Free testosterone

What it is

Free testosterone is the biologically active, unbound fraction of circulating testosterone, representing approximately 1–3% of total testosterone in the bloodstream, and is considered the primary driver of androgenic action at the cellular level. Also called unbound testosterone or free T, it is the portion not attached to sex hormone-binding globulin (SHBG) or albumin, giving it direct access to androgen receptors in muscle, bone, brain, and other tissues. Because total testosterone can appear normal even when free testosterone is low — particularly when SHBG is elevated — clinicians increasingly rely on free T measurements to diagnose testosterone deficiency (hypogonadism) in both men and women. In Canada, free testosterone testing is available through major laboratory networks including LifeLabs and Dynacare, though ordering practices and covered indications vary by province.

Causes and mechanism

Most testosterone in circulation (roughly 44–65%) binds tightly to sex hormone-binding globulin (SHBG), a liver-produced protein that renders it biologically unavailable. Another 30–50% binds loosely to albumin and is considered weakly bioavailable. Only the remaining 1–3% circulates as free testosterone, unattached to any carrier protein, and can diffuse freely into target cells to activate androgen receptors.

Several factors shift the free-to-total testosterone ratio:

Factor	Effect on SHBG	Effect on Free T
Aging (men, after ~30)	Increases ~1–2%/year	Decreases
Obesity / insulin resistance	Decreases	May increase or be disproportionately low
Hypothyroidism	Decreases	May increase
Hyperthyroidism	Increases	Decreases
Liver disease	Decreases	Variable
Exogenous estrogen / OCP use	Increases	Decreases
Anabolic steroid use	Decreases	Variable

In men, total testosterone declines at roughly 1% per year after age 30–40, but free testosterone often falls faster because SHBG rises with age. This divergence is why two men with identical total testosterone levels can have very different free T values and symptom profiles.

Symptoms and diagnosis

Low free testosterone can produce fatigue, reduced libido, erectile dysfunction (in men), decreased muscle mass, mood changes, and difficulty concentrating. In women, low free T is associated with reduced sexual desire, fatigue, and diminished sense of well-being, though the evidence base for women is less developed than for men.

Diagnosis involves two steps. First, a morning total testosterone measurement is taken (testosterone peaks in the early morning). If total T is borderline or SHBG is suspected to be abnormal, free testosterone is calculated or directly measured. Two approaches are used:

• Calculated free testosterone — derived from total testosterone, SHBG, and albumin using the Vermeulen equation; widely available and generally preferred for clinical decision-making.
• Direct (analog) immunoassay — measures free T directly but is considered less accurate than the calculated method; results can vary significantly between laboratories.

The Endocrine Society and Canadian guidelines from researchers at Mount Sinai and Women's College Hospital in Toronto recommend using calculated free testosterone when SHBG abnormalities are suspected, rather than relying on total testosterone alone.

Reference ranges differ by sex, age, and laboratory method, so results must always be interpreted against the specific lab's reference interval.

Treatment options

When low free testosterone is confirmed alongside consistent symptoms, treatment depends on the underlying cause and the patient's sex and clinical context.

In men: Testosterone replacement therapy (TRT) — available in Canada as topical gels (AndroGel, Testim), intramuscular injections (testosterone cypionate, testosterone enanthate), or subcutaneous pellets — raises both total and free testosterone. The goal is to restore free T to the mid-normal range for age, not simply to normalize total T. Canadian patients can access TRT through a family physician, urologist, or endocrinologist, or through telehealth platforms such as Felix, Maple, Phoenix, or Cleo, which offer virtual assessment and prescription.

In women: Low-dose testosterone therapy (typically topical) is used off-label in Canada for hypoactive sexual desire disorder (HSDD) and other androgen-insufficiency symptoms. The SOGC and international menopause societies acknowledge the evidence for testosterone in postmenopausal women, though no Health Canada-approved female testosterone product currently exists; compounded formulations are used.

Addressing root causes: Optimizing thyroid function, reducing obesity, or discontinuing SHBG-raising medications (such as oral contraceptives) can meaningfully raise free testosterone without direct hormone therapy.

When to see a clinician in Canada

Seek assessment if you experience persistent fatigue, low libido, mood changes, or unexplained loss of muscle mass — especially if symptoms have lasted more than a few months. A morning blood draw for total testosterone and SHBG is the standard first step; your family physician can order this through LifeLabs or Dynacare. If total testosterone is borderline (roughly 8–12 nmol/L in men) or SHBG is known to be abnormal, ask specifically about calculated free testosterone, as it may better reflect your androgen status. Referral to an endocrinologist or urologist is appropriate when initial results are ambiguous or when TRT is being considered.

Limitations and open questions

Research is still emerging on several fronts. There is no universally agreed lower threshold for free testosterone that defines deficiency — reference ranges vary between laboratories and between the calculated and direct-assay methods, making cross-lab comparisons unreliable. The clinical significance of low free testosterone in women is particularly uncertain; no Canadian or international guideline has established a diagnostic cut-off for female androgen insufficiency. The relationship between free testosterone levels and specific health outcomes (bone density, cardiovascular risk, cognitive function) in older adults remains an active area of investigation, with the National Academies of Medicine noting as early as 2004 that causality is difficult to establish. Health Canada has not yet approved any testosterone product specifically indicated for women, leaving prescribers reliant on off-label or compounded options whose long-term safety data are limited. Finally, the superiority of free testosterone over bioavailable testosterone (free T plus albumin-bound T) as a clinical marker has not been definitively established.