Testosterone for the aging male; current evidence and recommended practice - 0 views
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Total serum testosterone consists of free testosterone (2%–3%), testosterone bound to sex hormone binding globulin (SHBG) (45%) and testosterone bound to other proteins (mainly albumin −50%)
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Testosterone binds only loosely to albumin and so this testosterone as well as free testosterone is available to tissues and is termed bioavailable testosterone
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Bioavailable and free testosterone are known to correlate better than total testosterone with clinical sequelae of androgenization such as bone mineral density and muscle strength
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The reliable measurement of serum free testosterone requires equilibrium dialysis. This is not appropriate for clinical use as it is very time consuming and therefore expensive.
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With increasing age, a greater number of men have total testosterone levels just below the normal range or in the low-normal range. In these patients total testosterone can be an unreliable indicator of hypogonadal status.
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It is advised that at least two serum testosterone measurements, taken before 11 am on different mornings, are necessary to confirm the diagnosis.
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Patients with serum total testosterone consistently below 8 nmol/l invariably demonstrate the clinical syndrome of hypogonadism and are likely to benefit from treatment. Patients with serum total testosterone in the range 8–12 nmol/l often have symptoms attributable to hypogonadism and it may be decided to offer either a clinical trial of testosterone treatment or to make further efforts to define serum bioavailable or free testosterone and then reconsider treatment. Patients with serum total testosterone persistently above 12 nmol/l do not have hypogonadism and symptoms are likely to be due to other disease states or ageing per se so testosterone treatment is not indicated.
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Total testosterone levels fall at an average of 1.6% per year whilst free and bioavailable levels fall by 2%–3% per year.
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With advancing age there is also a reduction in androgen receptor concentration in some target tissues and this may contribute to the clinical syndrome of LOH
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Gonadotrophin levels rise during aging (Feldman et al 2002) and testicular secretory responses to recombinant human chorionic gonadotrophin (hCG) are reduced
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There are changes in the lutenising hormone (LH) production which consist of decreased LH pulse frequency and amplitude, (Veldhuis et al 1992; Pincus et al 1997) although pituitary production of LH in response to pharmacological stimulation with exogenous GnRH analogues is preserved
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the decreases in testosterone levels with aging seem to reflect changes at all levels of the hypothalamic-pituitary-testicular axis