Sunday, February 5, 2012

Hormone Transport in the Blood

Most peptide and all catecholamine hormones are water-soluble. Therefore, with the exception of a few peptides, these hormones are transported simply dissolved in plasma. In contrast, the poorly soluble steroid hormones and thyroid hormones are transported in the blood largely bound to plasma proteins. Even though the steroid and thyroid hormones exist in plasma mainly bound to large proteins, small concentrations of these hormones do exist dissolved in the plasma. The dissolved, or free, hormone is in equilibrium with the bound hormone: Free hormone + Binding protein = Hormone-protein complex. The total hormone concentration in plasma is the sum of the free and bound hormone. It is important to realize, however, that only the free hormone can diffuse across capillary walls and encounter its target cells. Therefore, the concentration of the free hormone is what is physiologically important rather than the concentration of the total hormone, most of which is bound. The degree of protein binding also influences the rate of metabolism and the excretion of the hormone.