Testosterone, Free Testosterone and Oestradiol Blood Test

Testosterone is the primary male sex hormone, produced mainly by the Leydig cells in the testes. This test measures total testosterone—the sum of testosterone bound to proteins (SHBG and albumin) plus the small fraction that is free. Total testosterone is the standard first-line measurement for assessing testosterone status, though it doesn't tell the whole story because most of it is bound and inactive. Testosterone regulates libido and sexual function, erectile function, sperm production and fertility, muscle mass and strength, bone density, fat distribution (preventing central obesity), red blood cell production, mood and mental wellbeing, energy and motivation, and cognitive function. Testosterone levels naturally decline gradually after age 30 (approximately 1-2% per year), though this varies significantly between individuals. Low testosterone (hypogonadism) can occur at any age. Normal total testosterone in men is typically 8.6-29 nmol/L in the morning. Levels below 8 nmol/L are generally considered low; levels 8-12 nmol/L are borderline; levels above 12 nmol/L are usually adequate. However, total testosterone can be misleading when SHBG is abnormal—this is why free testosterone calculation is valuable. Results outside the normal range may need a follow-up with your GP.

Free testosterone is the fraction of testosterone that is not bound to proteins and is therefore immediately available to enter cells and exert biological effects. Only about 2-3% of total testosterone is free; the rest is bound to sex hormone-binding globulin (SHBG, ~60-70%, tightly bound and inactive) and albumin (~25-35%, loosely bound and partially available). Free testosterone is calculated using a validated algorithm from the measured total testosterone, SHBG, and albumin values. Free testosterone is often more clinically meaningful than total testosterone because it represents what's actually available to your tissues. Total testosterone can be misleadingly normal when SHBG is elevated (as occurs with ageing, hyperthyroidism, liver disease, and oestrogen exposure), resulting in low free testosterone and symptoms despite "normal" total testosterone. Conversely, total testosterone can appear borderline-low when SHBG is low (as in obesity, insulin resistance, hypothyroidism), but free testosterone may actually be adequate. Normal free testosterone in men is typically 0.2-0.6 nmol/L (200-600 pmol/L). If free testosterone is low while total testosterone is normal or borderline, it suggests that SHBG is high and less testosterone is available to tissues—this can explain symptoms despite apparently adequate total testosterone. Understanding both total and free testosterone, along with SHBG, gives a much more complete picture of your androgen status. Results outside the normal range may need a follow-up with your GP.

Oestradiol is the most potent oestrogen and, while often considered a "female hormone," plays important roles in men. In men, oestradiol is produced primarily through conversion (aromatisation) of testosterone by the enzyme aromatase, found mainly in fat tissue. This is why oestradiol levels tend to increase with higher body fat. Men need some oestradiol—it's important for bone health, brain function, libido, and cardiovascular health—but too much can cause problems. Elevated oestradiol in men can cause breast tenderness or enlargement (gynaecomastia), water retention, mood changes (irritability, depression), reduced libido, and erectile dysfunction. The testosterone-to-oestradiol ratio matters: a man might have normal testosterone but if oestradiol is elevated, the effective androgen action is reduced. This is particularly relevant for men on TRT, where some of the administered testosterone gets converted to oestradiol; if oestradiol rises too high, it can negate some of the benefits of testosterone therapy. Normal oestradiol in men is typically 40-160 pmol/L. Levels above this, particularly if accompanied by symptoms, may warrant attention. For men on TRT with elevated oestradiol, options include reducing body fat (which reduces aromatase activity), adjusting testosterone dose, or occasionally using aromatase inhibitors (though these should be used cautiously due to effects on bone health and lipids). Results outside the normal range may need a follow-up with your GP.

Sex hormone-binding globulin (SHBG) is a protein produced by the liver that binds and transports sex hormones (testosterone, oestradiol, and dihydrotestosterone) in the blood. Testosterone bound to SHBG is tightly held and unavailable to cells—essentially inactive. SHBG therefore acts as a regulator of how much hormone is biologically available, independent of total hormone levels. SHBG levels are affected by many factors. SHBG is increased by ageing, hyperthyroidism, liver disease (cirrhosis), oestrogen exposure, and certain medications. SHBG is decreased by obesity, insulin resistance (metabolic syndrome, type 2 diabetes), hypothyroidism, androgen use, and high-dose glucocorticoids. These variations explain why total testosterone can be misleading—two men with identical total testosterone can have very different free testosterone depending on their SHBG levels. Normal SHBG in men is typically 15-55 nmol/L. Low SHBG (common in obesity/metabolic syndrome) means more free testosterone is available despite possibly lower total testosterone—symptoms may not match what total testosterone suggests. High SHBG means less free testosterone is available despite apparently adequate total testosterone—symptoms may be present despite "normal" total testosterone. SHBG is essential for calculating free testosterone and understanding the complete picture of androgen status. Results outside the normal range may need a follow-up with your GP.

Albumin is the most abundant protein in blood, produced by the liver. It has many functions including maintaining osmotic pressure (keeping fluid in blood vessels), transporting various substances (hormones, fatty acids, drugs, bilirubin), and buffering blood pH. In the context of hormone assessment, albumin's role as a hormone transport protein is most relevant. Approximately 25-35% of testosterone is bound to albumin. Unlike the tight binding to SHBG, albumin binding is loose and reversible—testosterone can dissociate from albumin and become available to tissues relatively easily. For this reason, albumin-bound testosterone is sometimes considered "bioavailable" along with free testosterone. Measuring albumin allows the calculation of free testosterone using validated algorithms that account for binding to both SHBG and albumin. Normal albumin is typically 35-50 g/L. Low albumin can occur in liver disease, kidney disease (nephrotic syndrome), malnutrition, and severe illness. Very low albumin affects hormone calculations and may indicate underlying health issues requiring investigation. In most healthy individuals, albumin is stable and within the normal range. Results outside the normal range may need a follow-up with your GP.