What is Free Hormonal Index?
The Free Hormonal Index refers to the fraction of a hormone that is unbound to carrier proteins and thus available to bind receptors and exert biological effects. For testosterone, only 1–2% is typically "free"; the rest is bound to SHBG or albumin.
Why It Matters
- •Bioavailability — free hormone determines tissue-level activity
- •Total vs. free — total testosterone can be normal while free is low (e.g., high SHBG)
- •Performance — cognitive and physical performance correlate with free hormone availability
Why does the Free Hormonal Index matter?
The free hormonal index matters because only the unbound fraction of a hormone can easily enter tissues and act on them. Total hormone levels can look normal while the active fraction is high or low, depending on how much binding protein is present.
The most common example is the free androgen index, which relates total testosterone to sex hormone-binding globulin (SHBG). It is used as a rough screening tool, for instance when assessing signs of excess androgens in women. Its accuracy is limited, especially in men and when SHBG is very low or high, so it is interpreted alongside other results.
What affects the Free Hormonal Index?
Anything that changes either the hormone or its binding protein can shift the index. SHBG levels are influenced by body weight, insulin resistance, thyroid function, liver function, age, pregnancy, and some medications, including oral contraceptives.
Timing also matters. Testosterone varies across the day and, in women, across the menstrual cycle, so sample timing affects results. Lab methods differ in accuracy at low concentrations. For these reasons, clinicians often confirm unusual values with repeat testing or more direct estimates of free hormone levels.
In ONDA Life
The CHM article covers performance window optimization based on free hormone levels.