Iodine: Properties, Uses and Dangers

Introduction:

Iodine is an indispensable micronutrient required in small amounts for the normal physiological function of the human body [1]. Iodine is a critical component of the thyroid hormones, which are necessary for various metabolic and enzymatic processes. These processes include control of the body’s metabolic rate, growth and development, neuron function and development. The recommended dietary intake for adult men and non-pregnant women is 150 µg/d, 220-250 µg/d for pregnant women and 250-290 µg/d for breastfeeding women (American thyroid Association) [2]. Seafood, dairy products, and plants grown in iodine-rich soils are decent sources of iodine as food [3]. Most other foods contain low amounts of iodine so individuals require additional sources to meet the recommended amounts. Insufficient intake of iodine results in a variety of disorders referred to as iodine deficiency disorders (IDD). They include mental impairment, goiter (enlargement of the thyroid gland), hypothyroidism, and dwarfism [1-3]. IDD is especially destructive during the early stages of pregnancy and in early childhood. In their most severe form, IDD include cretinism (extreme case of neurological damage from fetal hypothyroidism), stillbirth and miscarriage, and increased infant mortality. IDD is a noteworthy public health problem in more than 50 countries. According, to the World Health Organization (2004) [4] an estimated 2 billion people worldwide (35.2% of the world population) suffer from inadequate iodine intake. Iodide is crucial to thyroid function in vertebrates, with vital implications for human health. It is important to recall that more than 95% of total iodine necessary for humans is accumulated in the thyroid gland.

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The history of therapeutic use of iodine dates back centuries. It has been described as the most potent antiseptic available. However, its therapeutic uses require careful evaluation because of its narrow range of intake to support optimal thyroid function [5]. One of the most notable features of iodine as an antiseptic is the lack of selection of resistant strains. Only one report of iodine resistance has been published [6]. The behaviour of iodine-based antiseptics on skin was investigated by Gottardi in 1995. Iodine antiseptics have wide scope of antimicrobial activity, killing all principal pathogens and given enough time even spores. Iodine based preparations and iodine salts are widely used as antimicrobial remedies, especially in the form of alcoholic solutions. This is due to the broad antimicrobial activity of iodine and the low cost of iodine components [7]. However, there is considerable controversy about the maximum safe iodine and duration of use. Povidone-iodine preparations are widely used as an antiseptic to prepare patient’s skin before surgery and are also used by surgeons and theatre staff as a skin cleaner and antiseptic in preoperative hand scrubs [8]. Many other wound dressing material containing iodine are Inadine, Iodosorb, Iodoflex, Iodozyme, Repithel etc. [9].

The antimicrobial property of iodine has been using for water disinfection since First world war to disinfect drinking water for troops in France and subsequently, US army during second world war used Globalin (tetraglycine hydroperiodide) tablets [10]. Iodine based disinfectant has been in use by NASA in space flights [11]. Today iodine based disinfection materials are mainly available in two forms, as iodine solution and iodine resin [12]. Iodine resins are solid-phase iodine disinfectants. Iodine resins are generally regarded as demand release disinfectants [12].

Starting in the early 1990’s increasing interest in the chemistry of polyvalent organic reagents is also notable and since then an innumerable of reports on the use of such compounds in organic synthesis have been published [13]. Wijs reagent, a solution of iodine monochloride in acetic acid and can be prepared from a mixture of iodine and iodine trichloride, is used for the estimation of the iodine value of fats and oils [14].

However there are various problems associated with the use of free iodine, like excess iodine ingestion cause thyroid disorders, irritation of tissues and short duration of action when used as antimicrobial remedies and high degree of instability [15]. These problems were overcome by the development of iodophores. Iodophores are complexes between iodine and a solubilising agent or carrier to increase the solubility and endure the release of iodine. In an aqueous iodophore solution, iodine is present in the form of different thermodynamically stable anionic iodine species and diatomic iodine [16]. Anionic species of iodine could interact with cationic groups of various polymers and form polymeric iodophores as ionic complexes. Four types of carriers have been generally used:-

i) Polyoxymer iodophores.

ii) Cationic surfactant iodophores.

iii) Non-ionic surfactant iodophores.

iv) Polyvinyl-pyrrolidine iodophores (also known as povidone iodine or PVP-I).

In most of these carriers iodine is present in aggregates (or micelles) of surfactants, which act as reservoir of iodine. Polyoxymer iodophore are complexes of non-ionic copolymers with iodine. In case of non-ionic surfactant iodophore the complex formation take place through donor-acceptor mechanism with the surfactant ethereal oxygen (as donor) and iodine (as acceptor) [17]. PVP with iodine forms a stable charge-transfer complex. In PVP-I solutions, free species of iodine are formally controlled by the mass action law including a coupled reversible interaction between iodine–iodide, triiodide–polymer and iodine–triiodide–polymer complexes [18]. In 1981, Cadexomer iodine was developed as another means of delivering benign iodine. Cadexomer is a derivative of dextrines (containing some number of carboxyl groups) cross-linked with epichlorohydrin and exits in the form of water-insoluble microbeads; 0.9% of molecular iodine is physically (not chemically) trapped in the core of these microbeads [19]. Solubility of elemental iodine increases in the presence of iodide ions, such as potassium iodide, where iodine reacts to form tri-iodide ions. Aqueous solutions of iodine are not stable and, depending on conditions, many different species may be present. Of these, it is believed that molecular iodine (I₂) has the highest antimicrobial potential. Stability is influenced by pH and activity diminishes with increased alkalinity and storage time [20]. The seven principal iodine species found in aqueous solution are I₂, HOI, OI^─, H₂OI⁺, I₃^─, I^─, of which only hydrated iodine (I₂), hypoiodous acid (HOI) and iodine cation (H₂OI⁺) possess bactericidal activity. At physiologically compatible pH and low concentrations, the only species of importance are I^─, I₂ and I₃^─ [21]. The type and nature of the iodine species present in the iodine-polymer complexes depends on the nature of the polymer. Many iodine- synthetic polymers complexes were known to regulate the release of iodine like povidone-iodine (Betadine), iodine incorporated resins (quaternary ammonium polymers), iodpolycom complex (vinylpyrrolidone and butyl methacrylate) [7, 18, 19] etc. Medical research of the Sklifosovskiy Research Institute and the National Centre for Toxicological and biological Safety of Medical Products (Report No. 5-10, Jan 11. 2012) [7] demonstrated that the complexed iodine exerted no burning effects on surrounding tissues. However there are some disadvantages associated with iodine-artificial polymers like non-biodegradability, toxicity, expensive etc. which can be overcome by using natural gums.

References:

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2. American Thyroid Association, news release, June, 5, (2013).
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