Testosterone (transdermal) — The Primary Male Sex Hormone
Testosterone is the primary male sex steroid hormone, essential for maintaining muscle mass, bone density, libido, and cognitive function. In the context of longevity, transdermal testosterone replacement therapy (TRT) is used to address age-related hypogonadism, aiming to restore physiological levels and mitigate frailty, metabolic dysfunction, and cardiovascular risk associated with androgen deficiency.
Mechanism of Action
Testosterone exerts its effects primarily by binding to the intracellular androgen receptor (AR), which then translocates to the nucleus to act as a transcription factor, modulating the expression of target genes involved in protein synthesis and cellular growth. It also undergoes local conversion to estradiol via aromatase, which is crucial for bone health and cognitive function, and to dihydrotestosterone (DHT) via 5-alpha reductase, which mediates effects in the prostate and skin. By enhancing muscle protein synthesis and inhibiting protein breakdown, testosterone promotes hypertrophy and strength. Furthermore, it influences metabolic pathways by improving insulin sensitivity and reducing fat mass.
Human Trial Evidence
Extensive human trials have evaluated transdermal testosterone in older men with hypogonadism. The landmark Testosterone Trials (TTrials), published in the New England Journal of Medicine (2016), demonstrated that transdermal testosterone gel improved sexual function, mood, and bone density, while modestly increasing hemoglobin levels. The recent TRAVERSE trial (2023) showed that testosterone replacement therapy did not increase the incidence of major adverse cardiovascular events in men with hypogonadism and high cardiovascular risk.
Dosing Protocol
Transdermal testosterone is typically administered as a 1% or 1.62% gel, applied daily to the shoulders, upper arms, or abdomen. Starting doses usually range from 20 to 50 mg of testosterone per day, adjusted based on serum testosterone levels to target the mid-normal physiological range (e.g., 400–800 ng/dL). This is a prescription-only medication requiring regular monitoring of hematocrit, prostate-specific antigen (PSA), and lipid profiles.
Safety & Contraindications
Common adverse effects include application site reactions, acne, and increased hematocrit (erythrocytosis), which may require dose reduction or therapeutic phlebotomy. It is contraindicated in men with prostate cancer, breast cancer, severe lower urinary tract symptoms, or uncontrolled heart failure. Transdermal application carries a black box warning for the risk of secondary exposure to women and children, necessitating careful handwashing and covering of the application site.