Iodine

Iodine is an essential trace element required for the synthesis of thyroid hormones (thyroxine [T₄] and triiodothyronine [T₃]), which regulate basal metabolism, growth, and neurodevelopment. In humans, it is primarily obtained from the diet and, when deficient, can impair thyroid function and downstream physiological processes.

• Thyroid hormone production: Adequate iodine ensures optimal synthesis of T₄ and T₃, preventing hypothyroidism and associated fatigue, weight gain, and cold intolerance (WHO, 2022).
• Neurocognitive development: Maternal iodine status is strongly linked to fetal brain development; deficiencies increase the risk of lower IQ and neurodevelopmental disorders (Hynes et al., 2021).
• Metabolic regulation: Thyroid hormones modulate basal metabolic rate, lipid metabolism, and thermogenesis, influencing body weight and cardiovascular health (Klein & Danzi, 2020).
• Immune support: Iodine has antimicrobial properties, contributing to innate immunity, especially in the respiratory and gastrointestinal tracts (Baker et al., 2023).
• Pregnancy outcomes: Adequate iodine reduces the risk of miscarriage, pre‑eclampsia, and neonatal hypothyroidism (American Thyroid Association, 2023).

• Uptake: Iodine is taken up by the thyroid gland via the sodium‑iodide symporter (NIS), a membrane transporter that concentrates iodide up to 30‑fold relative to plasma.
• Oxidation and Iodination: Inside the follicular lumen, thyroid peroxidase (TPO) oxidizes iodide to I⁺, which then iodinates tyrosine residues on thyroglobulin, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
• Hormone Synthesis: Coupling of MIT and DIT yields T₃, whereas two DIT molecules form T₄.
• Hormone Release and Action: These hormones are released into circulation, where they bind to nuclear thyroid hormone receptors (TRα, TRβ) and regulate transcription of genes involved in mitochondrial activity, lipid metabolism, and neuronal development.
• Other Roles: Iodine also participates in the iodination of other proteins (e.g., lactoperoxidase) and acts as a substrate for antimicrobial iodine‑based defenses.

• Elemental Iodine: Iodine (elemental) is a diatomic halogen with the molecular formula I₂; the IUPAC name is simply iodine.
• Supplement Form: In supplements, the biologically active form is the iodide ion (I⁻), often supplied as potassium iodide (KI) or sodium iodide (NaI).
• Properties: The atomic weight of iodine is 126.90 g/mol, and it has a melting point of 113.7 °C and a boiling point of 184.3 °C. Iodine is a non‑metal with a violet‑brown color in solid form and sublimates readily.
• Ion Characteristics: The ion has a monovalent negative charge, a relatively large ionic radius (2.06 Å), and high polarizability, which underlies its strong affinity for the NIS transporter.
• Redox Properties: In aqueous solution, iodide can be oxidized to iodine, which can then be reduced back, making it a useful redox couple in biological systems.

• Extraction: Commercial iodine is primarily extracted from iodine‑rich brine deposits (e.g., Chilean and Japanese salt lakes) where it is recovered by oxidation of iodide with chlorine, followed by purification and crystallization.
• Natural Sources: Marine algae (kelp, seaweed) provide natural organic forms of iodine (as iodophenols and iodinated lipids) and are used in “kelp extract” supplements; these require careful processing to remove heavy metals and ensure consistent iodine content.
• Synthetic Production: Synthetic production of KI and NaI is achieved via the reaction of elemental iodine with potassium or sodium hydroxide, yielding high‑purity salts suitable for pharmaceutical use.
• Quality Control: Quality control focuses on pharmacopoeial specifications (e.g., USP <467>), ensuring ≤ 10 ppm heavy metal content, accurate labeling of iodine content (± 5 %), and the absence of contaminants such as bromide or arsenic.