Low testosterone is diagnosed through a simple blood test, but the timing, preparation, and interpretation matter more than most people realize. The standard approach is a total testosterone blood test drawn in the early morning, with a reading below 300 ng/dL serving as the widely accepted threshold for a low testosterone diagnosis. Getting an accurate result requires more than just showing up at a lab, though. Here’s what the process actually looks like.
The Primary Blood Test
The first step is a total testosterone test, which measures both the testosterone circulating freely in your blood and the testosterone bound to proteins. This is the most common and most important test in the process. Your doctor will order it as a standard blood draw, typically from a vein in your arm, and results usually come back within a few days.
Total testosterone gives the big-picture number. In some cases, your doctor may also order a free testosterone test, which measures only the small percentage of testosterone that isn’t attached to proteins and is actively available for your body to use. Free testosterone testing is typically reserved for situations where total testosterone results don’t clearly match your symptoms, or when conditions like obesity or liver disease might be affecting how much testosterone binds to proteins.
Why Morning Timing Is Critical
Testosterone levels follow a daily rhythm, peaking in the early morning and dropping throughout the day. This isn’t a minor fluctuation. In men younger than 45, average levels run around 600 ng/dL at 7 a.m., drop to about 500 ng/dL by 10 a.m., and fall to 400 to 450 ng/dL by 2 p.m. That’s a potential swing of 150 to 200 ng/dL over a few hours, enough to push a borderline result from normal to low depending on when blood is drawn.
Because of this, clinical guidelines recommend testing between 8 and 10 a.m. If you get your blood drawn in the afternoon, you could receive a falsely low reading that doesn’t reflect your actual baseline. This is one of the most common reasons for inaccurate results, and it’s entirely avoidable by scheduling your appointment first thing in the morning.
How to Prepare for the Test
Your doctor may ask you to fast for 10 to 12 hours before your blood draw. Fasting overnight and getting tested in the morning covers both the timing and preparation requirements at once. You can continue taking daily prescription medications like blood pressure drugs or blood thinners unless your doctor specifically tells you to stop. Over-the-counter pain relievers like ibuprofen are also fine.
One thing worth doing before the test: tell your doctor about every medication and supplement you’re currently taking. Certain substances can shift hormone levels or interfere with lab results, and your doctor needs that information to interpret your numbers correctly.
What the Numbers Mean
The American Urological Association sets 300 ng/dL as the diagnostic cutoff for low testosterone. A total testosterone level below that threshold, combined with symptoms, supports a diagnosis. This number was chosen to balance the potential benefits of treatment against the risks, based on the best available clinical evidence.
A single low reading isn’t enough for a formal diagnosis, though. Testosterone can fluctuate day to day based on sleep, stress, illness, and other factors. Your doctor will want to confirm the result with at least one additional morning blood draw on a separate day before moving forward. If both readings come back below 300 ng/dL and you have symptoms, that’s when the conversation about treatment typically begins.
Symptoms That Warrant Testing
Doctors don’t usually order testosterone testing as part of routine bloodwork. They order it when specific symptoms point toward a deficiency. The physical signs include erectile dysfunction, loss of interest in sex, shrinking testicles, loss of body hair, reduced muscle mass, thinning bones, and breast tissue swelling. On the emotional side, low testosterone can cause persistent fatigue, depressed mood, difficulty concentrating, irritability, and disrupted sleep. Some men also experience hot flashes.
It’s normal for sex drive to decrease gradually with age. But a complete loss of interest in sex, or a cluster of the symptoms above appearing together, is worth bringing up with your doctor. These symptoms overlap with many other conditions, which is exactly why blood testing is necessary to pin down whether testosterone is actually the issue.
Follow-Up Tests to Find the Cause
If your testosterone comes back low on two separate morning draws, the next step is figuring out why. Your doctor will order additional blood tests to measure luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin. These hormones reveal where the problem originates.
Your brain sends signals to your testicles telling them to produce testosterone. LH and FSH are those signals. If your testosterone is low but LH and FSH are high, your brain is shouting at the testicles to produce more, but they can’t keep up. This points to a problem in the testicles themselves, which doctors call primary hypogonadism. Causes include prior injury, infection, or genetic conditions.
If testosterone, LH, and FSH are all low, the problem is upstream. The brain isn’t sending the right signals in the first place, which points to an issue with the pituitary gland or hypothalamus. This is secondary hypogonadism, and it can be caused by obesity, certain medications, pituitary tumors, or other conditions. Elevated prolactin levels in particular suggest the pituitary gland needs further investigation, often with imaging.
This distinction matters because it changes the treatment approach. A testicular problem and a pituitary problem require different management strategies, so these follow-up tests aren’t optional. They’re a necessary part of getting the right diagnosis and the right treatment plan.