Education
Testosterone Blood Test: What It Checks and How to Prepare
Written and reviewed by Dr. Slava Fuzayloff, DO · Board-Certified Internal Medicine · Last updated: July 2026
Education
Written and reviewed by Dr. Slava Fuzayloff, DO · Board-Certified Internal Medicine · Last updated: July 2026
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A testosterone blood test measures the amount of testosterone in your blood to check for low testosterone (low T). It's a simple blood draw, done in the morning, and it's the only way to confirm whether your testosterone is actually low — symptoms alone can't.
Below is what the test checks, how to prepare, and what your results mean.
A testosterone blood test is a standard blood draw that measures how much testosterone is in your blood, reported in nanograms per deciliter (ng/dL). Providers order it when a man has symptoms of low testosterone — low sex drive, fatigue, or trouble getting or keeping an erection.
You may also see it called a "total testosterone test" or "serum testosterone test" ("serum" just means the liquid part of your blood). It's the same test.
The test alone can't diagnose a condition. An abnormal number, combined with your symptoms, tells your doctor whether to look further.
Most men start with one number — total testosterone. If that number is borderline, or your symptoms don't match it, two more measurements help clarify the picture: free testosterone (the usable part) and SHBG (a protein that controls how much testosterone your body can actually use).
| Measurement | What it shows | When it's used |
|---|---|---|
| Total testosterone | All the testosterone in your blood, bound and unbound | The first, main screening number for everyone |
| Free testosterone | The small fraction not attached to proteins — the part your body can actually use | When total T is borderline, or symptoms don't match the total number |
| SHBG (sex hormone-binding globulin) | A protein that grabs onto testosterone and controls how much stays “free” and usable | When total T looks off, or things like extra weight, thyroid problems, or aging may be shifting it |
For most men, total testosterone is enough. Free testosterone and SHBG come in when the total number doesn't tell the full story. On your lab report, testosterone shows up as a number measured in ng/dL next to the lab's normal range.
Sometimes. Endocrine Society guidelines recommend a fasting morning sample when checking for low testosterone, because eating — especially sugar — can temporarily lower your total testosterone. Many labs don't strictly require it unless your test is bundled with glucose or cholesterol. Follow the instructions your clinic gives you.
A few things that can skew a result, worth avoiding beforehand:
Heavy meals right before the draw — food and glucose can push the number down.
Intense exercise the day before — hard workouts can temporarily shift your levels.
Biotin supplements — high-dose biotin (often in hair and nail products) can throw off lab results. Tell your provider what supplements you take.
One common worry: masturbation or sex beforehand does not meaningfully change your total testosterone result. You don't need to avoid it.
Between 7 and 10 a.m. Testosterone follows a daily rhythm — it peaks in the early morning and drops through the day. A test drawn in the afternoon can read falsely low, even in a man whose morning level is normal.
This is why a low afternoon result often gets repeated in the morning before any diagnosis. If you can only test later in the day and the number comes back low, expect your doctor to recheck it in the morning.
One low number does not equal low testosterone. Your testosterone naturally rises and falls from day to day, so a single low reading can be a fluke rather than a real problem — which is why one test is never the whole story.
Endocrine Society guidelines call for two separate morning tests, both low, before diagnosing low testosterone. If your first result is low, a second confirming draw is standard — not a sign something went wrong. This is the step that separates a real diagnosis from a one-off dip.
If your testosterone is confirmed low, your doctor looks at why. That means a few more blood tests beyond testosterone itself, usually drawn from the same sample. These check whether the problem starts in your testicles or in the brain signals that control them, and screen for other conditions that cause the same symptoms.
| Test | What it checks |
|---|---|
| LH and FSH | Pituitary hormones that signal your testicles. They show whether low T starts in the testicles or higher up in the brain's signaling. |
| Estradiol | A form of estrogen. Helps interpret symptoms and, later, guide treatment. |
| Prolactin | A pituitary hormone; high levels can suppress testosterone and point to a specific cause. |
| General health labs (CBC, metabolic panel, thyroid, blood sugar) | Rule out other conditions — thyroid trouble, anemia, diabetes — that can mimic or worsen low-T symptoms. |
The point isn't to run every test. It's to find the cause and rule out look-alikes before starting any treatment.
Your results come back as a number, and you read it against the lab's normal range (called the reference range) printed beside it. A number below that range points to low testosterone; a number inside it counts as normal for that lab. Ranges vary slightly between labs, so always use the range on your report.
Two things a results page can't do: it can't diagnose you on its own (that takes your symptoms plus a confirmed second test), and the single cutoff for "low" and how levels shift by age are their own topics. What is low testosterone explains the diagnostic cutoff. Testosterone levels by age shows normal ranges by decade.
At Testosterone Clinic NYC, testing is done in person. Your first visit includes an exam and an in-office blood draw, sent to Quest or Northwell. Results come back in a few business days, and Dr. Fuzayloff reviews them with you to decide whether treatment makes sense.
Ready to get your levels checked? A $175 evaluation covers the visit and determines whether low testosterone is behind your symptoms — same-day appointments are often available.
Dr. Slava Fuzayloff, DO evaluates every new patient in person.
Yes. If you start testosterone therapy, ongoing blood tests track your levels and check for side effects — most importantly your red blood cell count.
• Endocrine Society. Testosterone Therapy in Men With Hypogonadism: Clinical Practice Guideline (2018). View source 
• Mayo Clinic Laboratories. Testosterone, Total and Free, Serum. View source
• MedlinePlus (NIH). Testosterone Levels Test. View source
• NCBI / Endotext. Laboratory Assessment of Testicular Function. View source
• FDA. The FDA Warns that Biotin May Interfere with Lab Tests (2017). View source
