Calculation of Free Testosterone

Free Testosterone Calculator

Total Testosterone (ng/dL):

SHBG (Sex Hormone Binding Globulin) (nmol/L):

Albumin (g/dL):

function calculateFreeTestosterone() { var totalTestosterone = parseFloat(document.getElementById('totalTestosterone').value); var shbg = parseFloat(document.getElementById('shbg').value); var albumin = parseFloat(document.getElementById('albumin').value); var resultDiv = document.getElementById('result'); if (isNaN(totalTestosterone) || isNaN(shbg) || isNaN(albumin) || totalTestosterone <= 0 || shbg <= 0 || albumin <= 0) { resultDiv.innerHTML = "Please enter valid positive numbers for all fields."; resultDiv.style.color = '#dc3545'; // Red for error return; } // Constants for the simplified Vermeulen formula approximation // These constants implicitly handle binding affinities and unit conversions var K_SHBG = 0.036; // Binding constant for SHBG (nmol/L) var K_ALBUMIN = 0.013; // Binding constant for Albumin (g/dL) // Formula for Calculated Free Testosterone (cFT) // cFT = Total Testosterone / (1 + (SHBG * K_SHBG) + (Albumin * K_ALBUMIN)) var freeTestosterone = totalTestosterone / (1 + (shbg * K_SHBG) + (albumin * K_ALBUMIN)); resultDiv.innerHTML = "Calculated Free Testosterone: " + freeTestosterone.toFixed(2) + " ng/dL"; resultDiv.style.color = '#28a745'; // Green for success }

Understanding Free Testosterone and Its Calculation

Testosterone is a crucial hormone, primarily known for its role in male health, but also important for women. While total testosterone measures all testosterone in your blood, it's the free testosterone that is biologically active and readily available for your body's tissues to use. This calculator helps you estimate your free testosterone levels based on your total testosterone, Sex Hormone Binding Globulin (SHBG), and albumin levels.

What is Free Testosterone?

Testosterone circulates in the bloodstream in three main forms:

Free Testosterone: This is the unbound form, representing about 1-4% of total testosterone. It's the biologically active form that can easily enter cells and exert its effects.
Albumin-Bound Testosterone: About 40-60% of testosterone is loosely bound to albumin. This form is also considered bioavailable, meaning it can dissociate from albumin and become available to tissues.
SHBG-Bound Testosterone: The remaining 30-50% is tightly bound to Sex Hormone Binding Globulin (SHBG). This form is generally considered biologically inactive because it's not easily released to tissues.

Therefore, free testosterone, along with albumin-bound testosterone, makes up what is often referred to as "bioavailable testosterone." However, free testosterone specifically refers to the unbound portion.

Why is Free Testosterone Important?

Measuring free testosterone can provide a more accurate picture of your hormonal status than total testosterone alone, especially when SHBG levels are abnormal. Symptoms of low testosterone (e.g., fatigue, low libido, erectile dysfunction, mood changes, loss of muscle mass) can occur even with normal total testosterone if SHBG levels are high, leading to low free testosterone. Conversely, high free testosterone can also lead to symptoms like acne, hair loss, or aggression.

How is Free Testosterone Calculated?

Direct measurement of free testosterone (e.g., via equilibrium dialysis) is complex and expensive. Therefore, it's often calculated using a formula that takes into account total testosterone, SHBG, and albumin levels. The calculator above uses a widely accepted approximation of the Vermeulen formula, which considers the binding affinities of testosterone to SHBG and albumin.

The formula used is:

Free Testosterone (ng/dL) = Total Testosterone (ng/dL) / (1 + (SHBG (nmol/L) * 0.036) + (Albumin (g/dL) * 0.013))

Where:

Total Testosterone: The total amount of testosterone in your blood, typically measured in nanograms per deciliter (ng/dL).
SHBG (Sex Hormone Binding Globulin): A protein that binds to sex hormones, including testosterone, measured in nanomoles per liter (nmol/L). Higher SHBG means less free testosterone.
Albumin: Another protein in the blood that binds to testosterone, measured in grams per deciliter (g/dL).

Normal Ranges for Free Testosterone

Normal ranges can vary slightly between labs and depend on age and sex. However, general reference ranges are:

Adult Males: Approximately 50-210 pg/mL (or 5-21 ng/dL). Note: The calculator provides results in ng/dL. To convert pg/mL to ng/dL, divide by 10.
Adult Females: Approximately 0.3-1.9 ng/dL.

It's important to discuss your results with a healthcare professional, as these ranges are guidelines, and individual health status and symptoms are crucial for interpretation.

Factors Affecting Free Testosterone

Several factors can influence your free testosterone levels:

Age: Free testosterone naturally declines with age.
Obesity: Can lower SHBG and total testosterone, affecting free testosterone.
Thyroid Disorders: Both hyper- and hypothyroidism can alter SHBG levels.
Liver Disease: Can affect SHBG production.
Medications: Certain drugs (e.g., corticosteroids, anticonvulsants, oral estrogens) can impact SHBG or testosterone production.
Insulin Resistance/Diabetes: Often associated with lower SHBG.

Disclaimer: This calculator provides an estimate of free testosterone based on a common formula. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare provider for interpretation of your lab results and for any health concerns.