Hormones

What are hormones?

In plain words: a hormone is a chemical message. One group of cells releases it into the blood, and cells elsewhere read it and change what they do.

Hormones as intercellular messengers

• Hormones carry messages between cells: they are produced by endocrine glands, poured straight into the blood and carried to their site of action (NCERT page 300).
• Their messages keep the biological activities of the body in balance (NCERT page 300).
• NCERT’s Class 11 Biology book calls this the classical definition of a hormone: a chemical produced by endocrine glands, released into the blood and carried to a distant target organ. Its current definition is wider: hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts, and some organs that are not endocrine glands, such as the heart, kidney and gastrointestinal tract, make them too (Chapter 19, pages 239–240). The Class 11 Biology page on endocrine glands and hormones sets the two definitions side by side.

Steroid, polypeptide and amino acid derivative hormones

By chemical nature, NCERT names three kinds (page 300):

• Thyroxine, made in the thyroid gland, is an iodinated derivative of the amino acid tyrosine (NCERT page 300). NCERT’s list of the three kinds does not name it; by its origin it is an amino acid derivative. NCERT’s Class 11 Biology book instead sorts hormones into four groups and gives the thyroid hormones a group of their own, the iodothyronines, beside the peptide, steroid and amino-acid-derivative hormones (Chapter 19, page 248). So in a Chemistry answer describe thyroxine as NCERT does here, an iodinated derivative of tyrosine; in a Biology answer, call it an iodothyronine.
• Steroid hormones are made by the adrenal cortex and the gonads, the testes in males and the ovaries in females (NCERT page 301).
• The four Biology groups, and how each kind acts on its target cells, are on the Class 11 Biology page on the mechanism of hormone action.

Functions of hormones

• Blood glucose: insulin is released when the blood glucose level rises quickly, and it keeps the level within a narrow limit. Glucagon tends to raise the level. Together the two hormones regulate blood glucose (NCERT page 300).
• External stimuli: epinephrine and norepinephrine mediate the body’s responses to them (NCERT page 300). They are also called adrenaline and noradrenaline, and the adrenal medulla makes them (NCERT Class 11 Biology, Chapter 19, page 244).
• Growth and development: growth hormones and sex hormones (NCERT page 300).
• Thyroxine: too little causes hypothyroidism, with lethargy and obesity; too much causes hyperthyroidism. Too little iodine in the diet can cause hypothyroidism and enlargement of the thyroid gland. This is largely controlled by adding sodium iodide to table salt, giving “iodised” salt (NCERT pages 300–301).
• Adrenal cortex (NCERT page 301):
  • glucocorticoids control carbohydrate metabolism, modulate inflammatory reactions and are involved in reactions to stress;
  • mineralocorticoids control how much water and salt the kidney excretes;
  • if the adrenal cortex fails, one result may be Addison’s disease, with hypoglycaemia, weakness and greater susceptibility to stress. It is fatal unless treated with glucocorticoids and mineralocorticoids.
• Sex hormones (NCERT page 301):
  • testosterone, the main male sex hormone, develops secondary male characteristics such as a deep voice, facial hair and general physique;
  • estradiol, the main female sex hormone, develops secondary female characteristics and helps control the menstrual cycle;
  • progesterone prepares the uterus for implantation of the fertilised egg.

Solved examples

Common mistakes to avoid

Quick revision summary

• Hormones: intercellular messengers made by endocrine glands and carried by the blood.
• Kinds: steroids (estrogens, androgens), polypeptides (insulin, endorphins) and amino acid derivatives (epinephrine, norepinephrine). Thyroxine is an iodinated derivative of tyrosine; the Class 11 Biology book puts thyroid hormones in a fourth group, the iodothyronines.
• Insulin lowers blood glucose and glucagon raises it. Low thyroxine causes hypothyroidism; too little iodine in the diet causes hypothyroidism and an enlarged thyroid (goitre), largely controlled by iodised salt.
• Adrenal cortex: glucocorticoids and mineralocorticoids; their failure causes Addison’s disease. Gonads: testosterone, estradiol, progesterone.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is iodine added to table salt?

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   A lack of iodine in the diet can cause hypothyroidism and enlargement of the thyroid gland, because the thyroid needs iodine to make thyroxine, an iodinated derivative of tyrosine. Adding sodium iodide to table salt (“iodised” salt) largely controls this (NCERT pages 300–301).

2. Q2. Which hormones are made by the adrenal cortex, and what happens if it does not work properly?

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   Glucocorticoids and mineralocorticoids, which are steroid hormones. Failure can cause Addison’s disease, with hypoglycaemia, weakness and greater susceptibility to stress; it is fatal unless treated with these two kinds of hormone (NCERT page 301).