Iodine and the Thyroid Gland
J V Evans, Emeritus Professor
of Physiology. University of New England. Armidale NSW 2864
(Mohair newsletter Technical issues Vol
7 # 3 Sept 1998)
The thyroid gland
Iodine deficiency has
been described as the most commonly encountered mineral deficiency in
grazing stock. Iodine is related to the proper functioning of the thyroid
gland which in turn is related to the production of hormones (eg thyroxine)
which in their turn play a multitude of roles in the well-being of the
Angora and it's progeny, both before and after birth. Thyroid hormones
are very important.
The thyroid gland is
located in the neck of the animal just below the “Adam's apple”.
This gland is rather unusual in that it produces hormones which contain
iodine. The thyroid gland can be thought of as an organ which has been
designed to concentrate and store the iodine which an animal takes in
in its diet. The gland makes a precursor of the thyroid hormones which
it stores for future use.
The amount of iodine
which can be stored in the thyroid of an Angora is quite considerable.
This means that large amounts of iodine are not required every day.
It is important to realise
that the foetus also has a thyroid gland and that it is functioning
from at least mid-term. It is equally important to realise that thyroid
hormones are required for the normal development of the foetus and for
the normal growth of the various skin components in the unborn kid.
They are also involved in the process which brings about the differentiation
of secondary fibre follicles. Therefore they are involved in the nature
of the fleece in the adult. Remember also that the thyroid hormones
in the mother do not pass across the placenta to the foetus. The foetus
has to make its own hormones using the iodine which the mother gives
it, and its own functioning thyroid gland.
Nearly all the iodine
in the body of the Angora goat is in the thyroid gland. However, very
small quantities can be detected in most cells of the body. This is
because the thyroid gland hormones go to, and have an effect on, most
cells in the body.
The thyroid gland has
a very efficient iodine trapping mechanism. (If a man is given a slug
of iodine, 20-50% of that iodine will have been securely trapped in
the thyroid gland within 24 hours.) If this trapping mechanism does
not work properly then the goat is in trouble. There are chemicals in
various feed-stuffs which can inhibit the trap (see later).
Once the iodine is trapped
in the thyroid gland it joins onto an amino acid called tyrosine. Either
one iodine atom is added to form monoiodotyrosine (MIT) or two atoms
are added to form diiodotyrosine (DIT). Once this is done the gland
can take two DIT's and join them together to make thyroxine (a hormone
most of us have heard of - biologists usually refer to it as T4).
One DIT and one MIT may join together to make another hormone (a very
important hormone but one not so well known) called triiodothyroxine
Small quantities of these
two hormones circulate in the blood attached to special proteins which
act as carriers. In time they find their way into most cells of the
body. It is in these body cells that they do their work and it is this
work that interests us as Angora husbandmen. We want that work to be
done properly and efficiently.
What do thyroid hormones do?
In general we can say
that normal development, growth and reproduction cannot occur without
them. Thyroid hormones are involved in a very great number of the important
chemical processes in the body. They are the most functionally diverse
of all the anabolic hormones (hormones that are involved in the “building
up” process in the body). Also, the full effect of many other hormones
cannot be realised without the presence of thyroid hormones. Importantly,
a lack of them is most damaging to young goats - particularly during
development in utero and soon after birth.
No wonder we want these
hormones to work well and be in sufficient supply. Perhaps an outline
of their effects will enable their importance to be appreciated.
Effects on growth and development.
(Remember, the foetus
has to depend to a very large extent on the thyroid hormones it can
make in its own thyroid gland.)
- Thyroid hormone deficiency
in the foetus affects brain development related to teat finding etc
- Deficiency in the foetus results
in bones not being properly made.
- Deficiency at certain times
in the growth of the foetus affects the growth of teeth.
- Deficiency in the foetus affects
the development of the skin, including the follicles which grow mohair.
Also affected is the growth of sweat and sebaceous glands in the skin.
- Thyroxine in the foetus is
known to hasten the development of primary follicles and the fibre production
in them, and also to initiate secondary follicle development and the
growth of fibre in them.
- Thyroid deficiency at birth
is said to prevent the branching of immature secondary follicles, a
process which usually occurs before weaning.
- Thyroid deficiency may also
be involved in lung disorders of new born kids (surfactant synthesis).
It is evident from these
examples that thyroid hormones are very much involved in normal growth
and development of the foetus. Growth can be an increase in cell size
or an increase in the number of cells. Both of these things are affected
by thyroid hormones. Thyroid gland deficiencies result in a higher proportion
of weak or stillborn kids being born than would otherwise be the case.
Effects on body chemistry.
Thyroid gland hormones
tend to stimulate the making of proteins. This is important to breeders
and the ability to do this may have been determined early in the foetal
life of the kid. Thyroid gland hormones are also involved in the metabolism
of fats and carbohydrates.
A properly working thyroid
gland is needed if some of the vitamins (the water soluble ones like
the B vitamins) are to be used effectively.
Thyroid gland hormones 2
increase the use of oxygen by cells of the body.
Thyroid gland activity
increases in times of cold. Thyroid gland hormones are thought to be
involved in increasing the internal heat production of the Angora. However,
it is thought that this effect is not a direct one in that other hormones
(such as adrenalin) are necessary if thyroid hormones are to exert,
to the full, their functional effects.
The initial stimulation
of the body warming mechanism which a cold snap brings about may not
be a direct thyroid effect either, but rather the result of thyroid
hormones making other hormones better able to do their job.
Whatever the real truth,
an animal that does not have a thyroid gland which works well will be
at a disadvantage in cold weather.
As readers can see from
this very superficial coverage of what the thyroid hormones do, it is
important that a good husbandman has some understanding of their functions.
Deficient thyroid activity.
Basically this is caused by:
- A deficiency in iodine in
the diet. This is by far the most common cause.
- Substances in foodstuffs (goitrogens)
which cause thyroid gland malfunction.
- An increased need for thyroid
hormones above that which the animal can supply.
- Genetic factors involving
Dealing with each of
these in turn.
Deficiency in the diet.
There are many areas
in the world where iodine is in short supply in pasture. This situation
is not uncommon in Australia and New Zealand. Iodine deficient areas
have been diagnosed in Northern New South Wales, the central NSW Coast,
and the Hunter Valley. Also areas are known in Tasmania, Victoria and
elsewhere. As goats are more susceptible to iodine deficiency than other
ruminants, there seems every likelihood that as the Angora industry
expands, more areas will be shown to be marginal or deficient.
Obviously, if iodine
is needed to make the thyroid hormones (which it is) then these hormones
- which we now can appreciate are rather important - cannot be made
if iodine is in short supply.
In iodine deficient areas
of Australia, therefore, iodine supplementation (particularly to Angoras)
is essential. Supplementation should be encouraged in many Angora raising
It has been estimated
that animals will tolerate 50-100 times the actual requirement without
any ill effects. If hand feeding is practised and the feed-stuffs contain
goitrogens then iodine requirement will be above that for animals on
Foodstuffs containing goitrogens.
Goitrogens are found
in many different plants such as rape, mustard, turnip, cabbage, soybeans,
linseed, peas, peanuts, lentils etc. Goitrogens don't all act in the
same way. Some slow down the actual production of thyroxine in the thyroid
gland itself. Others have their effect by slowing down the uptake of
iodine by the gland. That is, they make the trapping system less efficient.
The best thing is to
avoid these foodstuffs, but if they are used, an iodine supplement is
a wise precaution, although the effects of some goitrogens cannot readily
be overcome by this approach.
Increased need for thyroid
If there is an increased
need for thyroid hormones (a prolonged period of cold, wet weather)
and the animal is only just getting enough under normal circumstances,
then iodine supplementation and the avoiding of goitrogens in supplementary
feed becomes rather important.
Some animals have a better
thyroid hormone making mechanism than others. Very occasionally a line
of animals may have a genetic defect which leads to low thyroid gland
- Supplementation with iodine.
- Feed out as a supplement in
troughs, a mixture of 28 g. of Potassium Iodate in 140kg of stock salt.
- Feed out proprietary salt
blocks containing iodine.
- Drench all Angoras three times
a year with a solution of iodine in water. Make up a solution of 20g
of potassium iodide in one litre of water and dose all animals with
this solution at the rate of 10ml per 20kg of body weight. Drench four
weeks before mating, six weeks before kidding and 2 weeks before kidding.
This option is considered by many mohair producers to be the best strategy
as it gives the iodine at times when it is required and ensures that
all animals get it.
- Don't feed brassicas if
you have other options.
- If you do feed foodstuffs
with known goitrogen activity consider the need for extra iodine.
If a mohair producer
has unexplained deaths in his young kids or he produces weak kids which
require a lot of attention, or the effects of cold weather are more
severe than he thinks they should, then he should think of iodine deficiency
as a possible factor and arrange for some sort of supplementation to
be initiated as soon as possible.
Don't overdo it. Although
the safety margin is large it is possible to overdose. The main signs
of overdose are:
- A fine bran-like scurf (dandruff)
on the skin (pityriasis).
- Loss of appetite (anorexia).
© 2000 Mohair Aust Ltd