Hormone receptors can be divided broadly into membrane receptors and nuclear receptors. Membrane receptors primarily bind peptide hormones and small molecules that cannot traverse the plasma membrane (e.g., catecholamines, dopamine). Nuclear receptors bind small, lipid-soluble molecules that diffuse or are transported across the cell membrane (e.g., thyroid hormone, steroids, vitamin D). Hormones bind to both classes of receptors with specificity and high affinity. These characteristics are often described by Scatchard plots, which allow estimation of equilibrium dissociation constants (K_d) and maximum binding (B_max) (Fig. 1-6).

Binding affinity generally coincides with the concentration of circulating hormones and is typically in the subnanomolar range. Receptor occupancy at any given moment is a function of hormone concentration and the receptor's affinity for the hormone. Receptor numbers vary greatly in different target tissues, providing one of the …