Testosterone diffuses from the testes into the bloodstream for transport to target tissues. Approximately 1% to 2% of all testosterone circulates in the blood as free T, while the other 98% to 99% is protein-bound. Most protein-bound T is strongly associated with SHBG (60%), while the other 38% fraction is bound to albumin. Only the albumin-bound and fT forms are available for binding at target tissues, and are thus called bioavailable T (bioT).1Advancing age is accompanied with changes in testosterone binding due to an increase in SHBG.2
References:
Dandona P, Rosenberg MT. A practical guide to male hypogonadism in the primary care setting.Int J Clin Pract. 2010;64:682-696. PMID: 20518947.
Kaufman J-M, T'Sojen G, Verleulen A. Androgens in male senescence. In: Nieschlag E, Behre HM, eds. Testosterone: Action, Deficiency, Substitution. Cambridge, UK: Cambridge University Press; 2012: Chapter 16.
Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males(Habito et al., 2000)
Intensive insulin therapy increases sex hormone binding globulin in newly diagnosed type 2 diabetic patients (Tong et al., 2013)
Lower serum testosterone is independently associated with insulin resistance in non-diabetic older men: the Health In Men study (get it? H.I.M.) (Yeap et al., 2009)
Testosterone is an important hormone within the male body with many relevant biologic functions. Testosterone is produced in the testes as a final step in a well-coordinated series of events. The hypothalamus secretes gonadotropin releasing hormone (GnRH), which travels to the anterior pituitary and triggers the pulsatile secretion of gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).1 Follicle-stimulating hormone binds to the Sertoli cells within the seminiferous tubules of the testes and regulates the production of spermatozoa (S). Bursts of LH travel to the Leydig cells, found adjacent to the seminiferous tubules, and stimulate the production of testosterone. Approximately 7 mg of testosterone is produced per day by the testes, although this level declines with age.2 Testosterone production is regulated by a negative feedback mechanism, with testosterone inhibiting release of GnRH from the hypothalamus and LH from the pituitary gland.1
ReplyDeleteReferences:
Dandona P, Rosenberg MT. A practical guide to male hypogonadism in the primary care setting. Int J Clin Pract. 2010;64:682-696. PMID: 20518947.
Luetjens CM, Weibauer GF. Testosterone: an overview of biosynthesis, transport, metabolism and non-genomic actions. In: Nieschlag E, Behre HM, eds. Testosterone: Action-Deficiency-Substitution. Cambridge, UK: Cambridge University Press; 2012:15-32.