(5) The function of the hormone thyroxine

Thyroxine is a hormone made from iodine and amino acids, it is produced in, and released (secreted) by, the thyroid gland in the neck, which is part of the endocrine system.

Thyroxine has an important role in regulating the basal metabolic rate, the basic rate (speed) at which the chemical reactions of your body occur while your body is at rest.

Thyroxine increases the rate of metabolism of all the body's cells.

 Thyroxine increases the rate of metabolism e.g. increases the rate of respiration, powering the cell's chemistry and releasing more chemical potential energy for cell chemistry or thermal energy to maintain the right temperature.

Thyroxine is also important for many other biochemical processes including facilitating protein synthesis - essential for growth and development.

Problems with an underactive thyroid gland - symptoms of thyroxine deficiency

Tiredness, sluggishness, increase in weight, slower heart rate,

If a child has too little thyroxine it leads to slower growth and mental development.

This potentially harmful situation begins in the uterus, continues in the embryo, through infancy and into childhood, if there is insufficient thyroxine

 

A negative feedback system maintains the thyroxine concentration in the blood at the correct level.

So, how does the negative feedback system regulate the level of thyroxine in the blood?

Both the pituitary gland and hypothalamus (a small region at the base of the brain) control the thyroid and it is the hypothalamus, using TRH (thyrotropin releasing hormone), that alerts the pituitary gland to produce TSH (thyroid stimulating hormone).

Please note, from now on I'll just use the abbreviations TRH and TSH.

Thyroxine is produced in the thyroid gland, in response to the actions of two principal hormones:

The hormone TRH (from the hypothalamus), stimulates the production of TSH which is made and secreted from the pituitary gland into the bloodstream.

In turn, the production of TSH stimulates the thyroid gland to make more thyroxine.

TSH binds to receptors located on the cells of the thyroid gland to stimulate production of thyroxine.

We now put these two hormonal actions, 'forward and reverse' into the negative feedback system.

I've also added a graph to go with the text below.

 

If your body detects that the level of thyroxine has risen above 'normal', the hypothalamus stops releasing TRH.

This tells the pituitary gland to stop producing TSH (blocks secretion) inhibiting the production of thyroxine in the thyroid.

In reducing the amount of thyroxine secreted from the pituitary gland, the thyroxine level falls down to normal (1st half of graph below) and your metabolic rate is reduced to 'normal' i.e. becomes stabilised again.

Apparently, a higher than normal thyroxine level also reduces the secretion of TSH from the pituitary gland (i.e. without the intervention of the level of TRH from the hypothalamus).

 

If your body detects the level of thyroxine has fallen below 'normal', the hypothalamus is stimulated to release TRH.

The release of TRH stimulates the pituitary gland to release TSH.

The TSH stimulates the thyroid gland to produce more thyroxine, whose level rises back to normal (2nd half of graph) and your metabolic rate increases to 'normal' i.e. becomes stabilised again.

Note:

If the body temperature falls, the body produces more thyroxine to increase the rate of respiration and release more thermal energy.

But, since the increase in respiration releases more thermal energy and your body temperature rises, if it becomes too high, the thermoregulatory centre in the brain detects this and the adrenaline secretion is blocked.

 

The negative feedback system is illustrated in the graph below.

General comment on the graph and negative feedback systems

Using a negative feedback system, your body controls the levels of hormones, and other substances in the blood.

When your body detects that the level of a substance X is too high above the 'normal' level, or too low below the 'normal' level, it triggers a response to bring the level of substance X back up/down to its normal level.

 

Thyroid gland problems

e.g. if you have an underactive thyroid gland, it can cause your body to gain unnecessary weight.

This is because too little thyroxine is produced and your metabolic rate falls.

As a result, less of the glucose from your food intake is used up in respiration, so the excess glucose is converted to, and stored as, fat.

Fortunately, the remedy, in most cases, is to take thyroxine tablets every day.

 

Reminder: Homeostasis the function of self-regulating processes by which biological systems (organisms) tend to maintain physical and chemical stability while adjusting to conditions that are optimal for survival i.e. counteracting unfavourable conditions. These process can involve hormones secreted from the glands of the endocrine system.

---

Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key points about the action of thyroxine from the thyroid gland - part of the endocrine system

Thyroxine and Metabolic Regulation

1. What is Thyroxine?

• Thyroxine is a hormone produced by the thyroid gland, which is located in the neck, just in front of the trachea.

• It contains iodine, an essential element obtained from the diet (e.g. iodized salt or seafood).

• The active form of thyroxine in cells is triiodothyronine (T₃), but it is mostly secreted as T₄ (thyroxine), which is then converted into T₃ in target tissues.

---

2. Function of Thyroxine in the Body

• Primary Role: It regulates the body's basal metabolic rate (BMR)—the rate at which cells carry out chemical reactions while the body is at rest.

• It does this by increasing oxygen consumption and stimulating cellular respiration in nearly all cells.

• Thyroxine has wide-reaching effects on:

  • Growth and development (especially the brain during childhood)

  • Heart rate and blood flow

  • Body temperature regulation

  • Metabolism of proteins, carbohydrates, and fats

---

3. Negative Feedback Control

• The pituitary gland releases thyroid-stimulating hormone (TSH), which signals the thyroid gland to produce thyroxine.

• High thyroxine levels in the blood inhibit further TSH secretion, reducing thyroxine production—this is a classic example of negative feedback.

• This keeps metabolic rate within a stable range, crucial for homeostasis.

---

4. Effects of Imbalance

• Hypothyroidism (underactive thyroid):

  • Low thyroxine levels lead to slower metabolism.

  • Symptoms: fatigue, weight gain, cold intolerance, depression, slow heart rate.

• Hyperthyroidism (overactive thyroid):

  • Excess thyroxine causes increased metabolism.

  • Symptoms: weight loss, anxiety, rapid heart rate, heat intolerance.

---

5. Importance in Understanding Human Biology

• Thyroxine exemplifies how hormones act on target organs and regulate cellular processes.

• It connects endocrine control to energy balance, growth, and development—core concepts in human physiology.

• Its role in homeostasis shows how the body maintains internal conditions, despite external changes.

• A strong understanding of thyroxine allows students to grasp:

  • Hormonal feedback loops

  • Hormone action at the cellular level

  • The wider impact of endocrine dysfunction on human health

---

Summary of learning objectives and key words or phrases

Be able to describe and understand the function of the hormone thyroxine produced in the thyroid gland.

Explain how it regulates metabolic rate, can increases metabolism in body's cells, and its connection with the endocrine system.

---

WHAT NEXT?

TOP OF PAGE

INDEX of notes on the endocrine - hormone systems

Big website and use [SEARCH BOX] below, maybe quicker than the indexes

INDEX of all my BIOLOGY NOTES

HOME PAGE of Doc Brown's Science website

UK KS3 Science Quizzes for KS3 science students aged ~11-14, ~US grades 6, 7 and 8

Biology * Chemistry * Physics UK GCSE level students aged ~14-16, ~US grades 9-10

Advanced Level Chemistry for pre-university age ~16-18 ~US grades 11-12, K12 Honors

Find your GCSE/IGCSE science course for more help links to all science revision notes

email doc brown - comments - query?

---

Explaining importance of function of the hormone thyroxine in GCSE level biology, What you need to know about function of the hormone thyroxine for GCSE level biology, Explaining use of function of the hormone thyroxine knowledge in GCSE level biology, Examples of function of the hormone thyroxine explained when studying GCSE level biology, What is the significance of function of the hormone thyroxine in GCSE level biology, describing explaining theory of function of the hormone thyroxine when studying GCSE level biology, exam revision notes for function of the hormone thyroxine, online help for understanding function of the hormone thyroxine in GCSE biology, what do I need to learn about function of the hormone thyroxine? what do I need to know about function of the hormone thyroxine for GCSE biology exams, how to prepare for questions on function of the hormone thyroxine in GCSE biology examination? function of the hormone thyroxine for syllabus-specifications for students taking the IGCSE/GCSE level biology examinations, summary revision notes key points on function of the hormone thyroxine for students studying AQA igcse/gcse biology notes on function of the hormone thyroxine, Edexcel gcse biology notes on function of the hormone thyroxine,  OCR 21st century GCSE biology notes on function of the hormone thyroxine, OCR gateway GCSE biology notes on function of the hormone thyroxine, WJEC gcse biology notes on function of the hormone thyroxine, CCEA gcse biology notes on function of the hormone thyroxine, CIE Cambridge igcse biology, notes on function of the hormone thyroxine useful for US grade 9-10 biology

---

SITEMAP Website content © Dr Phil Brown 2000+. All copyrights reserved on Doc Brown's biology revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial.