Unit Converter
Testosterone free
(Biologically Active Fraction of Testosterone - Crucial for Accurate Diagnosis of Hypogonadism & Androgen Excess)
Synonyms
- Free testosterone
- FT
- Unbound testosterone
- Free T
- Bioactive testosterone fraction
(Note: “Bioavailable testosterone = Free + Albumin-bound”; not the same as free.)
Units of Measurement
- pmol/L
- pg/mL
- pg/dL
- pg/100 mL
- pg%
- pg/L
- ng/L
Unit Conversions
Molecular Weight of Testosterone = 288.42 g/mol
pmol/L ↔ pg/mL
1 pmol/L=0.288 pg/mL1\ \text{pmol/L} = 0.288\ \text{pg/mL}1 pmol/L=0.288 pg/mL 1 pg/mL=3.47 pmol/L1\ \text{pg/mL} = 3.47\ \text{pmol/L}1 pg/mL=3.47 pmol/L
pg/mL ↔ pg/L
1 pg/mL=1000 pg/L1\ \text{pg/mL} = 1000\ \text{pg/L}1 pg/mL=1000 pg/L
pg/mL ↔ ng/L
1 pg/mL=1 ng/L1\ \text{pg/mL} = 1\ \text{ng/L}1 pg/mL=1 ng/L
pg/dL = pg%
(Older clinical volumetric units.)
pg/dL → pg/mL
1 pg/dL=0.01 pg/mL1\ \text{pg/dL} = 0.01\ \text{pg/mL}1 pg/dL=0.01 pg/mL
Description
Free testosterone (FT) represents the ~1–2% of circulating testosterone that is:
- Not bound to sex hormone-binding globulin (SHBG)
- Not bound to albumin
- Freely available to enter tissues and activate androgen receptors
FT is the biologically active hormone - therefore more clinically meaningful than total testosterone in many scenarios.
Free testosterone can be measured by:
- Equilibrium dialysis (gold standard)
- Ultrafiltration (reference method)
- Calculated formulas (Vermeulen, Sodergard) using total T, SHBG, albumin
Direct immunoassays for FT are unreliable and should be avoided.
Physiological Role
Free testosterone directly drives:
1. Sexual & Reproductive Function
- Libido
- Erectile function
- Spermatogenesis
2. Muscle, Strength & Body Composition
- Lean mass
- Fat distribution
- Basal metabolic rate
3. Bone Metabolism
- Bone formation
- Anti-resorptive effects
4. Mood & Cognition
- Motivation
- Confidence
- Energy
- Memory
5. Erythropoiesis
Stimulates red-cell production.
Clinical Significance
LOW FREE TESTOSTERONE
True marker of clinical hypogonadism.
Primary hypogonadism
- Testicular failure
- Klinefelter syndrome
- Chemotherapy/radiation
- Orchitis
Secondary hypogonadism
- Pituitary disease
- Prolactinoma
- Hypothalamic disorders
- Systemic illness
- Obesity
- Type 2 diabetes
Symptoms
- Low libido
- Erectile dysfunction
- Low mood
- Fatigue
- Reduced muscle mass
- Central obesity
- Osteopenia/osteoporosis
FT is low even with normal total testosterone if SHBG is high.
HIGH FREE TESTOSTERONE
Men
- Exogenous testosterone use
- Androgen-secreting tumors
- Congenital adrenal hyperplasia (CAH)
Women
- Polycystic ovary syndrome (PCOS)
- Ovarian/adrenal androgen-secreting tumors
- Severe insulin resistance
- Hirsutism
- Acne
- Menstrual irregularities
Children
- Precocious puberty
Reference Intervals
(Tietz 8E + Endocrine Society + Mayo + ARUP - equilibrium dialysis values)
Adult Men
- Free Testosterone: 180 – 250 pmol/L
(= 5 – 7.2 pg/mL)
Typical laboratory range: 150 – 250 pmol/L
Adult Women
- Free Testosterone:
0.5 – 7 pmol/L
(= 0.02 – 0.2 pg/mL)
Postmenopausal Women
- FT tends to be very low (<2 pmol/L)
Children
Ranges vary by Tanner stage; very low prepubertally.
Diagnostic Uses
1. Confirming Hypogonadism (Men)
Indicated especially when:
- Total T is borderline (200–350 ng/dL)
- SHBG is abnormal (high or low)
- Symptoms are strong despite normal total T
2. PCOS Evaluation (Women)
FT is often more elevated than total T.
3. Androgen-producing Tumors
Markedly elevated FT.
4. Monitoring Testosterone Replacement Therapy
Ensures physiological free levels without supraphysiology.
5. Investigating Infertility
6. CAH or Adrenal Hyperplasia Patterns
Analytical Notes
- Always collect early morning (7–10 AM) in men
- Avoid testing during acute illness
- Prefer equilibrium dialysis FT or calculated FT
- SHBG is mandatory for accurate free T interpretation
- Albumin value needed for calculated FT
Clinical Pearls
- Free testosterone correlates better than total T with sexual & metabolic symptoms.
- Obesity → ↓ SHBG → low total T but normal FT (important distinction).
- High FT with normal total T usually indicates low SHBG.
- Women with PCOS almost always have elevated free androgen index (FAI) or elevated FT.
- Direct FT immunoassays are not reliable — use calculated or equilibrium dialysis.
Interesting Fact
Although free testosterone makes up only 1–2% of total testosterone, it mediates nearly all biological actions of the hormone.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Steroid Hormones
- Endocrine Society Clinical Practice Guideline - Hypogonadism in Men (2018)
- AACE Guidelines - Androgen Disorders
- Mayo Clinic Laboratories - Free Testosterone (Equilibrium Dialysis)
- ARUP Consult - Androgen Testing & Free Testosterone Calculation
- NIH / MedlinePlus - Testosterone