**1. Introduction**

Iodine is an important micromineral that has a vital role in the synthesis of thyroid hormones like triiodothyronine and thyroxine. These hormones have a role in thermoregulation, increasing cellular respiration and energy generation and have widespread effects on intermediary metabolism, growth, development, reproduction, muscle function, immune defense, and circulation [1].

The synthesis of thyroid hormones takes place in the thyroid gland of all species of animals and also in goats. The size of the gland is approximating 0.20% of bodyweight [2]. It contains the highest concentration of iodine (0.2–5%) of dry weight and in the largest amount (70–80%) of total body iodine.

It is a bilobed structure situated slightly behind the larynx. Right and left lobes of thyroid gland lie lateral to the trachea and are joined by a thin isthmus that passes across the ventral aspect of the trachea [3]. The thyroid gland is a highly vascularized tissue constituted by the functional unit called thyroid follicle. Each follicle has a spherical structure composed of an

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

outer monolayer of follicular cells surrounding an inner core of colloid, the thyroglobulinhormone complex, which is the storage reservoir of thyroid hormone. The colloid stored in the lumen is a clear, viscous fluid. The size of the follicles and the height of their cells vary according to the functional state of the gland. The cells may vary from an inactive squamous cell to the highly active, tall columnar cell.

**3. Etiology**

Iodine deficiency is of two types, that is, primary and secondary or conditional deficiency.

groundwater containing iodine less than 2 μg/l leads to iodine deficiency.

increase the usual dietary requirement of dairy animals by four to five times.

the iodine uptake by plants growing on such soils.

cause goiter in lambs and goats, respectively.

Primary deficiency is an environmental deficiency due to the low level of iodine in soil, water feed, and fodder of animals. A deficiency of iodine in soil and subsequently in fodder crops is the primary reason for iodine deficiency in animals. Soil deficiency may be due to leaching of iodine from surface soil and poor replenishment with airborne iodine [8]. Drinking of

Iodine Deficiency in Goats

77

http://dx.doi.org/10.5772/intechopen.72728

In general sandy soils are low in iodine. High clay content and high pH of soil interfere with

Iodine content of plant varies with species, strains, climatic and seasonal conditions, and chemical fertilizer supplemented to plants. Cereals, wheat bran, and oil cakes are poor in iodine, whereas straws and green fodders contain marginally adequate content of iodine as per requirement by livestock. Stage of maturity and cutting time significantly affect the iodine content of the fodder. Iodine content of fodder decreases with fall in environment temperature and vice versa [9, 10]. The excessive use of chemical fertilizers like DAP and potash decreases the uptake of iodine from soil; conversely, supplementation of seaweeds in soil will increase the iodine content of soil. Secondary or conditional deficiency is due to the presence of certain substances present in some plants called goitrogens. It results in iodine deficiency disorders in animals despite of normal iodine intake (1–4 ppm of dry matter), because it interferes with the utilization of dietary iodine or with its metabolism in hormone synthesis. Goitrogens in feed and fodders of animals

The presence of goitrogens in diets consisting largely of cruciferous plants like cabbage, *Brassica* spp., peanut, soybean, and yellow turnip contains cyanogenetic glycosides that are goitrogenic because on hydrolysis yields of hydrocyanic acid due to structural damage of the cell wall of plants and further converted to thiocyanates by ruminal microbes. Action of thiocyanates can be overcome by increasing supplementation of dietary iodine. Goitrin (thiooxazolidone) is a thiouracil type of goitrogen present in the seeds of rape, kale, and other *Brassica* spp. It inhibits hormone synthesis, and its action cannot be overcome by extra supplementation of dietary iodine. Linseed meal contains glycoside arachidoside (linamarin) which is converted into thiocyanates in the rumen. Subabool (*Leucaena leucocephala*) contains an alkaloid mimosine (3–6%) which not only inhibits the utilization of iodine but also prevents the availability of iodine from other ingredients in the ration fed to animals. Drinking of grossly bacterial-contaminated water and ingestion of such feedstuffs reported to develop goiter in ruminants. A continued intake of the grass (*Cynodon aethiopicus*) and African pearl millet (*Pennisetum typhoides*) with low iodine and high cyanogenetic glycoside contents may

Excess intake of calcium decreases the absorption of iodine from the gastrointestinal tract. High fluoride ingestion is also implicated as one of the important factors for development of goiter in animals. Deficiency of cobalt and thus vitamin B12 reported to increase thyroxine levels accompanied by marked hypertrophy and hyperplasia of thyroid gland [11, 12].

Thyroid hormones (T4, T3, and rT3) immediately on entering the circulation are bound to transport proteins, mainly to thyroxine-binding globulin (TBG), in lesser amounts to thyroxine-binding prealbumin (TBPA), and to albumin. There is a wide spectrum of species variation in hormone binding by serum proteins. TBG is the major binding protein for hormone, but not all species have TBG; however, TBPA is present in all species [4].

The soil in large geographic areas of the world is deficient in iodine. About 29% of the world's population, living in approximately 130 countries, is estimated to live in areas of deficiency.

Iodine deficiency is more prevalent primarily in mountainous regions such as the Himalayas [5], the European Alps, and the Andes, where iodine has been washed away by glaciations and flooding. Iodine deficiency also occurs in lowland regions far from the oceans, such as Central Africa and Eastern Europe. Globally, 2.2 billion people are at risk for iodine deficiency disorders (IDD). Of these persons, 30–70% have goiter and 1–10% have cretinism. The clinical disorders of iodine deficiency tend to be more profound in geographic areas associated with coexisting selenium and vitamin A deficiencies and in regions where goitrogens are fed in diet [6].

Iodine deficiency in large areas of the world is associated with iodine cycle in nature. Iodine occurs in the soil and the sea as iodide. Iodide ions are oxidized by sunlight to elemental iodine which is highly volatile. The concentration of iodide in seawater and air is about 50 μg/l and 0.7 μg/m<sup>3</sup> , respectively. Iodine in atmosphere is returned to the soil by rain and snow which has a concentration in the range of 1.8–8.5 μg/l. The return of iodine is slow and small in amount compared to the original loss, and repeated flooding further decreases iodine in the soil. High rainfall, snow, and floods increase the loss of soil iodine due to melting of glaciers in hilly area due to global warming [7].

Nutritional iodine deficiency in livestock is the leading cause of thyroid gland disorders/goiter. It generally occurs in farm animals wherever human goiter is endemic. Goat is considered as indicator species of iodine deficiency because of browsing habits and less ingestion of soil compared to other grazing animals [3].
