Thyroxine is a hormone made from iodine and amino
acids, it is produced in, and released (secreted) by, the thyroid gland
in the neck.
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
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
If your body detects that the level of
thyroxine has risen above 'normal', the hypothalamus stops
This tells the pituitary gland to stop
producing TSH (blocks secretion) inhibiting the production of thyroxine in the
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
If your body detects the level of thyroxine has
fallen below 'normal', the hypothalamus is stimulated to release
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.
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
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.
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.