Hormone Transport In Blood
When hormones are
secreted into the blood, many are immediately bound to plasma proteins.
The proteins may recognize
the hormone specifically and bind it with high affinity and specificity, for
example the binding of sex hormones by sex hormone - binding globulin (SHBG).
Other proteins, such as albumin, also bind many hormones, including thyroid hormone
and the sex hormones, with much lower affinity. Equilibrium is set up between
the free and bound hormone, so that a fixed proportion of the hormone travels
free and unbound, while most is carried bound. It is currently believed that
only the free fraction of the hormone is physiologically active and available
to the tissues and for metabolism. When a hormone is bound to plasma proteins
it is physiologically inactive and is also protected from metabolic enzymes in
organs such as the liver. Some drugs, such as aspirin, can displace other substances
such as anticoagulants from their binding sites, which in the case of
anticoagulants may cause haemorrhage.