Serum testosterone

When serum testosterone levels are used to judge the quality of testosterone substitution it is necessary to be aware of the pharmacokinetic profiles of the different testosterone preparations (Chapter 14). Moreover, in longitudinal surveillance of testosterone therapy it is important to use assay systems that strictly undergo internal and external quality control (Chapter 21). Generally, testosterone serum levels should be measured just before the injection of the next dose of long-acting preparations. The time point of the last injection or administration of oral or transdermal testosterone must be recorded to interpret the serum levels measured.

Levels below the lower normal limit at the end of a three-week interval after testosterone enanthate injection or a 12-week interval after testosterone undecanoate injection should prompt shorter injection frequency of two-week intervals. Conversely, if the levels are in the high physiological range at the end of the injection interval, the dosing intervals maybe extended. Low serum testosterone levels two to four hours after ingestion of oral testosterone undecanoate should prompt counseling of the patient so that the capsule is taken together with a meal and testosterone is better absorbed. However, it is difficult to base monitoring of treatment with oral testosterone undecanoate on serum testosterone levels and other parameters are of more importance if this mode of therapy is chosen.

When transdermal preparations are applied, serum testosterone levels may be measured just before the next dose is administered. Initial measurements, however, are only meaningful after two or three weeks following initiation of therapy since it takes time until the skin builds up a reservoir and steady state serum levels are reached. Transdermal patches may show poor adhesiveness, in particular in warm weather and when the patient sweats e.g. during athletic activity. This is not the case with gels which show good tolerability and only rarely skin irritation.

After initiation of testosterone substitution, measuring serum testosterone under the conditions mentioned above is recommendable after 3, 6 and 12 months and thereafter annually.

In blood, testosterone is bound to sex hormone binding globulin (SHBG) and other proteins. Only about 2% of testosterone is not bound and is available for biological action of testosterone (free testosterone). Since total testosterone correlates well with free testosterone, separate determination of free testosterone is not necessary for routine monitoring (see also Chapter 21).

Testosterone can also be determined in saliva. The concentrations correlate to free testosterone concentrations in serum. Saliva collection can be easily performed without the help of medical staff and thus provides a useful procedure for monitoring substitution therapy (Navarro et al. 1994; Schurmeyer et al. 1983; Tschop etal. 1998). However, since the available assays are not very robust, measurement of saliva testosterone has not become a widespread methodology and remains reserved for research projects.

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