What Causes Low Testosterone?

Two places low testosterone can start

Testosterone production works like a two-part relay. The brain (the hypothalamus and pituitary gland) sends hormone signals called LH and FSH, and the testicles answer by making testosterone and sperm. When the level runs low, the first question is which runner dropped the baton.

• Primary low testosterone means the testicles can't keep up even though the signal is fine. A 2026 review in JAMA puts it at under 1% of men, with Klinefelter syndrome (an extra X chromosome that often goes undiagnosed) as the most common cause. Injury, chemotherapy and radiation can also cause it.
• Secondary low testosterone means the signal from the brain is weak. It's the more common type, and several of its causes can be reversed.

Knowing which type you have matters, because it changes what helps.

Common causes of low testosterone you can change

The same JAMA review lists the potentially reversible causes of secondary low testosterone, and they're a familiar crowd:

• Excess weight. Obesity is a common and often reversible cause. The NIH notes that a healthy weight can improve testosterone levels.
• Serious illness. A hospital stay, a bad infection or a major surgery can push levels down for a while.
• Medications. Opioid pain medicines, corticosteroids like prednisone, some cancer immunotherapy drugs, and medicines that raise a hormone called prolactin can all lower testosterone.
• Pituitary problems. Less common, but a pituitary issue that raises prolactin is one of the things labs look for.

Permanent causes of the brain-side type exist too, such as head and neck radiation and severe head injury. Those are less common, and they need a different conversation.

Does weight or sleep lower testosterone?

Weight, yes, as above. Sleep, also yes, and faster than most people expect. In a University of Chicago study published in JAMA, young healthy men who slept under 5 hours a night for about a week saw their testosterone drop 10% to 15%, which the researchers compared to aging 10 to 15 years. The lowest readings came in the afternoon and evening.

So if your week involves a newborn, a night shift and a 6 a.m. alarm, your hormones have noticed. That's another reason one low test isn't a diagnosis.

Can low testosterone be temporary?

Often, yes. When the cause is illness, short sleep, a medication or weight, the level can come back up once that cause improves. That's why guidelines insist on repeating the test before calling it low.

A single low number after a rough week is a data point. Two low early-morning results on different days, plus symptoms that fit, is when the conversation changes. The Endocrine Society recommends diagnosing low testosterone only when symptoms and consistently low levels show up together.

Is low testosterone just part of getting older?

Partly. MedlinePlus notes that it's normal for testosterone to drift down with age, and the University of Chicago researchers put the usual decline at about 1% to 2% a year. Birthdays alone rarely explain a level that's truly low, though.

When a man in his 40s or 50s has a clearly low result, there's usually a co-pilot: weight gain around the middle, a sleep problem, a new medication, diabetes or a recent illness. Blaming age can hide a cause you could actually fix, and that's the part we care about most.

Age also shapes what symptoms mean. Low energy at 52 could be testosterone, but it could just as easily be sleep apnea, low thyroid, depression, iron deficiency or plain old overwork. Labs keep everyone honest, including us.

Which labs find the cause

Here's the usual order, and why each test earns its spot:

• Total testosterone, twice. Fasting, drawn between 7 and 10 a.m., on two different days. Testosterone peaks in the morning, so an afternoon draw can make normal look low.
• Free testosterone when needed. Weight and diabetes change a carrier protein called SHBG, which can make the total number misleading.
• LH and FSH. High signals with low testosterone point to the testicles. Low or normal signals point to the brain side.
• Prolactin and a closer pituitary look when the brain-side pattern shows up, plus a review of every medication you take.

Our at-home lab panels include total and free testosterone with SHBG for men, along with heart, blood sugar and thyroid markers, and a written review of your results from your DonoMed provider. The at-home panels don't include LH or FSH for men, so if your testosterone comes back low, your provider orders those as a separate test.

What happens after the labs

If the cause is something fixable, that comes first: adjusting a medication with the prescriber who started it, treating sleep apnea or poor sleep, working on weight, or giving your body time to recover from an illness.

If your labs show the brain-side pattern, some men are candidates for enclomiphene, a daily capsule designed to raise LH and FSH, which can prompt the testicles to make more of their own testosterone. It only makes sense when the testicles can still respond, which is exactly why the LH and FSH results matter. Enclomiphene is compounded by a state-licensed 503A pharmacy and is not FDA-approved, and the FDA does not review compounded medications for safety, effectiveness or quality. Labs come before and during treatment, every time.

And if your numbers are normal, we'll tell you so. You'll leave with a real answer, which beats a guess every time.

When to see someone sooner

Everything else (low energy, low sex drive, poor sleep, mood changes) is worth a calm workup. Those symptoms have many possible causes, and testosterone is only one of them.

Sources & further reading

• JAMA (2026): Adult Male Hypogonadism, A Review
• Endocrine Society: Testosterone Therapy in Men With Hypogonadism guideline
• MedlinePlus: Testosterone Levels Test
• University of Chicago Medicine: Sleep loss lowers testosterone in healthy young men
• NIDDK: Treatment for Erectile Dysfunction