Steroids and Thyroid Hormones Are Lipophilic and Carried in Plasma in Association with Both Specific and Nonspecific Binding Proteins; the Amount of Unbound, Active Hormone Is Relatively Small
The transport of steroid and thyroid hormones is more complicated than that of protein hormones, because the steroid and thyroid hormones are lipophilic and thus have limited solubility in aqueous solutions.
These hormones are transported in the blood through association with various types of proteins. Some of the proteins that bind steroids have a high affinity for a particular steroid; for example, a globulin, trans- cortinf has a high affinity for cortisol and corticosterone but also serves as an important transport vehicle for progesterone, even though it has a lower affinity for this hormone. The carrier proteins that have high affinities have low capacity because of their low plasma concentration. In contrast, the general class of plasma proteins called albumins have low affinities for steroid hormones but have a high capacity for steroid transport because of their high concentration in plasma.A hormone must be in the free, or unbound,∕orm before it can penetrate a target cell and elicit biological activity. This is accomplished by the establishment of equilibrium between bound and free hormone levels in the plasma. The free form usually represents only about 1% of the total amount of hormone in the plasma (up to 10% of cortisol may be in the free form). The system is responsive to use of the free form, and the free form is replenished quickly by dissociation of bound hormone from the protein. The total amount of the hormone is usually measured, with the exception of thyroid hormone, for which attempts are usually made to estimate the amounts of bound and free. As indicated for steroid hormones, synthesis and release are tightly linked, and because metabolic clearance rates are usually constant, concentrations of steroids in plasma are usually a good reflection of the secretion rate. Under certain physiological conditions, such as pregnancy in humans, metabolism of estrogens can change because of the increased production of estrogen-binding proteins.