Testosterone testing is typically performed to diagnose hormonal imbalances, such as hypogonadism, or to monitor the effectiveness of hormone replacement therapies. While total testosterone serves as a foundational measurement, other tests are often necessary to understand the amount of hormone that is truly available to the body’s tissues.
Total Testosterone: The Screening Standard
Total testosterone represents the entire quantity of the hormone circulating in the bloodstream, encompassing both the amount bound to proteins and the small fraction that is unbound. This measurement is the established initial screening tool when a patient presents with symptoms that might suggest low testosterone, such as decreased libido, fatigue, or unexplained muscle loss. The typical reference range for total testosterone in adult men is generally between 300 and 1,000 nanograms per deciliter (ng/dL).
To ensure the most accurate reading, the blood sample for this test is routinely collected in the morning, usually between 7 a.m. and 10 a.m. This timing is standard practice because testosterone levels naturally fluctuate throughout the day, peaking in the early hours. The total testosterone value alone is often sufficient to rule out or confirm a deficiency in straightforward cases.
Free and Bioavailable Testosterone: Measuring Activity
The total testosterone result can sometimes be misleading because most of the hormone is bound to transport proteins and is not immediately active. Roughly 97 to 98% of circulating testosterone is bound, primarily to Sex Hormone Binding Globulin (SHBG) and, to a lesser extent, to albumin. SHBG binds testosterone very tightly, rendering that portion biologically inactive and unable to enter cells to exert its effects.
The small percentage of testosterone that remains unbound is called free testosterone, and this is the biologically active form that can readily bind to tissue receptors. Bioavailable testosterone includes this free portion plus the hormone that is loosely bound to albumin, which can easily dissociate and become available for use. Measuring these fractions becomes necessary when the total testosterone result is borderline or when symptoms of deficiency persist despite a seemingly normal total count.
Conditions that alter SHBG levels can skew the interpretation of total testosterone, making the free or bioavailable measurement more informative. High SHBG levels can bind an excessive amount of the hormone, resulting in low free testosterone despite a normal total level. Conversely, low SHBG levels can lead to a higher free fraction, potentially masking a true deficiency in total testosterone.
Methodology Comparison: Blood vs. Other Samples
Serum blood testing, typically performed via venipuncture, remains the gold standard for measuring both total and free testosterone due to its accuracy and reliability. Laboratory techniques, such as liquid chromatography-tandem mass spectrometry (LC-MS), are highly sensitive and specific, ensuring precise quantification of the hormone in the serum. For definitive diagnosis and monitoring of treatment, the use of a blood sample is the preferred approach in clinical practice.
Alternative collection methods, such as saliva or dried blood spot testing, offer convenience as they are non-invasive and can be performed at home. Saliva testing often measures the free, or bioavailable, fraction of the hormone, which can be useful for tracking changes over a short period. However, these methods are generally considered less reliable for initial diagnosis. The reduced hormone concentration in these samples can introduce variability, making serum blood draws the more trusted method for establishing baseline levels.
Selecting the Right Test for Specific Needs
The choice between a total or free testosterone test is guided by the patient’s individual clinical situation and medical history. For initial screening in a patient presenting with classic symptoms of hypogonadism, total testosterone is the appropriate starting point. If this result is clearly low (e.g., below 300 ng/dL), a diagnosis may be made, often followed by a repeat test to confirm the finding.
If the total testosterone value falls in a gray area or if the patient’s symptoms are inconsistent with the result, a measurement of free or bioavailable testosterone is indicated. This is particularly relevant in patients with conditions known to affect SHBG, such as significant obesity, chronic liver disease, or thyroid dysfunction. In these complex cases, the free or bioavailable measurement offers the most accurate reflection of the patient’s true hormonal status.
For patients undergoing Testosterone Replacement Therapy (TRT), both total and free testosterone levels are often monitored to ensure the hormone is within a therapeutic range without causing adverse effects. The laboratory results must be interpreted in conjunction with the patient’s symptoms and overall health profile by a medical professional.