Hormones

The essentials

Hormones are molecules that act as chemical messengers. They are produced by endocrine (ductless) glands, released into the bloodstream and carried to target sites where they regulate body activities. They help maintain the balance of biological activities in the body (homeostasis). Chemically, hormones fall into three main groups.

Key points

• Steroids: e.g. oestrogens and androgens (sex hormones), and hormones of the adrenal cortex.
• Polypeptides: e.g. insulin and endorphins.
• Amino acid derivatives: e.g. epinephrine (adrenaline) and norepinephrine, and thyroxine.
• Insulin and glucagon together keep blood glucose within a narrow range: insulin lowers it, glucagon raises it.
• Epinephrine and norepinephrine mediate the body's responses to external stimuli.
• Thyroxine, made in the thyroid gland, is an iodinated derivative of the amino acid tyrosine. Low levels cause hypothyroidism (lethargy and obesity); high levels cause hyperthyroidism. Low iodine in the diet can lead to hypothyroidism and enlargement of the thyroid gland (goitre), which is controlled by adding sodium iodide to table salt (iodised salt).
• Adrenal cortex steroids: glucocorticoids control carbohydrate metabolism and modulate inflammatory reactions; mineralocorticoids control the level of excretion of water and salt by the kidney. Poor functioning of the adrenal cortex causes Addison's disease.
• Sex hormones: testosterone is the major male sex hormone, responsible for male secondary characteristics. Oestradiol is the main female sex hormone, responsible for female secondary characteristics and the menstrual cycle; progesterone prepares the uterus for implantation of the fertilised egg.

Exam tips

• Learn one example for each chemical class and the gland that makes it.

Common mistakes

• Calling insulin a steroid; it is a polypeptide hormone.