Hormones, the Hypothalamus, the Pituitary and the Thyroid: Quick Revision

A brief note on Hormones, the Hypothalamus, the Pituitary and the Thyroid. Read it before a test. Try each question at the end before you open its answer.

Hormones and the hypothalamus

• Hormone: a non-nutrient chemical messenger made in trace amounts. Endocrine glands are ductless.
• Hypothalamus: neurosecretory cells (nuclei). Releasing hormones (for example GnRH) and inhibiting hormones (for example somatostatin) reach the anterior pituitary by the portal system.
• Oxytocin and vasopressin: made in the hypothalamus, carried along nerve fibres, stored and released by the posterior pituitary.

The pituitary gland

• Pars distalis: growth hormone, prolactin, TSH, ACTH, LH, FSH.
• Pars intermedia: MSH for the melanocytes.
• Pars nervosa: oxytocin (smooth muscles, milk ejection) and vasopressin or ADH (distal tubules, water).
• Disorders: too much GH gives gigantism in a child and acromegaly in an adult. Too little GH in a child gives pituitary dwarfism. A lack of ADH gives diabetes insipidus.

The pineal and the thyroid

• Melatonin (pineal): 24 hour rhythm, sleep and body temperature.
• Thyroid: two lobes and an isthmus, follicles. T4 (four iodine atoms), T3 (three) and thyrocalcitonin.
• Thyroid hormones: regulate the basal metabolic rate. TSH controls the thyroid, and the hormones act back by negative feedback.
• Disorders: a lack of iodine gives goitre with a high TSH. Hypothyroidism in the mother gives cretinism in the baby. Graves' disease is exophthalmic goitre.

A worked example

A gland releases 800 units of a hormone. The half-life is 4 min. How much is left after 8 min?

1. Eight minutes hold $8 \div 4 = 2$ half-lives.
2. The amount is $800 \div 2^2 = 200$ units.

Practice with new numbers

A program to try

The program halves the amount of a hormone at each half-life.

Mistakes to avoid

• Saying that acromegaly is a disease of children. The excess in a child is gigantism.
• Mixing up cretinism and pituitary dwarfism. The first is a lack of thyroid hormone, the second a lack of growth hormone.
• Putting oxytocin and vasopressin in the anterior pituitary. They are made in the hypothalamus and released from the posterior pituitary.
• Saying that goitre comes from too much hormone. Simple goitre comes from a lack of iodine.

Check yourself

1. Which hormone acts on the melanocytes?

2. What happens to TSH when the iodine in the food is low?

3. Which hormone is called the antidiuretic hormone?

4. Which protein hormone of the thyroid helps to regulate the calcium of the blood?

Sources

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17.
• World Health Organization, UNICEF and ICCIDD (2007). Assessment of iodine deficiency disorders and monitoring their elimination, 3rd edition.
• NCERT (2023). Biology, Textbook for Class XI, chapter 19.