What is total testosterone?
Total testosterone measures the entire amount of testosterone circulating in your blood — bound and unbound combined. In a typical adult male, this ranges from 300 to 1,000 ng/dL, depending on the laboratory's reference range and the assay method used. It is the standard diagnostic marker for hypogonadism and the primary number most providers use to evaluate whether TRT is working.
Testosterone in the bloodstream exists in three forms. Approximately 60% is tightly bound to sex hormone-binding globulin (SHBG), a liver-produced protein with high affinity for testosterone. Around 38% is loosely bound to albumin, a general-purpose transport protein. The remaining 2-3% circulates freely, unattached to any protein. Total testosterone captures all three fractions in a single number.
The advantage of total testosterone is standardization — it is reproducible, widely available, and the basis for virtually all clinical guidelines on diagnosing and treating hypogonadism. The Endocrine Society's 2018 guidelines use total testosterone as the primary diagnostic criterion: two morning measurements below 300 ng/dL (or the lab's lower reference limit) confirm a biochemical diagnosis of testosterone deficiency.
The limitation is context. Total testosterone tells you how much is in the blood but not how much your tissues can actually use. Two men with identical total testosterone levels can have very different symptom profiles if their SHBG levels differ significantly.
What is free testosterone?
Free testosterone is the 2-3% of total testosterone that circulates unbound to any protein. This unattached fraction diffuses freely across cell membranes, binds to androgen receptors, and drives the biological effects of testosterone — muscle protein synthesis, bone density maintenance, libido, mood regulation, and cognitive function.
Because free testosterone is the biologically active form, it often correlates more closely with symptoms than total testosterone does. A 2010 study published in The Journal of Clinical Endocrinology & Metabolism found that free testosterone was a stronger predictor of sexual function, lean body mass, and bone mineral density than total testosterone in a cohort of older men.
Typical reference ranges for free testosterone in adult men are 9-30 ng/dL, but these ranges vary substantially between laboratories and methods. The narrow absolute values make free testosterone more sensitive to assay quality — small measurement errors represent a larger percentage of the result compared to total testosterone.
What is bioavailable testosterone?
Bioavailable testosterone includes both free testosterone and the albumin-bound fraction — roughly 40% of total testosterone. The rationale is that albumin binds testosterone loosely enough that it readily dissociates at the tissue level, making albumin-bound testosterone functionally available. SHBG-bound testosterone, by contrast, does not release easily and is considered biologically inactive.
Bioavailable testosterone is not routinely ordered in clinical practice, partly because it requires an ammonium sulfate precipitation assay that is less commonly offered by commercial labs. In most clinical scenarios, the combination of total testosterone plus free testosterone (or total testosterone plus SHBG, from which free T can be calculated) provides sufficient information. Your provider may order bioavailable testosterone if they use it as part of their standard evaluation, but it is not required by any major clinical guideline.
How does SHBG change the total vs. free testosterone picture?
SHBG is the bridge between total and free testosterone. When SHBG is high, it binds more testosterone, lowering the free fraction. When SHBG is low, more testosterone remains unbound. This is why two men with the same total testosterone can have meaningfully different free testosterone levels — and meaningfully different symptom experiences.
Consider two men, both with a total testosterone of 500 ng/dL. Man A has an SHBG of 20 nmol/L (low). His calculated free testosterone is approximately 15 ng/dL. Man B has an SHBG of 60 nmol/L (high). His calculated free testosterone is approximately 7 ng/dL — roughly half of Man A's, despite identical total testosterone. Man B may experience symptoms of low testosterone that Man A does not.
Several factors influence SHBG levels. Aging, liver disease, hyperthyroidism, estrogen therapy, and low-calorie diets tend to raise SHBG. Obesity, insulin resistance, type 2 diabetes, hypothyroidism, and exogenous testosterone (including TRT) tend to lower it. Understanding your SHBG level is essential for correctly interpreting your testosterone numbers. For a deeper dive, read our guide on SHBG and testosterone.
Key takeaway:If your total testosterone looks normal but you still have symptoms of low T — fatigue, low libido, brain fog — check your SHBG and free testosterone. High SHBG can make a “normal” total T misleading.
When does free testosterone matter more than total?
Free testosterone becomes the more important marker in several clinical situations. The Endocrine Society specifically recommends measuring free testosterone when total testosterone is near the lower limit of normal (borderline), when SHBG abnormalities are suspected, and in conditions known to alter SHBG — including obesity, aging, hepatic cirrhosis, and thyroid disorders.
Borderline total testosterone
A total testosterone of 320 ng/dL in a man with normal SHBG likely reflects genuine low-normal testosterone. The same number in a man with elevated SHBG of 70 nmol/L may correspond to a free testosterone well below the reference range — making TRT clearly indicated despite the “acceptable” total number.
Obesity and low SHBG
Obese men often have low SHBG due to insulin resistance. This means their total testosterone may appear low while free testosterone is actually normal — since less is bound to SHBG. Treating the obesity and insulin resistance may normalize total testosterone without requiring TRT. Free testosterone helps clarify whether the low total T is clinically significant.
Aging
SHBG increases approximately 1-2% per year after age 40. A 65-year-old man with a total testosterone of 450 ng/dL and age-related SHBG elevation may have meaningfully lower free testosterone than a 40-year-old with the same total T. Age-adjusted interpretation using free testosterone provides a more accurate clinical picture.
How are total and free testosterone tested?
Total testosterone is most accurately measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS), which is now the standard at major commercial laboratories including Quest Diagnostics and Labcorp. Older immunoassay methods are still used by some labs and are reasonably accurate for total testosterone in the normal male range, though less reliable at very low or very high levels.
Free testosterone testing is more complex. Three approaches exist, with different tradeoffs in accuracy, availability, and cost.
| Method | Accuracy | Availability | Cost |
|---|---|---|---|
| Equilibrium dialysis | Gold standard | Limited — reference labs only | $100-200+ |
| Calculated (Vermeulen equation) | Strong correlation with dialysis | Any lab that runs total T + SHBG | $50-100 (combined panel) |
| Direct analog immunoassay | Poor — known to be unreliable | Widely available | $30-60 |
The Endocrine Society recommends equilibrium dialysis or calculated free testosterone over the direct analog immunoassay. The analog method consistently underperforms — a 2004 validation study inClinical Chemistryfound it correlated poorly with dialysis across multiple assay platforms. If your lab report lists “free testosterone” without specifying the method, ask the lab or your provider which assay was used.
What are the optimal ranges on TRT?
Optimal ranges on TRT extend beyond simply clearing the lower reference limit. Most experienced TRT providers target the ranges where symptom resolution is most consistent, based on clinical observation and available data.
| Marker | Lab Reference Range | Typical TRT Target | Notes |
|---|---|---|---|
| Total testosterone | 300-1,000 ng/dL | 600-900 ng/dL (at trough) | Measured morning of next injection |
| Free testosterone | 9-30 ng/dL | 15-25 ng/dL (at trough) | Upper quartile of reference range |
| SHBG | 10-57 nmol/L | 20-40 nmol/L | Midrange preferred; extreme values warrant investigation |
These targets are clinical conventions, not rigid rules. Individual response varies. Some men feel optimal at a total testosterone of 550 ng/dL; others need 800 ng/dL to resolve symptoms. Free testosterone, estradiol balance, and overall symptom assessment should all factor into dose titration.
Important: Always compare your results to the specific reference range on your lab report. Ranges vary between laboratories and assay methods. A free testosterone of 12 ng/dL might be low-normal at one lab and mid-range at another.
Should you use calculated or direct free testosterone?
For routine TRT monitoring, calculated free testosterone using the Vermeulen equation is the most practical and reliable option. It requires three inputs — total testosterone, SHBG, and albumin — and produces a result that correlates well with the gold-standard equilibrium dialysis method. Multiple studies have validated the Vermeulen calculation across a wide range of testosterone and SHBG values.
The calculation can be done manually or through online calculators (the International Society for the Study of the Aging Male provides a free one). Many lab platforms now include calculated free testosterone as part of their standard hormone panels.
Avoid relying on the direct analog immunoassay for clinical decision-making. If your lab only offers the analog method for free testosterone, ordering total testosterone plus SHBG and calculating free T yourself is more accurate.
What should you do with your results?
Start by comparing your total and free testosterone to each other and to your SHBG. If both total and free testosterone are in your target range and symptoms are well-controlled, your protocol is working. No adjustments needed.
If total testosterone is adequate but free testosterone is low, investigate SHBG. Elevated SHBG is the most common explanation. Addressing the underlying cause — whether that is weight management, thyroid optimization, or a medication review — may improve free T without changing your testosterone dose. In some cases, more frequent injection schedules (which tend to lower SHBG) can help.
If both total and free testosterone are below target at trough, a dose increase or delivery method change may be warranted. Discuss with your provider — do not self-adjust.
If total testosterone is above range but free testosterone is normal, your SHBG is likely elevated and buffering the excess. This is not necessarily a problem, but it warrants monitoring and a conversation with your provider about whether the dose is appropriate.
For the complete picture of every marker your provider should be tracking alongside testosterone, return to our complete TRT blood work guide. To understand how estradiol fits into the equation, see our estradiol on TRT guide.
Frequently Asked Questions
Is free testosterone more important than total testosterone?
Free testosterone is more clinically relevant in certain situations — particularly when SHBG is abnormally high or low, or when total testosterone is borderline but symptoms persist. However, total testosterone remains the primary diagnostic marker. Most providers use both together for a complete picture.
What is a good free testosterone level for a man on TRT?
Most TRT-focused clinicians target a free testosterone of 15-25 ng/dL or the upper quartile of the lab's reference range, measured at trough. Ranges vary by laboratory and assay method, so compare your results to the specific reference range on your report rather than a universal number.
Can you have normal total testosterone but low free testosterone?
Yes. This is common when SHBG is elevated. High SHBG binds more testosterone, leaving less in the free (active) fraction. Aging, liver conditions, hyperthyroidism, and certain medications all raise SHBG. This is why the Endocrine Society recommends checking free testosterone when total T is borderline or SHBG is suspected to be abnormal.
How do you increase free testosterone without increasing total?
Lowering SHBG increases the free fraction without changing total testosterone. Weight loss, improving insulin sensitivity, treating hypothyroidism, and optimizing vitamin D can all lower SHBG naturally. More frequent injection schedules on TRT also tend to lower SHBG, increasing the free-to-total ratio.
Should I use the calculated or direct free testosterone test?
Calculated free testosterone (using total T, SHBG, and albumin via the Vermeulen equation) is generally more reliable and reproducible than direct analog immunoassays. Equilibrium dialysis is the gold standard for direct measurement but is expensive and not widely available. Most labs and providers use the calculated method.