Hormones begins by presenting an introduction to the broad topic of hormones. It then turns to an examination of hormones and disease, looking at what happens when things go wrong. Chapters that follow discuss hormones and development, hormones and reproduction, and hormones and behaviour. Finally, the text ends with a brief assessment of the use and abuse of hormones.
Joy Hinson and Peter Raven
Hormones and behaviour
This chapter assesses hormones and behaviour. It begins by looking at the mammalian hypothalamic hormones, oxytocin and vasopressin, which have counterparts in all vertebrates. All of these variations appear to have originated from a single gene, duplicated and copied forwards or backwards, and with individual changes in some amino acid positions. Many hormones have effects on behaviour in addition to their better-known effects on the body. Oxytocin affects a variety of complex behaviours, including maternal bonding, attachment, trust, empathy, and eating behaviour. In addition, several different hormones interact to control appetite and food intake: ghrelin stimulates hunger and increases food intake; leptin inhibits hunger and regulates fat stores; glucagon-like peptide (GLP), insulin, and oxytocin all work to stop you from eating when you are full. Oxytocin also works to control food preferences. Meanwhile, testosterone determines male behaviour; the absence of testosterone determines female behaviour.
Hormones and development
This chapter examines hormones and development. In mammals, including humans, hormones are essential for the development of a male phenotype. The female phenotype is the default and it is the presence of male hormones that overrides the default and allows the baby to become phenotypically male. Much of our understanding of normal sexual development comes from investigating situations where this process goes wrong. In other vertebrates, gender determination is more complex and can be affected by factors such as temperature and social hierarchy. The chapter then considers the role of thyroid hormone in development. Hormones control metamorphosis in vertebrates and insects. Lastly, the chapter also looks at endocrine disrupting chemicals. Many natural and man-made products contain chemicals which are similar to hormones and can interfere with normal hormonal processes. It is increasingly recognized that these chemicals have damaging effects.
Hormones and disease: What happens when things go wrong?
This chapter discusses hormones and disease. All of the diseases associated with hormones result from either too much hormone activity or too little. Too much hormone activity is usually the result of a disorder in the gland that produces the hormone, often due to uncontrolled growth of the gland. Meanwhile, too little hormone activity can be a fault in hormone production or it can be a problem with hormone action, where the hormone receptor does not work properly. Endocrine disease can have many causes, including autoimmune disease, defects in the receptor, enzyme defects, and hormone resistance syndromes. Some forms of cancer are also dependent on hormones and can be treated by blocking either production or action of the hormone.
Hormones and reproduction
This chapter focuses on hormones and reproduction. Both male and female reproductive tissues produce steroid hormones as well as gametes. Puberty is a process that is initiated by the development of kisspeptin neurons, which signal the onset of menarche in girls at a body weight of around 47 kg; the exact mechanism is uncertain in boys. The chapter then looks at ovulation in different animals, which may be on a continuous cycle, on a seasonal cycle, or induced by contact with a male. A woman has only a limited number of ova that mature during her reproductive life, whereas men continuously produce new sperm, from puberty until they die. The chapter also considers contraception. Hormones are essential for pregnancy to occur, continue normally, and result in the delivery of a healthy baby.
This chapter provides an overview of hormones, which constitute a range of different compounds, including peptides and steroids. Oversimplification of information about hormones can lead to some misleading ideas. For example, not all hormones are slow acting as is often assumed. The endocrine and nervous systems are closely connected, and hormones are released by a range of different tissues in the body, not just the 'classical' endocrine organs. Steroid hormones are all made from cholesterol by a process called steroidogenesis, while peptide hormones are made just like any other protein. The effects of hormones are all as a result of the hormone binding to its receptor. These receptors are found in target tissues, and their cellular location depends on the chemical type of hormone. The chapter then looks at negative feedback control, which is the most effective means of regulating blood levels of hormones and their effects.
The use and abuse of hormones
This chapter explores the use and abuse of hormones. When the body is not making enough of a hormone to keep healthy, it is usually necessary to give the hormone as a prescribed medicine. Steroid hormones and thyroxine are usually given as tablets, taken orally, while peptide hormones usually need to be given by injection. The term hormone replacement therapy (HRT) has come to mean the replacement of oestrogens in post-menopausal women, given as a daily tablet containing both oestrogen and progesterone. In spite of significant adverse effects, synthetic corticosteroids are widely used to treat inflammatory illnesses and asthma. The chapter then considers how anabolic androgenic steroids, corticosteroids, growth hormone, and erythropoietin have all been systematically abused throughout sports for decades. Although each has been identified with performance benefits, they also cause harm. Meanwhile, the use of hormones in agriculture remains controversial.