Testosterone

What testosterone actually does

Testosterone is the primary androgen in males, but calling it a "sex hormone" sells it short. It regulates muscle protein synthesis, bone mineral density, red blood cell production, fat metabolism, mood, motivation, and cognitive function. Women produce testosterone too, at roughly one-tenth the concentration, where it supports bone health, libido, and energy levels.

In men, total testosterone declines about 1-2% per year after age 30. But this isn't just aging at work. Population-level data show a roughly 25% drop in average testosterone over the past four decades, a trend linked to rising obesity rates, environmental endocrine disruptors like phthalates and BPA, chronic stress, and sedentary behavior. A 2025 multi-population study published in eClinicalMedicine found that higher serum testosterone was consistently associated with reduced biological age acceleration across Chinese, American, and British cohorts ^[1].

Testing: total T is not the full picture

Sex hormone-binding globulin (SHBG) locks up 60-70% of circulating testosterone, making it biologically inactive. Albumin loosely binds another 25-30%. That leaves just 1-3% as free testosterone, the fraction that enters cells and produces effects. A man with high total testosterone but elevated SHBG can still experience symptoms of deficiency.

A proper hormonal panel should include total testosterone, free testosterone, SHBG, estradiol, and LH. Blood draws should happen between 8 and 10 a.m. when levels peak, and the AUA recommends confirming low results with a second fasting sample. Clinically low testosterone is generally defined as below 300 ng/dL, while optimal ranges sit between 500-900 ng/dL with free testosterone above 15-20 pg/mL.

Natural testosterone optimization

Before reaching for pharmaceuticals, lifestyle changes can produce significant effects on testosterone output.

Resistance training is the most potent natural stimulus. Compound movements like squats, deadlifts, and bench presses at 70-85% of one-rep max generate the strongest acute hormonal response. A 2024 clinical trial in frail older men showed that 52 weeks of progressive resistance training combined with testosterone supplementation improved physical performance and reduced fatigue ^[2].

Sleep is non-negotiable. Testosterone production peaks during deep sleep. A meta-analysis of 18 studies found that total sleep deprivation significantly reduces serum testosterone, while restricting sleep to 5 hours per night drops levels by 10-15% ^[3]. The cortisol-testosterone seesaw matters here: sleep loss raises cortisol and suppresses gonadotropin-releasing hormone.

Body composition directly impacts hormonal balance. Each one-point drop in BMI raises testosterone by about 1 ng/dL, partly because excess adipose tissue contains aromatase, the enzyme that converts testosterone to estradiol.

Micronutrients support the biochemical machinery of steroidogenesis. Zinc (30 mg/day), magnesium, and vitamin D are the best-studied. A 2024 meta-analysis confirmed that vitamin D supplementation can raise total testosterone levels, particularly in men who are deficient ^[4]. Boron at 6-10 mg/day may lower SHBG, increasing the free testosterone fraction.

Testosterone replacement therapy (TRT)

When lifestyle optimization isn't enough and levels are persistently below 300 ng/dL with symptoms, TRT becomes a clinical option. Standard protocols use testosterone cypionate or enanthate injections (100-200 mg weekly, often split into two doses for stable blood levels), transdermal gels, or subcutaneous pellets.

The landmark TRAVERSE trial, published in the New England Journal of Medicine in 2023, randomized over 5,200 men and found that testosterone therapy did not increase major adverse cardiovascular events compared with placebo ^[5]. However, secondary outcomes revealed elevated rates of atrial fibrillation, acute kidney injury, and pulmonary embolism. TRT also raises haematocrit and requires regular monitoring of PSA, estradiol, and lipid profiles.

TRT and fertility

Exogenous testosterone suppresses the HPG axis through negative feedback, reducing intratesticular testosterone and FSH. Sperm concentrations can drop below 1 million/mL within 3-4 months ^[6]. Men who want to preserve fertility have alternatives: clomiphene citrate stimulates endogenous testosterone production by blocking estrogen receptors at the hypothalamus, and a 2024 systematic review found SERMs increased total testosterone by an average of 274 ng/dL while preserving or improving sperm parameters ^[7]. Human chorionic gonadotropin (hCG) is another option that maintains testicular function during or alongside TRT.

Testosterone in women

Women's testosterone levels matter more than commonly recognized. Androgens influence bone density, muscle maintenance, mood, energy, and sexual desire throughout the female lifespan. DHEA, the precursor hormone, converts partly to testosterone, and supplementation at 50 mg/day has been shown to increase testosterone and estradiol in postmenopausal women ^[8]. However, the clinical evidence for broad anti-aging benefits from DHEA supplementation remains limited, and decisions about androgen therapy in women should involve careful risk-benefit assessment with an endocrinologist.