Iodine and it’s Role in Animal Body

Iodine and it’s Role in Animal Body

Neelam Kumari¹ and Gourav Biswas²

¹Assistant Professor at Mahatma Gandhi Veterinary College Bharatpur Rajasthan

²Ph. D scholar at Indian Veterinary Research Institute Izatnagar Bareilly

Introduction

            Iodine is considered unique among all the mineral elements as its deficiency causes clinical abnormality- enlargement of the thyroid gland in the neck or goitre. The body contains a very minute quantity of iodine, half to two-thirds of this is found in the thyroid gland. The gland consists of two parts lying on each side of the trachea at its upper end (iodine content of thyroid gland on dry matter basis is 0.1%). The iodine in the thyroid gland exists as inorganic iodide, mono and diiodotyrosine, triiodothyronine, polypeptide containing thyroxine and thyroglobulin. Thyroglobulin (glycoprotein) is the main storage form of the iodine in the thyroid gland. Thyroxine (3,5,3’,5’-tetra-idothyronine) is secreted from the thyroid gland. It contains about 65 % of iodine.

Function of iodine

• The primary function of thyroid gland is to control the basal metabolic rate through the output of its hormone
• Structural studies of the thyroxine molecule have revealed that its four atoms of iodine are attached in two similar positions on linked ‘outer’ and ‘inner’ tyrosine rings.
• Thyroxine therefore became known as 3,3′,5,5′-tetraiodothyronine (T4), a relatively inactive prohormone requiring removal of one atom by two OR-deiodinase enzymes (5’ORD types I and II) to generate the active hormone, 3,3′,5-triiodothyronine (T3).
• Both T4 and T3 can be inactivated by an IR deiodinase (type III), and physiological activity of thyroid hormones is regulated peripherally by the three deiodinases rather than by the thyroid gland.
• Thyroid hormones have a thermoregulatory role, increasing cellular respiration and energy production and have widespread effects on intermediary metabolism, growth, muscle function, immune defence and circulation.
• The seasonality of reproduction in ewes is related to seasonal changes in thyroid activity and the required cooperation from the male is probably facilitated by a thyroid response to change in day length.
• As a determinant of metabolic rate, T3 interacts with hormones such as insulin, growth hormone and corticosterone and regulatory proteins of exocrine origin.

Metabolism of Iodine

• Iodine is absorbed very efficiently from the gastrointestinal tract and this enables any iodine secreted before absorptive sites to be extensively recycled.
• Absorbed iodine is transported in the bloodstream loosely bound to plasma proteins.
• A small proportion of extra-thyroidal iodine circulates in free ionic form, like chloride, and accumulates in soft tissues such as muscle and liver when excess iodine is consumed.
• Recycling of thyroidal iodine occurs via the iodide pool. Iodine is recycled via secretion into the abomasum.
• Excess dietary iodine is excreted predominantly via urine as iodide, but in lactating animals, significant amounts can be secreted in milk.

T4 Synthesis and Storage

• Approximately 80% of iodine in the mammalian body is found in the thyroid gland because 90% of the iodine entering that organ goes no further.
• Captured iodine is used to iodinate tyrosine to form mono – (T1) and diiodotyrosine (T2) and two molecules of T2 are then used to form T4.
• The efficiency with which iodine is captured varies according to need and is regulated by the secretion of two hormones with thyrotropin-releasing and T4-stimulating properties (TRH and TSH).
• Together, these hormones determine the levels of T4 under feedback control from the circulating levels of free (f) T4 and T3, the tiny fractions (<1%) of predominantly globulin- or albumin-bound, plasma T4 and T3 pools.
• Environmental temperature affects iodine uptake by the thyroid gland.

Activation of T4

• T4 is taken up by intracellular receptors on inner mitochondrial membranes and in the cell nucleus, where it is activated by two selenium-dependent deiodinases: ID1 and ID2.

Sources of iodine

• Iodine occurs in traces in most foods and is present mainly as inorganic iodide.
• The richest sources of this element are foods of marine origin (6 g/kg DM have been reported for some seaweeds)
• Fishmeal is also a rich source of iodine.
• The iodine content of land plants is related to the amount of iodine present in the soil, and consequently, wide variations occur in similar crops grown in different areas.
• Iodine content of milk and eggs can be enormously increased by feeding cows or hens with large quantities of seaweed.

Deficiency symptoms

• Severe goitre or enlargement of the thyroid gland.
• Calve and piglets from iodine-deficient cows and sows are often hairless with thick pulpy skin.
• The moulting process and pigmentation of feathers can be affected by iodine deficiency in birds.
• Infertility problems such as male sterility and a decline in libido may occur in the adult sheep suffering from iodine deficiency and hairless pups.

Goitre

• It is the enlargement of thyroid gland and simple goitre is the most common type.
• Simple goitre is caused primarily by a lack of iodine.
• Hyperplastic changes begin as a result of a failure of the thyroid tissue to supply enough secretion, owing either to a reduced supply of iodine for its manufacture or to an increased demand for the secretion by body.
• Simple goitre is most likely to develop in humans during pregnancy and puberty (Iodine requirement is more).

Other factors that contributes to occurrence of simple goitre

• High calcium content of the water in many goitrous regions.
• Specific goitrogenic substances in certain foods, notably various members of the Brassica family (cabbage), peanuts and soyabeans.
• Goitrogens interfere with thyroid hormone synthesis by limiting the capacity of the gland either to trap iodine or to incorporate this iodine into thyro-active substances. These are of two types:
• Organic goitrogens e.g., cruciferous plants, most brassicas and white clover, rapeseed meal.
• Inorganic goitrogenic e.g., high arsenic and high fluoride intake, excess ingestion of bivalent cobalt, rubidium etc.

Toxicity

• Prolonged consumption of large amounts of iodine can cause a markedly reduced thyroidal iodine uptake, causing iodide goitre.
• Most animals appear to start showing ill effects when fed rations containing from 200 to 500 ppm of iodine.

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