Free Testosterone

What Free Testosterone Measures

Free testosterone is the fraction not bound to SHBG (sex hormone binding globulin) or albumin. Because SHBG can fluctuate substantially with insulin resistance, alcohol, thyroid status, and obesity, two men with identical total testosterone can have very different free testosterone values - and free testosterone is what actually signals at the androgen receptor.

Free testosterone should always be interpreted alongside SHBG. A man with total T = 700 but SHBG = 85 may have free T below optimal range, while a man with total T = 550 and SHBG = 30 may have free T that's clinically optimal.

Assay method matters: equilibrium dialysis (LC-MS/MS) is the gold standard. Direct immunoassays are less reliable. If the assay isn't specified on the report, look it up.

What Affects This Biomarker

Free testosterone is influenced by: total testosterone production, SHBG level (higher SHBG = lower free T for same total), insulin resistance (tends to lower SHBG, raising free T modestly), alcohol (raises SHBG), thyroid status, and pharmacologic agents - TRT, HCG, enclomiphene raise it; aromatase inhibitors indirectly shift it.

In the Context of Peptide Protocols

On TRT, target free testosterone in the upper-optimal range - many men feel best at 18–24 ng/dL. Pair with total T, SHBG, and sensitive estradiol for a complete picture. For enclomiphene monotherapy, expect free T to rise in proportion to total T as the HPG axis restores function.

Deep Dive

The three pools: free, bioavailable, and bound

Circulating testosterone exists in three fractions, and only a sliver is truly "free." Roughly 1 to 2 percent circulates completely unbound. Around half is loosely bound to albumin, and most of the remainder, on the order of 40 to 45 percent, is tightly bound to SHBG. The free hormone hypothesis holds that only unbound hormone readily crosses the cell membrane to engage the androgen receptor, which is why the free fraction often tracks symptoms more closely than total does.

The albumin-bound fraction sits in a middle ground. Its binding is weak enough that much of it dissociates during a single pass through the capillary bed, so free plus albumin-bound is grouped together as bioavailable testosterone. SHBG-bound testosterone, by contrast, is held tightly and behaves more like a storage and buffering pool than an immediately available one. This is the mechanistic reason a man with high SHBG can look replete on a total testosterone reading while running short on what tissues can actually reach.

Measured versus calculated: your number may be a model, not a measurement

Most panels that print a free testosterone value did not measure it directly. They calculated it. Equilibrium dialysis by LC-MS/MS is the reference standard, but it is slower and costlier, so many labs instead report a calculated free testosterone derived from total testosterone, SHBG, and albumin using the Vermeulen equation (Vermeulen et al., 1999). That equation assumes a single class of testosterone binding site on SHBG with fixed affinity constants.

Newer work has questioned that assumption. Zakharov and colleagues (2015) described a multi-step allosteric binding model in which SHBG binds testosterone with more complexity than the classic equation captures, which means calculated free testosterone can diverge from dialysis values, especially at very high or very low SHBG. Separately, the inexpensive "direct" or analog immunoassay for free testosterone, still bundled into some panels, is widely regarded as unreliable. The practical takeaway for anyone tracking over time is simple: know which method produced your number, and never compare a calculated value from one lab against a dialysis value from another.

Units and reference-range traps

Free testosterone is reported in at least three unit systems, and mixing them produces alarming false readings. The conversions worth memorizing: 1 ng/dL equals 10 pg/mL, and 1 nmol/L of testosterone equals about 28.8 ng/dL. A "free T of 15" is an in-range 15 ng/dL on one report but a very different 15 pg/mL, which is only 1.5 ng/dL, on another. Some labs also express free testosterone as a percentage of total.

Because assays and reference intervals are lab-specific, a free testosterone result is only interpretable against that lab's own range and, ideally, against your own prior draws from the same lab. The Endocrine Society guideline (Bhasin et al., 2018) recommends measuring free testosterone in men whose total sits near the lower limit or who have conditions that shift SHBG, precisely because total alone can mislead in those settings.

Timing is the biggest source of a bad result

Testosterone follows a diurnal rhythm, peaking in the early morning and declining through the day. Morning values can run 20 to 30 percent higher than late-afternoon values in younger men, and free testosterone tracks that curve. This is why guidelines call for a fasting draw before roughly 10 to 11 am, and why a single low reading is meant to be confirmed on a repeat morning sample rather than read in isolation.

State matters too. Acute illness, a recent hard training session, marked psychological stress, and short sleep can all transiently depress the reading. In a controlled study, Leproult and Van Cauter (JAMA, 2011) found that one week of sleep restricted to five hours lowered daytime testosterone by 10 to 15 percent in healthy young men. To keep a clean trend line, standardize the conditions: same lab, same assay, same early-morning fasted window, and not in the middle of an illness.

What moves free testosterone: the SHBG lever and age

Free testosterone is total production filtered through SHBG, so it has two independent levers, and this is why age tends to hit it harder than it hits total. In the Massachusetts Male Aging Study (Feldman et al., 2002), total testosterone declined on the order of 1 to 2 percent per year while SHBG rose steadily, so the free and bioavailable fractions fell faster, roughly 2 to 3 percent per year. The European Male Ageing Study (Wu et al., 2010) used a free testosterone threshold near 220 pmol/L, about 6.3 ng/dL, alongside symptoms when characterizing late-onset hypogonadism.

Anything that moves SHBG moves the free fraction. Insulin resistance and higher body fat tend to lower SHBG, which raises the free ratio even as total may fall. Fat loss, alcohol, oral estrogen, and higher thyroid hormone raise SHBG, which lowers the free ratio. Sleep, resistance training, and avoiding prolonged low-energy-availability states are the lifestyle inputs research most consistently ties to the production side. None of this is a prescription; it is context for reading your own trend and discussing any changes with your clinician.

Tracking context for protocols

On testosterone therapy, exogenous testosterone suppresses hepatic SHBG synthesis, so SHBG typically falls and the free-to-total ratio rises. The consequence for tracking is that free testosterone can reach the upper-optimal range while total sits only mid-range, which makes the free value the more faithful readout of where a protocol has landed. On HPG-axis approaches such as HCG, gonadorelin, or enclomiphene, endogenous restoration lifts total and free together, and free testosterone rising in step with total is the expected pattern.

For the cleanest longitudinal read, pull free testosterone with SHBG, total testosterone, and a sensitive estradiol assay in the same morning draw, and hold the lab and assay constant across time points. Growth-hormone secretagogues such as MK-677, and repair peptides such as BPC-157 and TB-500, have no established direct effect on free testosterone in human data, so a shift while running those protocols usually points to something else worth reviewing with your clinician.

Related Reading

• Testosterone Panel Guide →
• Testosterone Optimization Guide →

Conditions That Track Free Testosterone

Frequently Asked Questions

Where This Fits in Your Panel

Free Testosterone is one marker on a fuller panel. The Peptide Blood Work Checklist lays out the complete baseline panel, what to add by protocol type, and when to retest.

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