Introduction

Abstract

The traditional and still major part of clinical endocrinology deals with the glands that produce hormones and in particular with the plasma concentrations of hormones to which cells expressing receptors are exposed. Glandular biosynthesis and secretion, the means of transport of hormone to target cells, and metabolic inactivation determine the effective hormone concentration. However, the capacity to form hormones is not limited to endocrine glands. Hormones may be activated by nonendocrine organs through proteolytic cleavage of protein prohormones (for example, in the vascular bed). Others, such as dihydrotestosterone, triiodothyronine, and estradiol, are in part secreted by endocrine glands and in part formed in peripheral tissues from circulating precursors. Some messengers circulate only in restricted compartments such as the hypothalamic-pituitary portal system and do not reach the systemic circulation in appreciable quantities. Many hormones, of which insulin and dihydrotestosterone are examples, have both paracrine actions in the tissues in which they are formed and classic endocrine actions at peripheral sites.