Regulation and Control of the Thyroid-Pituitary System

Abstract

The thyroid-pituitary system of warm-blooded animals does appear to behave as an effective regulatory system in which the controlled variable is the level of free (unbound) thyroid hormone in the plasma. A negative feedback loop illustrating the concept of the free hormone level in plasma acting on the pituitary gland to regulate the secretion of thyroid-stimulating hormone (TSH) is an essential feature of the biologist’s line-and-arrow diagram of this system and some function to express this action appears in any mathematical description of the system. However, the relationship between the rate of secretion of thyroid hormone and the concentration of free hormone in the plasma to which the pituitary is exposed is neither simple nor direct. The animal is a highly complex and variable plant, affecting the distribution and metabolism of thyroid hormone. Any adequate and realistic model of the system must consider the features of the plant in detail. A mathematical formulation of the process of hormone binding to carrier proteins in plasma is presented. Simulation of even a limited aspect of a complex biological system as it exists in real life may provide valuable insight into the overall behavior of the system.