The panel / Androgens

LH and FSH

If your testosterone is genuinely low, this is the test that decides what happens next. LH and FSH tell us whether the problem is in the testicles or in the brain that signals them. Those are different diseases with different treatments, and one of them is frequently reversible without any hormone at all.

What it measures

LH and FSH are the two signals your pituitary sends to your testicles. LH tells them to make testosterone. FSH drives sperm production. Your brain reads how much testosterone is circulating and adjusts both accordingly, so their level tells us whether the signal is being sent and ignored, or never sent at all.

What each signal does

LHThe signal to produce testosterone

FSHThe signal to produce sperm

BothRespond to how much testosterone the brain detects

This is a branch point, not a severity score. The number matters far less than which direction it moved relative to the testosterone.

One point worth flagging: biotin, sold widely as a hair and nail supplement, interferes with the immunoassays used for LH and FSH and can distort the result. It needs to stop at least three days before the draw. Men taking it for hair loss often do not think of it as a medication and do not mention it.

Why it is on the panel

Because low testosterone is a finding, not a diagnosis, and this is what turns one into the other. Without it you know a man is low but not why, and the why determines everything: whether to look for a pituitary tumour, whether the cause is a medication he could stop, whether weight loss would fix it, and whether starting testosterone would cost him his fertility permanently or temporarily.

How it is measured

SpecimenSerum, drawn with the testosterone

TimingMorning, 7 to 10am

FastingYes, with the testosterone draw

When drawnAfter low testosterone is confirmed on two samples

Stop firstBiotin supplements, 72 hours before

TurnaroundTypically 2 to 4 days

Interpretation

Reading the result

The pattern is read against the testosterone, never alone. There are two that matter and they point in opposite directions.

Bands used in practice

Low testosterone, high LH and FSHPrimary. The testicles are not responding despite being told to.

Low testosterone, low or normal LH and FSHSecondary. The signal itself is not being sent.

Low testosterone, mixed pictureCombined, which happens and is read on the rest of the panel.

Normal testosteroneLH and FSH are not usually informative on their own.

In primary hypogonadism the FSH rise reflects damage to the sperm-producing tubules, and sperm production is usually hit harder than testosterone production. In secondary hypogonadism both fall together, because the single missing signal drives both.

What produces a secondary pattern without pituitary disease

This matters because the secondary pattern is common and frequently has a cause that is neither a tumour nor permanent.

8 things that move it

Obesity. The most common cause by a wide margin, and weight loss of even 5 percent often raises testosterone meaningfully.

Opioids. Chronic use suppresses the pituitary signal and is routinely missed.

Glucocorticoids. Steroid courses suppress the axis for as long as they are taken.

Anabolic steroids or prior testosterone use. Exogenous androgen shuts the signal down directly.

High prolactin, from a pituitary cause or from medication.

Iron overload, which deposits in the pituitary and is easy to test for.

Acute or systemic illness, which suppresses the axis temporarily.

Head injury or previous cranial radiation.

What happens next

A primary pattern prompts consideration of karyotype testing, because Klinefelter syndrome is the most common cause and is frequently undiagnosed into adulthood.

A secondary pattern prompts prolactin, iron studies, a full medication review, and an assessment of the other pituitary hormones.

A secondary pattern in a man under 50, or with headaches, visual changes, very low gonadotropins or a testosterone below 150, prompts pituitary imaging.

If fertility matters, the branch decides the options. That conversation happens before any testosterone is prescribed, not after.

Straight answers

Because the treatments are different and one of them is reversible. A secondary pattern is often caused by something we can remove: weight, an opioid, a steroid course, a prolactin problem. Fix that and the testosterone frequently recovers on its own. A primary pattern means the testicles themselves are damaged, which is not reversible, and the conversation is about replacement rather than repair.