Learning Objectives
Explain the chemical composition of hormones and the mechanisms of hormone action.
By the end of this section, you will be able to:
- Identify the three major structural classes of hormones
- Compare and contrast intracellular receptor systems and 2nd messenger systems
- Identify factors that influence a target cell’s response
- Understand the various mechanisms for stimulating hormone release.
When released into the blood, a hormone circulates freely throughout the body. However, a hormone will only affect the activity of its target cells; that is, cells with receptors for that particular hormone. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell’s response. The major hormones of the human body and their effects are identified in Table 17.2.
| Endocrine Glands and Their Major Hormones (Table 17.2) | |||
|---|---|---|---|
| Endocrine gland | Associated hormones | Chemical class | Effect |
| Pituitary (anterior) | Growth hormone (GH) | Peptide | Promotes growth of body tissues |
| Pituitary (anterior) | Prolactin (PRL) | Peptide | Promotes milk production |
| Pituitary (anterior) | Thyroid-stimulating hormone (TSH) | Peptide | Stimulates thyroid hormone release |
| Pituitary (anterior) | Adrenocorticotropic hormone (ACTH) | Peptide | Stimulates hormone release by adrenal cortex |
| Pituitary (anterior) | Follicle-stimulating hormone (FSH) | Peptide | Stimulates gamete production |
| Pituitary (anterior) | Luteinizing hormone (LH) | Peptide | Stimulates androgen production by gonads |
| Pituitary (posterior) | Antidiuretic hormone (ADH) | Peptide | Stimulates water reabsorption by kidneys |
| Pituitary (posterior) | Oxytocin | Peptide | Stimulates uterine contractions during childbirth |
| Thyroid | Thyroxine (T4), triiodothyronine (T3) | Amine | Stimulate basal metabolic rate |
| Thyroid | Calcitonin | Peptide | Reduces blood Ca2+ levels |
| Parathyroid | Parathyroid hormone (PTH) | Peptide | Increases blood Ca2+ levels |
| Adrenal (cortex) | Aldosterone | Steroid | Increases blood Na+ levels |
| Adrenal (cortex) | Cortisol, corticosterone, cortisone | Steroid | Increase blood glucose levels |
| Adrenal (medulla) | Epinephrine, norepinephrine | Amine | Stimulate fight-or-flight response |
| Pineal | Melatonin | Amine | Regulates sleep cycles |
| Pancreas | Insulin | Peptide | Reduces blood glucose levels |
| Pancreas | Glucagon | Peptide | Increases blood glucose levels |
| Testes | Testosterone | Steroid | Stimulates development of male secondary sex characteristics and sperm production |
| Ovaries | Estrogens and progesterone | Steroid | Stimulate development of female secondary sex characteristics and prepare the body for childbirth |
Types of Hormones
The hormones of the human body can be structurally divided into three major groups: amino acid derivatives (amines), peptides, and steroids (Figure 17.2.1). These chemical groups affect a hormone’s distribution, the type of re