Adrenocortical Hormones Are Carried in Plasma in Association with Specific Binding Globulins (Corticosteroid- Binding Globulin)
Steroid hormones, as indicated previously, are lipids and depend on binding to plasma proteins for transport in the blood. A specific globulin that has a high affinity for cortisol has been identified: corticosteroid-binding globulin, or trans- cortin.
Of the cortisol carried in plasma, 75% is bound to transcortin and 15% to albumin, leaving 10% in the unbound, or free, state. This amount of free hormone is large compared with thyroid hormones: less than 0.1% of T4 is free. The transport of aldosterone is mainly associated with albumin (50%), and only 10% is associated with transcortin, leaving a very large amount (40%) in the free state.Changes in physiological or pathophysiological states can influence the amount of binding proteins present in plasma. Estrogen produced in increasing amounts by the fetoplacental unit during pregnancy results in an increase in hepatic synthesis of transcortin, whereas liver dysfunction can result in lower concentrations of transcortin. The large pool of hormone present in the bound state during pregnancy gives animals a good reserve from which to make appropriate adjustments in the amount of free hormone available for influencing biological activity. Because the total amount of glucocorticoid is determined in the assay of plasma concentrations, the veterinary clinician needs to be aware that total concentrations not only reflect secretion rate, but also can be influenced by the amount of glucocorticoid-binding plasma proteins.