discharged into the bloodstream but are released into connective tissue spaces. They may act on adjacent cells or diffuse to nearby target cells that express specific receptors for that particular hormone. This type of hormonal action is referred to as paracrine control. In addition, some cells express receptors for hormones that they secrete. This type of hormonal action is referred to as autocrine control. These hormones regulate the cell’s own activity. Figure 21.1 summarizes various hormonal control mechanisms.
Hormones include three classes of compounds.
Cells of the endocrine system release more than 100 hormones and hormonally active substances that are chemically divided into three classes of compounds:  Steroids, cholesterol-derived compounds, are synthesized and secreted by cells of the ovaries, testes, and adrenal cortex. These hormones (gonadal and adrenocortical steroids) are released into the bloodstream and transported to target cells with the help of plasma proteins or specialized carrier proteins such as androgen-binding protein. Hormone-binding carrier proteins protect the hormone from degradation during transport to the target tissue. When needed, the hormone is released from the carrier protein to become active.
 Small peptides, polypeptides,and proteins are synthesized and secreted by cells of the hypothalamus, pituitary gland, thyroid gland, parathyroid gland, pancreas, and scattered enteroendocrine cells of the gastrointestinal tract and respiratory system. This group of hormones (e.g., insulin, glucagon, growth hormone [GH], adreno corticotropic hormone [ACTH], follicle-stimulating hormone [FSH], luteinizing hormone [LH], antidiuretic hormone [ADH], oxytocin, interleukins, and various growth factors), when released into the circulation, dissolve readily in the blood and generally do not require special transport proteins. However, most if not all polypeptides and proteins have specific carrier proteins (e.g., insulin growth  factor–binding protein (IGFBP).
 Amino acids and arachidonic acid analogs, and their derivatives, including the catecholamines (norepinephrine and epinephrine–phenylalanine/tyrosine derivatives) and prostaglandins, prostacyclins, and leukotrienes (arachidonic acid derivatives). They are synthesized and secreted by many neurons as well as a variety of cells including cells of the adrenal medulla. Also included in this group of compounds are thyroid hormones, the iodinated derivatives of the amino acid tyrosine that are synthesized and secreted by the thyroid gland. When released into the circulation, catecholamines dissolve readily in the blood, in contrast to thyroid hormones that bind to the prealbumin fraction of serum proteins (transthyretin) and a specialized thyroxin-binding protein.
Hormones interact with specific hormone receptors to alter biologic activity of the target cells.