Your testicles make testosterone. Your pituitary tells them to. When the number comes back low, there are only two possible reasons: the factory has stopped, or the order stopped arriving. LH and FSH tell you which. They are two lines on the same blood form, and leaving them off is the single most common way a low result gets misread.
Two men walk in with the same result. Same age, same symptoms, same low testosterone. One of them has a problem that will not reverse and needs managing for life. The other has something that can often be undone, sometimes completely, without ever going near hormone treatment.
Nothing on the testosterone line distinguishes them.
What LH and FSH actually are
Your pituitary sits under your brain and sends two hormones down to your testicles. Luteinising hormone, LH, tells them to produce testosterone. Follicle stimulating hormone, FSH, drives sperm production. The testicles respond, testosterone rises, and the pituitary senses that rise and eases off. It is a thermostat.
Which means a low testosterone level has to be one of two situations, and the thermostat tells you which.
Primary: the testicles are not responding
If the testicles are struggling, testosterone falls, the pituitary notices, and it shouts louder. LH and FSH go up. Low testosterone with high LH and FSH is primary hypogonadism, and it means the problem is in the testicle itself.
Causes include past injury, past infection, undescended testes in childhood, chemotherapy or radiotherapy, and some genetic conditions that are usually diagnosed earlier in life but occasionally are not.
This version does not reverse. The testicles are not going to start again. What it does need is a proper diagnosis, because the reason matters, and some of the causes carry other health implications that have nothing to do with how you feel.
Secondary: the signal is not arriving
If the pituitary is not sending the message, testosterone falls and LH and FSH stay low, or sit in the middle of the range looking unremarkable. That is the part that catches people out. LH does not have to be flagged as abnormal for the picture to be abnormal. A normal LH alongside a clearly low testosterone is itself the finding, because in a working thermostat it should have climbed.
This is secondary hypogonadism, and it is the more common one in the men who read pages like this.
Why secondary matters more
Because a large share of it is caused by something else, and that something else can often be treated.
Carrying significant excess weight suppresses the signal. So do opioid painkillers, taken regularly. So does glucocorticoid medication, the anti-inflammatory tablets given for conditions like asthma, arthritis and inflammatory bowel disease. Untreated sleep apnea contributes. A pituitary producing too much prolactin will shut the system down and is entirely treatable once found. Iron overload can damage the pituitary quietly for years before anyone looks. Severe illness, major surgery and prolonged stress all push it down temporarily.
None of that is visible on the testosterone line. All of it changes what should happen next.
The guideline calls this mandatory
This is not a matter of opinion or of how thorough a particular doctor likes to be. The Society for Endocrinology guideline states that distinguishing primary from central hypogonadism by measuring LH and FSH, rather than making a non-specific diagnosis of low testosterone, is a mandatory clinical requirement under all circumstances.
Those are the guideline's own words. Under all circumstances. It is worth reading your own results with that sentence in front of you.
What a specialist does with this
The Endocrine Society guideline is explicit that once low testosterone is confirmed, LH and FSH should be measured to separate primary from secondary, and that secondary hypogonadism should prompt a search for the cause rather than a prescription.
In practice that means a low testosterone with low LH leads to more questions, not fewer. Prolactin. Iron studies. Thyroid. A review of every medication you take. And in some patterns, imaging of the pituitary.
A low testosterone with high LH leads somewhere quite different.
What this changes for you
If your result was secondary and the cause was never looked for, you were offered a treatment for a situation that might have resolved without one. If it was primary and nobody said so, you were left believing the problem might go away on its own when it will not.
Either way the missing information is the same two lines on the form.
The honest limitation
This is not always tidy. Plenty of men sit in a grey zone, with borderline numbers and a mixed picture, particularly when weight is a factor. LH and FSH narrow the question considerably. They do not always close it.
What they do is stop you being treated for the wrong problem, which is the outcome that a single testosterone value, on its own, cannot protect you from.
What else belongs on that blood form is covered in what a blood panel actually measures. Why a result can be wrong before it is even analyzed is in what makes a result wrong.
Bhasin S, et al. "Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline." The Journal of Clinical Endocrinology and Metabolism, 2018. doi:10.1210/jc.2018-00229
Jayasena CN, et al. "Society for Endocrinology guidelines for testosterone replacement therapy in male hypogonadism." Clinical Endocrinology, 2022. doi:10.1111/cen.14633