Metabolism and Excretion of Hormones The concentration of a hormone in the plasma is also influenced by the rate at which it is metabolized (inactivated) and/or excreted. Inactivation can take place at or near the receptor or in the liver, the major site for hormone metabolism. In addition, the kidneys can metabolize a variety of hormones, or excrete them in their free (active) form. In fact, the rate of excretion of a hormone in the urine has been used as an indicator of its rate of secretion during exercise (8, 34, 61 62). Because blood flow to the kidneys and liver de creases during exercise, the rate at which hormones are inactivated or excreted decreases. This results in an elevation of the plasma level of the hormone over and above that due to higher rates of secretion
Transport Protein The concentration of some hormones is influenced by the quantity of transport protein in the plasma. Steroid hormones and thyroxine are transported bound to plasma proteins. For a hormone to exert its effect on a cell, it must be "free to interact with the receptor and not "bound" to the transport protein. The amount of free hormone is dependent on the quantity of transport protein and the capacity and affinity of the protein to bind the hormone molecules. Capacity refers to the maximal quantity of hormone that can be bound to the trans port protein, and affinity refers to the tendency of the transport protein to bind to the hormone. An increase in the quantity, capacity, or affinity of transport protein would reduce the amount of free hormone and its effect on tissue (37, 74). For example, high levels of estrogen during pregnancy increase the quantity of thyroxine's transport protein, causing a reduction in free thyroxine. The thyroid gland produces more thyroxine to counteract this effect.