Transdermal testosterone gels were first introduced in the US in 2000. Since then, they have emerged as a favorable mode of testosterone substitution. Serum testosterone levels reach a steady-state in the first 24 hours of application and remain in the normal range for the duration of the application. This pharmacokinetic profile is comparable to that of testosterone patch but superior to injectable testosterone esters that are associated with peaks and troughs with each dose. Testosterone gels are as efficacious as patches and injectable forms in their effects on sexual function and mood. Anticipated increases in prostate-specific antigen with testosterone therapy are not significantly different with testosterone gels, and the risk of polycythemia is lower than injectable modalities. Application site reactions, a major drawback of testosterone patches, occur less frequently with testosterone gels. However, inter-personal transfer is a concern if appropriate precautions are not taken. Superior tolerability and dose flexibility make testosterone gel highly desirable over other modalities of testosterone replacement. Androgel and Testim, the two currently available testosterone gel products in the US, have certain brandspecific properties that clinicians may consider prior to prescribing. Keywords: testosterone gel, Androgel, Testim, hypogonadism
Physiology of hypothalamic-pituitary-gonadal axisTesticular function is dependent on the hypothalamic secretion of gonadotropin-releasing hormone (GnRH) which in turn stimulates the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary to act on the testis. This results in spermatogenesis and testosterone production via stimulation of seminiferous tubules and Sertoli cells, and Leydig cells, respectively. Testosterone is secreted at adult levels during three periods of male life: transient surge during the first trimester of intrauterine life, perinatal surge during early neonatal life, and continually after puberty to maintain androgenization. Testosterone exerts negative feedback both at the level of the hypothalamus and the pituitary gland. Additionaly, sertoli cells produce inhibin that exerts a negative feedback on FSH secretion.
Diagnosis of androgen deficiencyThe estimates of the prevalence of androgen deficiency, defined solely in terms of serum testosterone concentrations below the lower limit of the normal range for healthy young men, vary greatly among different studies. Longitudinal data from the Massachusetts Male Aging Study (MMAS) indicate that the prevalence of symptomatic androgen deficiency (Total testosterone 300 ng/dL) in men aged between 40-69 years is between 6% and 12%.