Free Androgen Index

What it is

The Free Androgen Index (FAI) is a calculation, not a direct measurement. It's used to estimate the amount of testosterone that's freely available in your blood to be used by your body's tissues. Testosterone is a key hormone, particularly important for men, but also present in women.

In your blood, most testosterone is bound to proteins: primarily sex hormone-binding globulin (SHBG) and albumin. A small fraction, however, remains unbound or "free." It's this free testosterone that's considered biologically active.

The FAI is calculated using the results of two other blood tests: your total testosterone level and your SHBG level. The formula essentially adjusts your total testosterone level based on how much SHBG is available to bind to it, giving an indication of how much testosterone is likely to be free and active. The result is usually expressed as a ratio or percentage.

Why it matters

Understanding your FAI can be helpful in assessing your hormonal balance, particularly concerning testosterone. Low levels of free testosterone can sometimes be associated with symptoms like reduced libido, fatigue, erectile dysfunction, and mood changes in men. In women, testosterone plays a role in energy levels, mood, and sexual function, and imbalances can also lead to symptoms.

By estimating the amount of bioavailable testosterone, the FAI can provide a more nuanced picture than total testosterone alone, especially when SHBG levels might be unusually high or low. This is because someone with a high total testosterone level might still have low free testosterone if their SHBG is very high, and vice versa. The FAI helps to clarify this relationship.

What can cause it to be high or low

Several factors can influence your FAI, leading to levels that are higher or lower than the typical reference range (which in the UK is generally considered to be between 35–92.6 ratio, though this can vary slightly between laboratories).

Factors that may be associated with a lower FAI:

• High SHBG levels: Conditions that increase SHBG, such as certain liver conditions, hyperthyroidism (an overactive thyroid), or sometimes with ageing, can lead to more testosterone being bound, thus lowering the FAI.
• Ageing: Testosterone levels naturally tend to decline with age, and SHBG levels can sometimes increase, both contributing to a lower FAI.
• Certain medications: Some medications can affect hormone levels or SHBG production.
• Obesity: While obesity can sometimes be associated with lower SHBG, it can also be linked to lower testosterone production, potentially impacting the FAI.

Factors that may be associated with a higher FAI:

• Low SHBG levels: Conditions that decrease SHBG, such as obesity, hypothyroidism (an underactive thyroid), or the use of anabolic steroids, can result in more testosterone being free, leading to a higher FAI.
• Increased testosterone production: Conditions that lead to an overproduction of testosterone, such as polycystic ovary syndrome (PCOS) in women or certain testicular conditions in men, can result in higher FAI.
• Certain medications: Some medications can directly increase testosterone or decrease SHBG.

It's important to remember that the FAI is one piece of the puzzle. Your GP will consider it alongside your symptoms, medical history, and other test results.

Lifestyle factors that may help

Certain lifestyle choices can play a role in maintaining healthy hormone balance, which may indirectly influence your FAI. Discussing these with your GP is a good starting point.

• Maintaining a healthy weight: Being significantly overweight or underweight can affect hormone production and SHBG levels. Aiming for a healthy body mass index (BMI) through a balanced diet and regular physical activity is often recommended.
• Regular exercise: Moderate, regular physical activity can contribute to overall health and well-being, potentially supporting healthy hormone levels.
• Balanced diet: A nutritious diet rich in whole foods, fruits, vegetables, and lean proteins supports general health. Limiting processed foods, excessive sugar, and unhealthy fats is also beneficial.
• Managing stress: Chronic stress can impact hormone regulation. Exploring stress-management techniques like mindfulness, yoga, or spending time in nature might be helpful.
• Adequate sleep: Getting sufficient, good-quality sleep is crucial for many bodily functions, including hormone production and regulation.

Questions to discuss with your GP

If you have concerns about your hormone levels or are experiencing symptoms that might be related, it's always best to have a conversation with your GP. Here are some questions you might consider discussing:

• What does my Free Androgen Index result mean in the context of my overall health and symptoms?
• Are my testosterone and SHBG levels within the expected range for my age and sex?
• Could my symptoms be related to my hormone levels, and if so, what are the next steps?
• Are there any lifestyle changes that might help support my hormone balance?
• Are there any other tests or investigations that would be helpful to understand my hormonal health better?
• What are the potential causes for my specific FAI result, and what are the implications?