problem is in the testes or in the brain that signals them, whether it might be reversible, and what it means for fertility. This is a plain-language guide for men trying to make sense of a low-T workup, written for the expats and travellers who get bloods done in Pattaya. It's general education, not a diagnosis.

The chain of command: the HPG axis

Testosterone isn't made in isolation — it's the end of a chain. The brain's hypothalamus signals the pituitary, which releases two messenger hormones to the testes ³:

• LH (luteinising hormone) tells the testes' Leydig cells to make testosterone.
• FSH (follicle-stimulating hormone) tells the Sertoli cells to support sperm production.

So LH and FSH are the signals, and testosterone (plus sperm) is the response. That simple relationship is what makes these two hormones so informative: by reading the signal alongside the response, you can locate exactly where the system is breaking down.

Primary vs secondary: where the problem is

This is the core of it. When testosterone is genuinely low, measuring LH and FSH splits the cause into two very different categories ¹:

• Primary hypogonadism (testicular failure): testosterone is low, but LH and FSH are high. The pituitary is signalling hard, but the testes can't respond — so the brain "shouts louder." Causes include Klinefelter syndrome, mumps, or chemotherapy/radiation ³.
• Secondary (central) hypogonadism: testosterone is low, and LH/FSH are low or inappropriately normal. Here the signal is missing — the problem is in the pituitary or hypothalamus. The Endocrine Society stresses identifying these men, because some have pituitary or hypothalamic disorders that need their own management beyond testosterone ¹.

Why it matters: in primary failure, testosterone replacement is the route. In secondary, the cause may be reversible, and fertility-sparing options (like clomiphene/enclomiphene or hCG, which raise your own LH/FSH) come into play ³.

Why prolactin is in the workup

Prolactin seems out of place on a male hormone panel until you see what it does. A high prolactin — most often from a small, benign pituitary tumour (a prolactinoma), or from certain medications — suppresses the whole axis, lowering LH, FSH and testosterone and causing low libido and erectile difficulty ⁵. Because it's a treatable cause, guidelines advise checking prolactin specifically when testosterone is low and LH is low or low-normal ². And when testosterone is very low (under about 150 ng/dL) with a low or normal LH, a pituitary MRI is often warranted to look for a tumour, regardless of the prolactin result ².

The fertility twist: what TRT does to this axis

Here's something many men don't realise until it matters. Testosterone therapy switches LH and FSH off. Giving the body testosterone from outside provides negative feedback to the pituitary, which stops releasing LH and FSH — and without FSH and the high intratesticular testosterone that LH drives, sperm production shuts down ⁶. That's why fertility-minded men are often steered away from standard TRT and toward options like hCG or clomiphene that work with the axis rather than overriding it (see our guide on TRT and fertility). The good news: after stopping testosterone, roughly two-thirds of men recover normal sperm counts within about a year ⁶ — but it isn't instant or guaranteed, which is exactly why the conversation should happen before starting.

What we see at the clinic

Plenty of men arrive in Pattaya with a single low testosterone number and an assumption that TRT is the automatic next step. Our first job is usually to slow down and ask why it's low — and LH, FSH and, where indicated, prolactin are how we answer that. It changes things: a man with high LH (primary) is a different conversation from one with low LH (secondary), where we want to know whether there's a reversible or pituitary cause before anyone commits to lifelong therapy. We're also careful to raise fertility early, because TRT quietly switches off sperm production and not every man has been told that. We don't diagnose or start hormone treatment from one number — it's interpreted with repeat morning testosterone, SHBG, symptoms and these pituitary markers, by a doctor. What the full picture buys is the right treatment, not just a treatment.

Common questions

My testosterone is low — why do I need LH and FSH tested? Because they tell you the cause. High LH/FSH points to the testes (primary); low or normal LH/FSH points to the pituitary or hypothalamus (secondary) — and those are managed very differently ¹. Without them, you know the level but not the diagnosis.

What does a high prolactin mean? It can be a treatable pituitary cause of low testosterone — often a small benign tumour or a medication — that suppresses the whole axis and lowers libido ⁵. It's why prolactin is checked when the picture looks central, and why a very low testosterone may prompt a pituitary MRI ².

Will testosterone therapy make me infertile? It suppresses fertility while you're on it — TRT switches off LH/FSH and sperm production ⁶. Most men recover within about a year of stopping, but not all, which is why fertility-sparing options exist and why this is discussed before starting ⁶.

Can low testosterone be fixed without TRT? Sometimes — particularly in secondary hypogonadism, where the cause may be reversible and medications that raise your own LH/FSH (clomiphene, hCG) can work ³. That's one practical reason to find out whether it's primary or secondary first.

When should these be measured? After a low testosterone is confirmed — guidelines advise repeating a morning testosterone first, then using LH/FSH (and prolactin if indicated) to find the cause ¹. They're the follow-up that turns a number into an answer.

Key takeaway

A low testosterone is a starting point, not a diagnosis — and LH, FSH and prolactin are what complete it. LH and FSH locate the problem: high means the testes are failing (primary, → replacement), low or normal means the brain's signal is missing (secondary, → look for a reversible or pituitary cause) ¹. Prolactin catches a treatable pituitary driver ⁵. And because TRT itself switches off LH/FSH and sperm production ⁶, these markers also shape your fertility choices. Read together with a repeat morning testosterone by a doctor, they turn "your T is low" into the right plan rather than a reflexive prescription.