Testosterone: A Complete Guide for Men and Women
Testosterone is the most widely recognized sex hormone, yet it remains one of the most misunderstood—particularly regarding its role in women's health and its decline in men well below the threshold of clinical hypogonadism. This guide covers testosterone's functions across all organ systems, how deficiency presents in men and women, how it is properly tested, and what testosterone replacement therapy involves.
What Is Testosterone?
Testosterone is a steroid hormone belonging to the androgen class, produced primarily in the testes in men, the ovaries in women, and by the adrenal glands in both sexes. It is synthesized from cholesterol through a sequence of enzymatic conversions involving pregnenolone, progesterone, and DHEA as intermediate steps. In men, testosterone is the dominant sex hormone. In women, testosterone is produced in smaller quantities but plays essential roles in energy, libido, mood, bone density, and muscle maintenance.
Testosterone's Functions in the Body
The functions of testosterone are far more extensive than its common association with sexual characteristics and libido:
Muscle and strength: Testosterone is the primary anabolic hormone driving muscle protein synthesis. It stimulates satellite cells (muscle stem cells) to proliferate and fuse with existing muscle fibers, increasing muscle cross-sectional area and strength. Without adequate testosterone, muscle mass declines regardless of exercise effort.
Bone density: Testosterone directly stimulates osteoblast activity (bone formation) and is converted to estradiol by aromatase in bone tissue, where estradiol prevents osteoclast-mediated bone resorption. Men with low testosterone have significantly higher rates of osteoporosis and fragility fractures.
Fat metabolism: Testosterone inhibits the activity of lipoprotein lipase in fat cells (reducing fat storage) and promotes lipolysis (fat breakdown). It also redistributes fat away from visceral depots toward lean tissue. Low testosterone is strongly associated with increased visceral adiposity and metabolic syndrome.
Insulin sensitivity: Testosterone improves insulin sensitivity through multiple mechanisms including increased muscle mass (the primary site of glucose disposal), reduced visceral fat (a major driver of insulin resistance), and direct effects on insulin signaling pathways. Low testosterone is independently associated with type 2 diabetes risk.
Cardiovascular health: Testosterone supports healthy red blood cell production, vascular endothelial function, and cardiac muscle metabolism. Low testosterone in men is independently associated with increased cardiovascular mortality.
Cognitive function: Testosterone receptors are present throughout the brain. Testosterone supports verbal memory, spatial processing, and executive function. In men with hypogonadism, testosterone replacement has been shown to improve multiple cognitive domains.
Mood and motivation: Testosterone supports dopamine neurotransmission and the reward system. Low testosterone is strongly associated with depression, loss of motivation, reduced assertiveness, and emotional flatness—often misdiagnosed as primary depression.
Libido and sexual function: In both men and women, testosterone is the primary hormonal driver of sexual desire. In men, it also supports erectile function through nitric oxide-mediated vasodilation. Low testosterone is the leading hormonal cause of reduced libido in both sexes.
Normal and Optimal Testosterone Ranges
For men, total testosterone reference ranges typically span 270–1,070 ng/dL (depending on the laboratory), but "optimal" for health and vitality is generally considered to be in the upper third of this range—roughly 600–900 ng/dL. Free testosterone is more clinically meaningful than total testosterone for assessing androgenic activity, as SHBG binds the majority of circulating testosterone and renders it inactive. Free testosterone should ideally be in the upper quarter of the reference range.
For women, total testosterone ranges from approximately 15–70 ng/dL, with free testosterone at the lower end of detection. Women's symptoms of testosterone deficiency are real and significant even when total testosterone is "in range"—particularly when SHBG is high, driving free testosterone to very low levels.
Symptoms of Low Testosterone
In men: fatigue, low motivation, reduced libido, erectile dysfunction, decreased muscle mass, increased body fat (especially abdominal), mood changes including depression and irritability, reduced cognitive sharpness, poor sleep, and reduced bone density.
In women: reduced libido, difficulty building or maintaining muscle, fatigue, mood flatness, reduced mental sharpness and motivation, difficulty with orgasm, and worsening body composition.
Testing Testosterone Properly
A complete testosterone evaluation includes: total testosterone (morning draw when levels are highest), free testosterone (by equilibrium dialysis, the gold-standard method), SHBG, estradiol (including conversion from testosterone), LH and FSH (to distinguish primary from secondary hypogonadism), DHEA-S, and PSA in men over 40.
Testosterone Replacement Therapy
TRT for men is available via intramuscular or subcutaneous injections of testosterone cypionate or enanthate, topical gels or creams, transdermal patches, buccal adhesive systems, and subcutaneous pellets. Each delivery method has distinct pharmacokinetic profiles and practical trade-offs. TRT for women typically involves low-dose topical testosterone cream or subcutaneous pellets at doses 10–20 times lower than male doses, precisely calibrated to restore female physiological levels without masculinizing effects.
Proper monitoring includes regular assessment of total and free testosterone, estradiol, hematocrit, and PSA (in men). Ongoing dose adjustments ensure sustained optimization.
Optimize Your Testosterone with Expert Guidance
Testosterone optimization is a nuanced medical intervention that requires comprehensive evaluation, individualized protocols, and consistent monitoring. Kenton Bruice, M.D., specializes in testosterone replacement for both men and women at his clinics in Denver, Aspen, and St. Louis. Schedule a consultation with Dr. Bruice to have your testosterone evaluated and optimized.