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Fig. 1. Lifetime curves of circulating testosterone (T) concentrations in males and females. A. Prenatal period. Androgens are produced by the fetal testis and adrenals, while T is predominantly biosynthesized in the testes. In the fetal testis, the Leydig cells start producing T for sexual differentiation of male sex organs at 6 weeks of gestation. T levels keep rising until the end of the second trimester, peaking with values similar to levels measured in adult men; thereafter T's concentration decrease until the end of the third trimester. T levels in a female fetus remain low during pregnancy. B. Neonatal Period. At birth (0-3 days of life), T levels are increased in males due to a surge of luteinizing hormone (LH)/follicular-stimulating hormone (FSH), and again rises with mini puberty between 2 and 3 months of life stimulated once again by LH. C. Pubertal and adult period. During adrenarche around the age of 6-8 years, T levels increase slightly in both sexes. Finally, activation of the hypothalamic-pituitary-gonadal axis at the age of 12 years (puberty in boys) results in a significant increase in T levels peaking between the second and fourth decade of life in men. With aging, however, T production slightly decreases in men. Although T levels also rise during puberty in females, after about 10 years of age, T remains markedly lower in middle-aged women and decreases further with menopause

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