Iodine – Properties, Deficiency Symptoms, and Sources

Iodine serves as a critical component in the biosynthesis of thyroid hormones, specifically thyroxine (T₄) and triiodothyronine (T₃), with daily production rates of approximately 80–90 micrograms and 32 micrograms, respectively. It constitutes 65% of the molecular weight of thyroxine and 58% of triiodothyronine, underscoring its indispensable role in endocrine system regulation. Dietary iodine is primarily obtained through inorganic compounds—iodides and iodates—which are absorbed predominantly in the small intestinal mucosa, with minor contributions from the stomach. Upon absorption, circulating iodine in the plasma undergoes selective uptake by thyroid follicular cells, with lesser accumulation in salivary glands, gastric mucosa, and the choroid plexus of the brain. During lactation, additional iodine deposition occurs in mammary gland tissue, followed by its secretion into breast milk. The primary route for iodine excretion is renal, via urine; trace amounts are also eliminated through exhaled air, feces, and sweat. Thyroidal iodine uptake is an active process occurring against an electrochemical gradient, indicative of its high specificity. Under conditions of adequate iodine intake, roughly 10% of the absorbed iodine is stored in the thyroid, whereas this proportion can escalate to as much as 80% during deficiency—a compensatory adaptive response of the organism [1].

The human body’s daily requirement for iodine exhibits substantial variability depending on life stage and specific physiological conditions, such as pregnancy or lactation. According to expert guidelines, the optimal intake for an adult individual is 150 micrograms per day, though this demand increases significantly during gestation and breastfeeding—as evidenced by reference studies [2]. The World Health Organization (WHO) emphasizes that chronic iodine deficiency ranks among the most critical global health determinants, yielding severe clinical consequences. The most prevalent manifestation of inadequate iodine intake is the development of goiter (thyroid enlargement), which, in regions with endemic iodine insufficiency, assumes a mass-scale character and persists at stable levels over decades. An effective solution involves modifying dietary patterns, with the systematic iodization of table salt standing as the most efficient, cost-effective, and widely implemented strategy—one explicitly endorsed by international expert panels [3].

The most clinically distinctive manifestation of chronic iodine insufficiency is thyroid enlargement, known as goiter, which develops when daily iodine intake falls below the 50-microgram threshold. Prolonged inadequate iodine supply triggers a cascade of severe health consequences that vary by age group. In adults, this deficiency may lead to cognitive impairment, reduced physical and mental performance, and the development of both non-toxic (euthyroid) and toxic (hyperthyroid) goiter, with severe or moderate deficiency often culminating in hypothyroidism. Children and adolescents experience delayed physical growth and intellectual disabilities as a result of iodine insufficiency. For fetuses and neonates, the risks escalate to include spontaneous miscarriage, stillbirth, congenital malformations, elevated perinatal mortality, and—particularly in endemic regions—the development of endemic cretinism. Notably, even mild to moderate iodine deficiency increases susceptibility to follicular thyroid carcinoma, which exhibits a more aggressive clinical course compared to papillary thyroid carcinoma, as corroborated by epidemiological evidence [1].

The foundational reservoir of iodine originates from iodine-rich geological formations, from which this trace element is liberated through erosion and weathering processes, ultimately accumulating in aquatic ecosystems—primarily seas and oceans. Consequently, both seawater itself and marine organisms—including seaweed, mollusks, crustaceans, and fish—exhibit exceptionally high iodine concentrations. In human nutrition, the predominant sources of iodine are marine-derived products, supplemented by dairy, eggs, and, to a far lesser extent, drinking water, which fulfills merely one-tenth of daily requirements. Among natural sources, seaweed stands out as the most iodine-dense, whereas in Poland, milk, dairy products, eggs, and marine fish play a pivotal role. Notably, eggs demonstrate a remarkable capacity for iodine accumulation, with an average content of approximately 21 micrograms per egg, predominantly concentrated in the yolk (the white contains up to eighteen times less). Furthermore, iodine supplementation in layer hen feed significantly elevates iodine levels in eggs. Meat, despite variations among species, cannot be considered a meaningful iodine source—its content remains negligible compared to classic iodine-rich foods such as fish, eggs, or milk. As a trace element, iodine is present in living organisms in minute quantities, yet its role in maintaining homeostasis and ensuring the proper functioning of biological systems is indispensable. Iodine deficiency entails severe health repercussions, underscoring the necessity of consciously incorporating iodine-rich foods—including marine fish, eggs, and iodized salt—into daily dietary patterns.