Serum testosterone is the total amount of testosterone measured in your blood. When a doctor orders a testosterone test, this is typically what they’re checking. The result captures all testosterone circulating in your bloodstream, including the portion actively available to your tissues and the portion bound to proteins and temporarily inactive. For men, the widely used diagnostic cutoff is 300 ng/dL, below which testosterone is considered low.
The Three Forms in Your Blood
Testosterone doesn’t float freely through your bloodstream in one uniform state. It exists in three distinct forms, and understanding the difference matters because a “normal” total number can sometimes be misleading.
About half of the testosterone in your blood is bound to a protein called sex hormone-binding globulin (SHBG). This portion is essentially locked up and unavailable to your tissues. Another large fraction is loosely bound to a different protein, albumin. Only 0.5% to 3% remains completely unbound, called free testosterone.
When your doctor orders a standard testosterone test, the result reflects all three forms added together. That’s your total serum testosterone. Free testosterone and the albumin-bound portion together are sometimes called “bioavailable testosterone” because both can be readily used by your body’s tissues. In certain situations, particularly when SHBG levels are unusually high or low, your doctor may order a free or bioavailable testosterone test to get a clearer picture of what’s actually reaching your cells.
Normal Ranges for Men
The American Urological Association sets 300 ng/dL as the threshold below which testosterone is considered low. Other medical societies around the world use cutoffs ranging from 230 to 350 ng/dL, but 300 ng/dL is the most commonly applied standard in the United States.
Levels vary by age. Data from national health surveys show that men in their early 20s typically fall in a range of about 409 to 558 ng/dL, while men aged 40 to 44 cluster between 350 and 473 ng/dL. The overall middle range for men 20 to 44 is roughly 374 to 511 ng/dL. Testosterone gradually declines with age, generally dropping about 1% to 2% per year after age 30, which means a level that would be unremarkable at 55 might warrant attention at 25. A study using NHANES data proposed age-specific cutoffs for low testosterone: around 409 ng/dL for men in their early 20s, dropping to about 350 ng/dL for men in their early 40s.
Normal Ranges for Women
Women produce testosterone too, just in much smaller amounts. For a healthy premenopausal woman around age 30, total testosterone typically falls between 15 and 46 ng/dL, with free testosterone ranging from 1.2 to 6.4 pg/mL. These levels fluctuate slightly across the menstrual cycle, with a small bump around ovulation, but the variation is minor enough that a blood draw on any day of the cycle gives a usable result.
Testosterone levels above the normal female range can point toward conditions like polycystic ovary syndrome (PCOS) or, less commonly, adrenal or ovarian tumors. Low levels in women may contribute to reduced energy, lower libido, or decreased bone density, though female testosterone deficiency is less clearly defined and more debated than in men.
What Testosterone Does in Your Body
Testosterone’s most recognized role is in male sexual development, but its functions extend well beyond that. It drives sperm production, signals your bone marrow to make new red blood cells, and keeps both bone density and muscle mass from declining. It plays a direct role in libido, mood regulation, and fat distribution. In women, testosterone contributes to bone strength, muscle maintenance, and sexual desire at the lower concentrations their bodies produce.
Why the Test Requires a Morning Blood Draw
Testosterone follows a daily rhythm. Levels peak between 5 and 8 a.m. and reach their lowest point in the evening, with a drop of 10% to 25% over the course of a single day. Because of this natural swing, the American Urological Association recommends blood draws before 10 a.m. An afternoon sample could read significantly lower than a morning one in the same person on the same day, potentially leading to an inaccurate diagnosis of low testosterone.
A single low reading also isn’t enough for a diagnosis. Guidelines call for at least two separate morning blood draws showing levels below the threshold before low testosterone is confirmed, since day-to-day variation is normal.
Factors That Shift Your Results
Your total serum testosterone number can be pushed higher or lower by factors that have nothing to do with how much testosterone your body actually produces. Many of these work by changing SHBG levels, which in turn change how much testosterone is “counted” versus how much is available to your tissues.
Body weight is one of the strongest influences. Overweight and obese individuals tend to have lower SHBG levels, which can pull total testosterone readings down even when bioavailable testosterone is adequate. Weight loss tends to normalize SHBG. On the other end, eating disorders like anorexia nervosa push SHBG higher, which can inflate total testosterone while reducing the amount your tissues can actually use.
Thyroid function also plays a role. Both an overactive and underactive thyroid alter SHBG production. Hyperthyroidism raises SHBG, while hypothyroidism lowers it. Liver conditions, particularly fatty liver disease and cirrhosis, reduce SHBG synthesis as well. Chronic inflammation from any source can suppress SHBG production through inflammatory signaling molecules that downregulate the gene responsible for making it.
Oral contraceptives raise SHBG significantly in women, which can make total testosterone appear normal while free testosterone drops. This is one reason clinicians sometimes order free or bioavailable testosterone when the total number doesn’t match a patient’s symptoms.
How the Test Is Measured
Most commercial labs use automated immunoassays to measure testosterone. These are fast and inexpensive but can be less precise at very low concentrations, which matters when testing women or children. The gold standard is a technique called liquid chromatography-tandem mass spectrometry (LC-MS/MS), which can detect testosterone levels as low as 0.28 ng/dL with high accuracy across all clinically relevant concentrations. If your results seem inconsistent with your symptoms, your doctor may retest using this more precise method.
It’s also worth knowing that reference ranges can vary between labs. A result of 280 ng/dL might be flagged as low at one lab and fall within “normal” at another, depending on the assay used and the population the lab based its reference range on. Comparing results across different labs or testing methods isn’t always straightforward.