Testosterone Therapy and Women's SexualityThe relative contribution of androgen and estrogen to female sexual function is controversial. Low libido, decreased wellbeing, blunted motivation, and fatigue are listed as major features of the proposed syndrome of female androgen deficiency.1,2 However, defining and elucidating this syndrome has been problematic. First, the symptoms are vague and difficult to objectify, and can all occur in other syndromes such as major depressive disorders. Second, there is no consensus on the definition of low testosterone (T) levels. This lack reflects assay difficulties and insufficient studies establishing the normal range of androgen levels in different phases of life. In fact, T levels reach an apparent peak in the early reproductive years (early 20s) and decrease with age. Women in their 40s have approximately half the level of circulating total T as that of women in their 20s. 3 The rate of the age-related decrease in total T then seems to slow, and is not specifically related to menopause. Dehydroepiandrosterone sulphate (DHEA-S) shows similar changes to those described for T, but has an even more pronounced age-related decrease after the early reproductive years that continues through to later life. Diurnal-and menstrual-cycle-linked changes in T and androstenedione (A) also occur. T (and A) levels are highest in the morning before 10am and in the middle third of the menstrual cycle, although one small study found that the mid-cycle ovarian increase in free T level did not occur in older menstruating women (those aged 43-47 years). 5,6 There have been conflicting reports concerning post-menopause. Some studies reported that T, A, and sex-hormone-binding globulin (SHBG) levels remain unchanged, whereas others reported a small yet significant difference in the first six months after menopause. However, after spontaneous menopause the ratio T/SHBG did not show any change, 8 even though in surgical menopause both T and A decrease by about 50%, suggesting an important role of ovaries in T secretion after menopause. On the other hand, in post-menopause the principal source of circulating T is derived from the peripheral conversion of A and DHEA(S).
9In addition, independently of menopausal transition, circulating delta5-androgens fall linearly with age, beginning in the 20s as a result of the age-related reduction of 17,20 desmolase activity, the enzyme that rules the biosynthesis of the delta5-adrenal pathway.
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Androgens in WomenThe biological activity of an androgen depends on its ability to bind to androgen receptors in target tissues and regulate gene transcriptional activity, the production rate, the metabolic clearance rate (which includes various metabolic conversions and excretion), and the quantitative amount that is available to the target tissues. The metabolic clearance rate and amount of androgen that is bioavailable in the blood to be transported into cells is dependent to a great extent on the degree of binding of the low capacity and high affinity to SHBG, and t...