From the Research
Yes, there is a formula to calculate total testosterone corrected for low SHBG, known as the Vermeulen equation or Free Androgen Index (FAI), which is FAI = (Total Testosterone ÷ SHBG) × 100, estimating the percentage of testosterone that is bioavailable. When SHBG is low, a greater proportion of total testosterone exists in the free or bioavailable form, which is the physiologically active fraction. This is clinically important because total testosterone levels might appear normal while free testosterone could be elevated due to low SHBG 1.
Key Points to Consider
- The Vermeulen equation calculates free testosterone by accounting for both SHBG and albumin binding, providing a more accurate assessment than FAI alone 1.
- Laboratory testing can directly measure free testosterone, but these calculations provide a reasonable estimate when only total testosterone and SHBG values are available.
- Clinicians should interpret these results in the context of clinical symptoms, as the relationship between testosterone, SHBG, and clinical effects is complex and individualized 2, 3.
- Recent guidelines, such as those from the Society for Endocrinology, emphasize a multidisciplinary approach to managing male hypogonadism, including the use of testosterone replacement therapy 4.
Clinical Implications
- The calculation of free testosterone, especially in the context of low SHBG, is crucial for accurately diagnosing and managing hypogonadism.
- Understanding the impact of SHBG on total testosterone levels can help clinicians make informed decisions about testosterone replacement therapy.
- It is essential to consider the patient's overall clinical presentation, including symptoms and other laboratory results, when interpreting the results of these calculations 5, 4.