Understanding Bioavailable Testosterone: A Practical Guide

When men come to True North Metabolic Kitchener-Waterloo Testosterone Replacement Therapy clinic, one of the most confusing topics is “bioavailable testosterone.” Most labs report total testosterone, but symptoms may correlate better with how much testosterone is actually usable by tissues. That “usable” portion is what we call bioavailable testosterone.

Total vs Free vs Bioavailable Testosterone

In the blood, testosterone exists in three main forms:

1. Tightly bound to SHBG (sex hormone–binding globulin)

2. Loosely bound to albumin

3. Completely unbound (free testosterone)

Total testosterone = SHBG-bound + albumin-bound + free.

But SHBG-bound testosterone is essentially “locked away.” The biologically useful pool is:

• Free testosterone

• Albumin-bound testosterone (weakly bound and easily released)

So: Bioavailable testosterone = free T + albumin-bound T

At True North Metabolic Kitchener-Waterloo Testosterone Replacement Therapy clinic, this is often more clinically meaningful than total T alone, especially when SHBG is abnormal.

Why SHBG Matters So Much

SHBG acts like a sponge. High SHBG (common in aging, hyperthyroidism, some liver conditions) grabs more testosterone, shrinking the free and bioavailable pool even if total T looks “normal.” Low SHBG (obesity, insulin resistance, some androgen use) does the opposite: total T may look low, but bioavailable T can be reasonably preserved.

This is why two men can both have a total testosterone of 15 nmol/L:

• Man A: SHBG 20 nmol/L → higher free/bioavailable T

• Man B: SHBG 60 nmol/L → much lower free/bioavailable T

Clinically, Man B is more likely to feel hypogonadal, and that’s exactly the kind of nuance we deal with at True North Metabolic Kitchener-Waterloo Testosterone Replacement Therapy clinic.

The Math Behind Bioavailable Testosterone

In practice, most labs don’t measure bioavailable T directly. Instead, we:

• Measure total testosterone

• Measure SHBG

• Assume a typical albumin level (or measure it)

• Plug into a validated equation (e.g., Vermeulen formula) to calculate free and bioavailable testosterone

Conceptually, the calculation uses:

• Known binding affinities of testosterone to SHBG and albumin

• The law of mass action (how much of a hormone is free vs bound in equilibrium)

You don’t need to do this by hand; online calculators and lab middleware do the math. But the key idea is that for a given total T, higher SHBG means proportionally more bound and less free/bioavailable testosterone.

A Simple Example

Imagine a man with:

• Total T: 20 nmol/L

• SHBG: 30 nmol/L

• Albumin: normal

His calculated free T might be in the mid-normal range, and his bioavailable T might also be comfortably normal. If another man has the same total T of 20 nmol/L but SHBG of 70 nmol/L, his calculated free and bioavailable T will be much lower. On paper they share the same total T; in real life they’re very different.

This is why, at True North Metabolic Kitchener-Waterloo Testosterone Replacement Therapy clinic, we pay close attention to SHBG and calculated free/bioavailable testosterone rather than chasing a single total T cutoff.

How This Guides TRT Decisions

When assessing men for TRT, looking at bioavailable testosterone helps in several ways:

• Explains symptoms in men with “borderline” total T but high SHBG

• Prevents overtreatment in low-SHBG men whose total T looks low but bioavailable T is adequate

• Helps titrate doses to achieve physiologic free/bioavailable levels, not just big total T numbers

The goal at True North Metabolic Kitchener-Waterloo Testosterone Replacement Therapy clinic is not to push numbers to supraphysiologic levels but to restore a healthy, symptom-relieving bioavailable testosterone range, while monitoring safety markers like hematocrit, PSA, and lipids.

Take-Home Message

Total testosterone is only the starting point. Bioavailable testosterone—free plus albumin-bound—is what most tissues actually “see.” SHBG is the key modifier, and calculated bioavailable T often explains why one man with a “normal” total T feels hypogonadal while another with a similar number feels fine. By using bioavailable testosterone intelligently, True North Metabolic Kitchener-Waterloo Testosterone Replacement Therapy clinic can individualize treatment, avoid unnecessary risk, and focus on both numbers and how patients actually feel.