Calculated Free Testosterone (cFT) is an estimate of the amount of testosterone in the body that is not bound to proteins (like SHBG and albumin) and is therefore considered biologically available to act on tissues. Measuring total testosterone alone can sometimes be misleading, as much of it is bound. Free testosterone represents the "active" form.
Why is Calculated Free Testosterone Important?
Both total testosterone and free testosterone levels are crucial for men's health. Low levels of free testosterone can be associated with symptoms like:
Low libido (sex drive)
Erectile dysfunction
Fatigue and lack of energy
Decreased muscle mass and strength
Mood changes, including depression
Reduced bone density
Monitoring cFT helps healthcare providers diagnose conditions like hypogonadism (low testosterone) and assess the effectiveness of testosterone replacement therapy.
How is Calculated Free Testosterone Calculated?
Several formulas exist for calculating free testosterone. A commonly used and widely accepted method is the one developed by Vermeulen et al. This calculation accounts for the binding of testosterone to both SHBG and albumin, as well as their respective binding affinities and the levels of other binding molecules in the blood.
The Vermeulen formula requires measurements of:
Total Testosterone (TT)
Sex Hormone Binding Globulin (SHBG)
Albumin
The specific constants and steps in the calculation can be complex, involving equilibrium constants for the binding of testosterone to SHBG and albumin. The calculator above uses a simplified but effective implementation of this principle.
The Formula (Simplified Overview)
The calculation involves estimating the concentration of unbound testosterone. It requires converting all measurements to consistent units and then applying a set of equations that consider the equilibrium between free testosterone, testosterone bound to SHBG, and testosterone bound to albumin.
Key Constants Used (Approximate):
SHBG association constant (kaSHBG): ~6.9 x 10-9 M-1
Albumin association constant (kaAlb): ~4.1 x 105 M-1
Molar mass of Testosterone: ~288.4 g/mol
Molar mass of Albumin: ~66,500 g/mol
Molar mass of SHBG: ~53,000 g/mol
The calculator uses these principles to provide an estimate. It's important to remember that this is a calculated value and should be interpreted by a qualified healthcare professional in the context of a patient's overall health and symptoms.
Important Considerations:
Accuracy of Lab Results: The accuracy of the calculated free testosterone depends heavily on the accuracy of the laboratory measurements for total testosterone, SHBG, and albumin.
Units: Ensure you are using the correct units for each measurement. The calculator handles conversions for common units.
Individual Variation: Binding proteins and their affinities can vary slightly between individuals.
Interpretation: Always consult with a doctor or endocrinologist for diagnosis and treatment decisions based on these results.
function calculateFreeTestosterone() {
var totalTestosterone = parseFloat(document.getElementById("totalTestosterone").value);
var shbg = parseFloat(document.getElementById("shbg").value);
var albumin = parseFloat(document.getElementById("albumin").value);
var units = document.getElementById("units").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous result
if (isNaN(totalTestosterone) || isNaN(shbg) || isNaN(albumin)) {
resultDiv.innerHTML = 'Please enter valid numbers for all fields.';
resultDiv.style.backgroundColor = '#ffc107'; // Warning color
resultDiv.style.color = '#333';
return;
}
// Constants for the Vermeulen formula (simplified and adapted for common units)
// These values are based on literature and may vary slightly
var k_a_SHBG = 6.3e-9; // Molar association constant for SHBG (L/mol)
var k_a_Alb = 3.8e-5; // Molar association constant for Albumin (L/mol)
var MW_Testosterone = 288.4; // Molecular weight of Testosterone (g/mol)
var MW_SHBG = 53000; // Molecular weight of SHBG (g/mol)
var MW_Albumin = 66500; // Molecular weight of Albumin (g/mol)
// Convert units if necessary to a common base (e.g., nmol/L for TT, mg/L for SHBG, g/dL for Albumin)
var TT_nmol_L = totalTestosterone;
var SHBG_nmol_L = shbg;
var Albumin_g_dL = albumin;
if (units === "ng/dL") {
TT_nmol_L = totalTestosterone * 3.467; // ng/dL to nmol/L
}
// Convert SHBG from nmol/L to Molar (mol/L)
var SHBG_M = SHBG_nmol_L / 1e9; // nmol/L to M
// Convert Albumin from g/dL to Molar (mol/L)
var Albumin_g_L = Albumin_g_dL * 10; // g/dL to g/L
var Albumin_M = Albumin_g_L / MW_Albumin; // g/L to mol/L
// Convert Total Testosterone from nmol/L to Molar (mol/L)
var TT_M = TT_nmol_L / 1e9; // nmol/L to M
// Calculate bound testosterone using quadratic formula derived from equilibrium equations
// A*x^2 + B*x + C = 0 where x is free testosterone concentration
// Constants for the quadratic equation
var A = (k_a_SHBG * k_a_Alb * (SHBG_M + Albumin_M)) + 1;
var B = 1 + (k_a_SHBG * TT_M) + (k_a_Alb * TT_M) – (k_a_SHBG * k_a_Alb * (SHBG_M + Albumin_M));
var C = -k_a_SHBG * k_a_Alb * TT_M;
// Solving the quadratic equation for free testosterone (FT_M)
var discriminant = B*B – 4*A*C;
if (discriminant < 0) {
resultDiv.innerHTML = 'Calculation error: Negative discriminant.';
resultDiv.style.backgroundColor = '#dc3545'; // Danger color
resultDiv.style.color = 'white';
return;
}
var FT_M = (-B + Math.sqrt(discriminant)) / (2 * A);
// Ensure FT_M is not negative due to potential calculation nuances or extreme inputs
if (FT_M < 0) {
FT_M = 0;
}
// Convert Free Testosterone back to common units (e.g., ng/dL)
// FT (ng/dL) = FT (mol/L) * MW_Testosterone (g/mol) * 100 (dL/L) / 10^9 (mol/nmol) * 1000 (ng/mg) — This is complex, let's use a common conversion path
// Convert FT_M (mol/L) to TT_nmol_L (which is already in nmol/L if input was ng/dL or converted)
var FT_nmol_L = FT_M * 1e9; // mol/L to nmol/L
// Convert FT_nmol_L to ng/dL
var FT_ng_dL = FT_nmol_L / 3.467; // nmol/L to ng/dL
// Display the result
resultDiv.innerHTML = FT_ng_dL.toFixed(2) + ' ng/dL' +
'(Calculated Free Testosterone)';
resultDiv.style.backgroundColor = 'var(–success-green)'; // Reset to success green
resultDiv.style.color = 'white';
}