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C4

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 or T3.

    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.

    1. 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.)

      1. Thyroid hormone deficiency in the foetus affects brain development related to teat finding etc at birth.
      2. Deficiency in the foetus results in bones not being properly made.
      3. Deficiency at certain times in the growth of the foetus affects the growth of teeth.
      4. 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.
      5. 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.
      6. Thyroid deficiency at birth is said to prevent the branching of immature secondary follicles, a process which usually occurs before weaning.
      7. 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.

    2. 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.

    3. Functional effects.

      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:

    1. A deficiency in iodine in the diet. This is by far the most common cause.
    2. Substances in foodstuffs (goitrogens) which cause thyroid gland malfunction.
    3. An increased need for thyroid hormones above that which the animal can supply.
    4. Genetic factors involving enzyme deficiencies.

    Dealing with each of these in turn.

    1. 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 areas.

      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 pasture.

    2. 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.

    3. Increased need for thyroid hormones.

      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.

    4. Genetic factors.

      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 hormone production.

    Prevention.

    1. Supplementation with iodine.
      1. Feed out as a supplement in troughs, a mixture of 28 g. of Potassium Iodate in 140kg of stock salt.
      2. Feed out proprietary salt blocks containing iodine.
      3. 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.
    2. Don't feed brassicas if you have other options.
    3. If you do feed foodstuffs with known goitrogen activity consider the need for extra iodine.

    Summary.

    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.

    Caution.

    Don't overdo it. Although the safety margin is large it is possible to overdose. The main signs of overdose are:

    1. A fine bran-like scurf (dandruff) on the skin (pityriasis).
    2. Loss of appetite (anorexia).

    © 2000 Mohair Aust Ltd