Testosterone is a steroid hormone produced by both men and women, though in vastly different amounts. This hormone plays a role in maintaining muscle mass, bone density, energy levels, and sexual function in all adults. When symptoms suggest an imbalance, a blood test is necessary to diagnose the hormonal status.
Clinical Signs That Warrant Testing
A variety of symptoms prompt a healthcare provider to order a testosterone test. In men, indicators of low testosterone include decreased libido, erectile dysfunction, and chronic fatigue. Physical changes such as reduced muscle mass, increased body fat, and loss of body hair may also suggest a deficiency. These symptoms indicate the need for objective laboratory confirmation.
In women, symptoms leading to testing are often related to testosterone excess or an imbalance in sex hormones. Signs of excess can include hirsutism (excessive hair growth in a male pattern), acne, and menstrual cycle irregularities. Unexplained infertility or a drop in libido and energy levels may also warrant a hormonal workup. A blood test provides the specific data required to guide diagnosis, as many of these symptoms overlap with other conditions.
Preparing for Accurate Measurement
The timing of a blood draw significantly influences the accuracy of testosterone measurement due to the hormone’s natural daily cycle. Testosterone levels typically peak in the early morning and can decline by up to 30% by late afternoon, demonstrating strong diurnal variation. The standard protocol requires blood collection between 7:00 AM and 10:00 AM to capture this peak level and ensure a reliable baseline reading.
Laboratories primarily measure Total Testosterone, which is the sum of all testosterone in the blood, including bound and unbound portions. The majority of circulating testosterone is bound to Sex Hormone Binding Globulin (SHBG) or albumin. Since SHBG-bound testosterone is largely inactive, measuring Free Testosterone or bioavailable testosterone is important in certain cases.
Free testosterone is the small percentage (typically 1–3%) of the hormone that is not bound to proteins and is immediately available to act on the body’s tissues. Testing for free testosterone is useful when total testosterone levels are borderline, or when conditions like obesity or hyperthyroidism affect SHBG levels. The gold standard for accurate measurement is Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). This method is more precise than older immunoassay methods.
To ensure an accurate result, acute illness or recent intense physical exertion should be avoided, as these factors can temporarily lower testosterone levels. Although fasting may not significantly affect total testosterone, fasting samples are often preferred for the initial diagnostic test. Consistency is achieved by repeating a low diagnostic test on a separate morning to confirm a pattern, rather than relying on a single fluctuating result.
Interpreting the Test Results
A laboratory report provides a numerical value for the testosterone level alongside a reference range that defines what is considered “normal” by that specific lab. Testosterone levels are commonly reported in nanograms per deciliter (ng/dL) in the United States, or nanomoles per liter (nmol/L) elsewhere. For adult men, a typical total testosterone range falls between 300 ng/dL and 1000 ng/dL, though this varies based on age and testing methodology.
The reference range is not a rigid boundary, and a number slightly outside this range does not automatically indicate a disease or the need for treatment. The result must always be interpreted in the context of the patient’s age, symptoms, and overall health status. A man with a total testosterone level of 350 ng/dL who is experiencing fatigue and sexual dysfunction may be considered deficient, while another man with the same number and no symptoms may not require intervention.
Interpretation of free testosterone depends on the overall clinical picture, especially when total testosterone is normal but symptoms persist. A high SHBG level can bind too much of the hormone, resulting in low free testosterone and associated symptoms despite a normal total count. For women, the normal total testosterone range is much lower, typically between 15 ng/dL and 70 ng/dL for premenopausal women, requiring careful interpretation of small deviations.
Ongoing Monitoring and Reassessment
Once treatment, such as testosterone replacement therapy, is initiated, regular follow-up testing is necessary to ensure the effectiveness and safety of the intervention. This monitoring phase addresses the long-term management of testosterone levels. Initial re-testing is typically recommended 3 to 6 months after starting therapy or after any dose adjustment.
The goal of ongoing treatment is to maintain levels within a mid-normal therapeutic range, often targeted between 450 ng/dL and 600 ng/dL, while mitigating side effects. Once levels have stabilized and symptoms are managed, testing frequency can be reduced to once or twice per year. The exact timing of the blood draw depends on the type of therapy used, such as measuring levels midway between injections or a few hours after applying a transdermal gel.
Monitoring also involves checking other health markers, such as hematocrit (to monitor red blood cell levels) and Prostate-Specific Antigen (PSA) in older men. This comprehensive approach ensures that hormonal treatment achieves the desired testosterone level while maintaining the patient’s health within safe parameters. The frequency of reassessment remains personalized, depending on the individual’s response and changes in medical status.