Steroid sex hormones are well absorbed through the skin (factory workers need protective clothing) and the gut. Most are subject to extensive hepatic metabolic inactivation (some so much that oral administration is ineffective or requires very large doses if a useful amount is to pass through the liver and reach the systemic circulation). There is some enterohepatic recirculation, especially of oestrogen, and this may be interrupted by severe diarrhoea to cause loss of efficacy. There are some nonsteroid analogues that are more slowly metabolised. Sustained-release (depot) preparations are used. The hormones are carried in the blood extensively bound to sex-hormone-binding globulin. In general the plasma t'/2 relates to the duration of cellular action, which is implied in the recommended dosage schedules.
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