DIAGNOSING LOW TESTOSTERONE: UNDERSTANDING TOTAL, FREE, AND BIOAVAILABLE TESTOSTERONE

Low testosterone, or hypogonadism, is a medical condition with profound implications for men’s physical, emotional, and psychological health. It can lead to symptoms like fatigue, decreased muscle mass, low libido, mood disturbances, and even cognitive changes. Diagnosing low testosterone involves complex measurements of the hormone, which can vary in the bloodstream depending on different factors. In clinical practice, testosterone levels are typically categorized into three types: total testosterone, free testosterone, and bioavailable testosterone. Understanding the differences between these forms, the methods used to measure them, and their significance in the diagnostic process is key to making accurate assessments.

The Basics: What is Testosterone?

Testosterone is the primary androgen hormone in males, produced mainly in the testes, and plays a crucial role in the development of male reproductive tissues, muscle mass, bone density, and secondary sexual characteristics. Testosterone exists in several states within the bloodstream:

1. Bound to Sex Hormone-Binding Globulin (SHBG) – About 60-65% of circulating testosterone is tightly bound to SHBG. This bound testosterone is not biologically active and is unavailable for tissue uptake.
2. Bound to Albumin – Around 30-35% of testosterone is loosely bound to albumin, which is a plasma protein. This form of testosterone is considered bioavailable because it can dissociate easily from albumin and become active.
3. Free Testosterone – Roughly 2-4% of testosterone remains unbound and is termed free testosterone. This form is the most biologically active, as it can readily enter cells and exert androgenic effects.

Total Testosterone: The Foundation of Diagnosis

Total testosterone refers to the sum of all testosterone in the blood—both bound (to SHBG and albumin) and unbound. It is typically the first measurement taken when diagnosing hypogonadism.

How is Total Testosterone Measured?

Total testosterone is most commonly measured using a blood test, with morning levels often being preferred due to the diurnal rhythm of testosterone, which peaks in the early morning. The standard method for measuring total testosterone is immunoassay or liquid chromatography-mass spectrometry (LC-MS). LC-MS is considered the gold standard due to its high accuracy, but immunoassays are more widely available and commonly used despite some concerns about accuracy in the lower ranges of testosterone.

Concerns with Total Testosterone Measurement

Total testosterone alone is often not enough to make a definitive diagnosis of low testosterone. A patient may present with "normal" total testosterone levels but still exhibit symptoms of hypogonadism, which could be due to alterations in the amount of SHBG or albumin in the blood. Conditions such as obesity, aging, liver disease, and certain medications can affect SHBG levels, leading to misleading total testosterone readings.

For example, a man with high SHBG may have a normal total testosterone level but have very little free testosterone available, leading to symptoms of low testosterone. Conversely, men with low SHBG (such as those with obesity or type 2 diabetes) may have lower total testosterone but higher levels of bioavailable testosterone, which may still be sufficient for physiological function.

Free Testosterone: The Active Hormone

Free testosterone is the unbound fraction of testosterone that can easily cross cell membranes and activate androgen receptors. Since free testosterone is the biologically active form, measuring it can provide a more direct insight into the hormonal status of a patient.

How is Free Testosterone Measured?

Free testosterone can be measured either directly or estimated using calculations based on total testosterone, SHBG, and albumin levels. Direct methods include equilibrium dialysis and ultrafiltration, which are considered the most accurate but are not always widely available due to the complexity and cost of these tests.

The most common method used in clinical settings is calculated free testosterone, where formulas such as the Vermeulen or Södergård equation are applied. These formulas use total testosterone, SHBG, and sometimes albumin to estimate the free testosterone fraction.

Concerns with Free Testosterone Measurement

While direct measurement of free testosterone is the gold standard, the indirect methods (formulas based on total testosterone and SHBG) may lack accuracy, especially in cases where SHBG levels are abnormal. Additionally, some experts argue that the reference ranges for free testosterone are less well-defined than those for total testosterone, which can complicate the interpretation of results.

Moreover, since only a small fraction of testosterone is free (2-4%), even minor inaccuracies in measurement can lead to significant variability. This makes it crucial to contextualize free testosterone levels with clinical symptoms.

Bioavailable Testosterone: A More Complete Picture

Bioavailable testosterone refers to the fraction of testosterone that is either free or loosely bound to albumin. This form of testosterone is readily available for uptake by tissues and can therefore provide a more comprehensive picture of hormonal activity.

How is Bioavailable Testosterone Measured?

Bioavailable testosterone can be measured using a specialized assay called the ammonium sulfate precipitation method. This test separates SHBG-bound testosterone from the albumin-bound and free testosterone, which can then be quantified. Alternatively, bioavailable testosterone can also be calculated using algorithms similar to those used for free testosterone.

Concerns with Bioavailable Testosterone Measurement

The measurement of bioavailable testosterone is complex and less standardized than total or free testosterone measurements. Additionally, this test is not as widely available, which limits its use in routine clinical practice. Despite this, bioavailable testosterone is often considered more relevant than total testosterone in situations where SHBG is altered, as it gives a more accurate reflection of the active hormone available to tissues.

Interpreting Testosterone Levels in Diagnosing Low Testosterone

Testosterone levels fluctuate throughout the day, and age plays a role in testosterone production, with levels typically declining as men age. Therefore, it's important to interpret testosterone levels in the context of the individual patient's health status, symptoms, and timing of the test.

Total Testosterone Cutoffs

According to the American Urological Association (AUA) and Endocrine Society guidelines, the normal range for total testosterone in adult males is typically between 300 ng/dL and 1,000 ng/dL. A level below 300 ng/dL is often used as a diagnostic cutoff for low testosterone. However, relying solely on total testosterone can be misleading due to variations in SHBG, which is why many experts recommend measuring free or bioavailable testosterone as well.

Free Testosterone Levels

There is no universally accepted normal range for free testosterone, but a common reference range is between 5 ng/dL and 21 ng/dL. A free testosterone level below 5 ng/dL may indicate hypogonadism, but symptoms and clinical presentation should always be considered alongside lab values.

Bioavailable Testosterone in Diagnosis

While total and free testosterone are commonly measured, bioavailable testosterone is particularly useful in cases where SHBG levels are abnormal. For instance, older men or those with liver disease may have elevated SHBG, which reduces the availability of free testosterone despite normal total testosterone levels. Conversely, obese men or those with insulin resistance may have lower SHBG, which could mask low total testosterone but still provide adequate bioavailable testosterone.

The Role of SHBG in Diagnosis

Sex Hormone-Binding Globulin (SHBG) plays a critical role in determining the levels of free and bioavailable testosterone. High levels of SHBG (seen in conditions like aging, hyperthyroidism, and liver disease) can falsely elevate total testosterone while reducing the free and bioavailable fractions. On the other hand, low SHBG (seen in obesity, diabetes, and hypothyroidism) can lead to an underestimation of hypogonadism if only total testosterone is measured. This is why SHBG measurement is often included in the diagnostic workup for low testosterone.

The Diagnostic Approach: Integrating Symptoms and Laboratory Findings

Diagnosing low testosterone involves more than just interpreting lab results. Physicians must also consider the patient's clinical presentation, including symptoms like fatigue, decreased libido, depression, and cognitive difficulties. Guidelines recommend repeating testosterone measurements, particularly total and free testosterone, on different days and under similar conditions (i.e., morning samples) to ensure accuracy.

The diagnosis of low testosterone is typically confirmed when a patient exhibits characteristic symptoms and has low total testosterone (below 300 ng/dL) or free testosterone (below the reference range), alongside other supporting lab values like low bioavailable testosterone and abnormal SHBG levels.

Conclusion

Diagnosing low testosterone is a nuanced process that requires a comprehensive understanding of the different forms of testosterone—total, free, and bioavailable—and how each measurement reflects hormonal activity in the body. While total testosterone provides an initial assessment, free and bioavailable testosterone levels offer a clearer picture in cases where SHBG is abnormal or when symptoms are incongruent with total testosterone levels alone. Clinicians must consider these factors and incorporate both laboratory data and clinical symptoms to accurately diagnose hypogonadism.