Most testosterone in the blood is bound to proteins, mainly SHBG, and cannot act. Only the free fraction reaches tissue. A man can have a total testosterone inside the normal range and a free level that is too low to support desire, energy or recovery, usually because his SHBG is high. The Endocrine Society's guideline says to measure free testosterone when the total is borderline or when SHBG is likely to be off, which describes a lot of men over 40.
Total, bound and free
Think of total testosterone as everything in the blood. Most of it is attached to SHBG, a protein made by the liver, and to albumin. The SHBG bound portion is locked. The small free portion is what enters cells, drives libido, and helps muscle recover. That fraction is what your body experiences.
Why SHBG changes the picture
SHBG rises with age, with thyroid changes, and with some medications. When it rises, it binds more testosterone, and the free level drops even if the total holds steady. So the same total reading means different things in two men with different SHBG. Reading total alone misses that.
What the guidelines say
The Endocrine Society recommends measuring free testosterone when total testosterone is near the bottom of the range or when SHBG is likely to be abnormal. It also asks for consistent results on more than one morning draw and matching symptoms before anyone is treated. A single reading, total or free, is not a diagnosis.
What it means for desire
Libido tracks the free fraction more closely than the total. A man told his testosterone is normal, who still has flat desire, slower recovery and less drive, is often looking at a high SHBG and a low free level. That is the case a full panel catches and a single test does not.
How Regener8 approaches it
The panel comes first, drawn at any LabCorp, and a board certified clinician goes through it with you on a 30 minute video visit. If a plan is right for you it is built from those numbers, shipped to your door, and retested every six months so it keeps matching where you are.
Last updated
September 6, 2026
