- Low testosterone with high LH points to the testes; low testosterone with low or inappropriately normal LH points to the brain or pituitary.
- I examine the testes of every man evaluated for low testosterone, usually with office ultrasound; a lump is imaged that week.
- Prolactin and estradiol are on my initial panel by practice, not guideline requirement, and I never treat an estradiol number without symptoms.
- 'Functional' means the pattern may reverse, not that it is mild; reversibility is tested, not promised.
- Pituitary MRI is selective; the AUA threshold is testosterone below 150 ng/dL with low or low-normal LH.
Low Is Established. Cause Is Not.
A man with two clearly low results usually assumes the next step is picking a product. Marketing sorts men by the number they want; physiology sorts them by which part of the system broke. Testes scarred by chemotherapy, an axis quieted by opioids, severe obesity, and a pituitary tumor all yield low serum testosterone, yet each makes a different claim about what treatment corrects. So before any formulation is discussed, I want LH, FSH, and an evaluation aimed at cause.
What LH and FSH Are Telling You
Low testosterone with an elevated LH means the pituitary is pushing harder and the testes are failing to answer. That is the primary, or hypergonadotropic, pattern. Low testosterone with a low or inappropriately normal LH means the signal is not climbing as it should; that is the secondary, central, or hypogonadotropic pattern, implicating the hypothalamus or pituitary. An LH inside the reference range can still be wrong, because an intact feedback loop should drive LH up as testosterone falls. None of this can be read until low testosterone is confirmed.
FSH speaks to reproduction. A high FSH can indicate poor function in the sperm-producing tubules, but when fertility is the concern, a semen analysis measures the outcome and FSH alone does not. Normal testosterone with high LH, sometimes called compensated hypogonadism, is a reason to keep watching, not to start TRT. And after exogenous testosterone or anabolic steroids, suppressed LH and FSH reflect drug feedback, not a failing pituitary.
Primary Failure: The Testes Cannot Respond
Primary hypogonadism comes from injury to the testes. Undescended testes, torsion, orchitis, chemotherapy, radiation, surgery, and certain genetic conditions often supply the explanation, and some of these men first present with infertility.
Every man I evaluate for low testosterone gets a testicular exam, and in my clinic it usually extends into a scrotal ultrasound at the same appointment. Palpation gives a rough sense of size and consistency; the probe gives measured volume, internal texture, a varicocele, and masses too small or deep for fingers to find. The guidelines mandate the exam, not routine imaging, and a clinician who scans only when something feels abnormal is well within them. My rationale is simply that the exam is already underway and the probe answers what the hand cannot. Nor do I defer: a lump gets imaged that week, not at the next hormone check.
Secondary Patterns and the Trouble With "Functional"
Secondary hypogonadism spans structural pituitary lesions, high prolactin, prior surgery or radiation, head trauma, critical illness, obesity, opioids, glucocorticoids, and other exposures. Calling every low-LH result "pituitary failure" exaggerates structural disease; calling every one "functional" risks overlooking a mass. Prolactin matters most when LH is low or low-normal, since persistently high prolactin suppresses GnRH. One elevated prolactin is read in context and rechecked; a modest rise does not prove a tumor.
My initial panel includes prolactin and estradiol, and estradiol stays on the monitoring panel after treatment begins. That is how I practice, not a guideline requirement. Some testosterone becomes estradiol, which helps protect bone and shapes desire. What I refuse to do is treat an elevated estradiol number in a man with no symptoms; that is the optimization error under a different name.
"Functional hypogonadism" is the term for a secondary pattern that could reverse with obesity, illness, medication, sleep loss, or energy deficit, provided no destructive brain disease has been identified. The European Male Ageing Study linked obesity strongly to the secondary pattern, but that does not mean obesity explains every case or that a high BMI excuses assessing the pituitary. Reversibility is tested, not promised. "Functional" is not a synonym for mild or self-inflicted, and a man with a severe, persistent value should not leave with "lose weight" as his whole plan. I identify the suppressor, address it when safe, remeasure, and if the pattern holds I revisit the explanation rather than defend the label.
Deciding on a Pituitary MRI
Scanning everyone with a low LH would turn up incidental changes; scanning too few would miss disease. The AUA offers a workable threshold: a total testosterone under 150 ng/dL, combined with a low or low-normal LH, justifies pituitary MRI regardless of a normal prolactin. That is a workup criterion, not a line for self-triage. In a specialty-center series of 281 imaged men with low testosterone and normal prolactin, 83.6 percent of scans were normal, and some abnormal ones were probably incidental. Normal prolactin does not fully rule out structural disease, but the yield does not justify imaging every low value.
The pituitary governs more than LH and FSH, and cortisol matters most, because central adrenal insufficiency can be dangerous. A man with a secondary pattern who develops new peripheral vision loss, double vision, or a severe new headache needs prompt attention.
Why Cause Reshapes the Discussion
A primary or secondary label does not select a formulation, but it changes what we discuss. In established primary failure, replacement is likely long term and no stimulation strategy revives an organ that cannot respond. In drug-induced central suppression, altering the exposure may restore signaling in some men, though abrupt withdrawal is hazardous and recovery is not guaranteed. In both patterns, exogenous testosterone suppresses whatever LH and FSH remain, so a man hoping to conceive needs a fertility-preserving plan first. An unexplained label is never a license to stop thinking.
Common questions
What is the difference between primary and secondary hypogonadism?
In primary hypogonadism, testosterone is low and LH is high: the pituitary is pushing harder and the testes are failing to respond. In secondary hypogonadism, testosterone is low and LH is low or inappropriately normal, so the signal from the hypothalamus or pituitary is too weak. Both require confirmed low testosterone, and the pattern locates the failure without naming its cause.
Does low testosterone mean I need a pituitary MRI?
Usually no. The decision weighs severity, LH pattern, prolactin, symptoms, and history together. The AUA supports MRI for a total testosterone under 150 ng/dL paired with a low or low-normal LH, whatever the prolactin shows. In one series of selected men, 83.6 percent of scans were normal. New peripheral vision loss, double vision, or a severe new headache alongside a secondary pattern needs prompt evaluation.
Can functional low testosterone reverse on its own?
Sometimes. Functional hypogonadism refers to a secondary pattern tied to obesity, illness, medication, sleep loss, or energy deficit when no destructive brain disease has been found. Losing weight, recovering from illness, treating a sleep disorder, or changing a medication under supervision can raise testosterone in some men, but how much and how fast varies. Reversibility gets tested rather than promised, and functional does not mean mild.