Iodine

Iodine is a naturally occurring chemical element that, in living organisms, functions as an essential micronutrient ^[1]. Its single indispensable job in the human body is to serve as a building block of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which govern metabolism, energy use, growth, and the development of the brain ^[1]. Because the body cannot synthesize iodine, it depends on a steady dietary supply ^[3].

Iodine enters the diet mainly through foods of marine origin, such as fish, shellfish, and seaweed, as well as dairy products, eggs, and crops grown in iodine-rich soil ^[1]. The amount in plant and animal foods varies widely with the iodine content of local soil and water, so populations far from the sea or in mountainous and flood-prone regions have historically been prone to shortage ^[3]. Iodized salt, ordinary table salt fortified with a small amount of iodine, was introduced in the twentieth century as a simple and inexpensive way to guarantee adequate intake ^[3].

Iodine deficiency is a major global health problem and the most common preventable cause of intellectual disability worldwide ^[4]. In its milder forms it causes goiter and an underactive thyroid, with symptoms such as fatigue, weight gain, and mental sluggishness ^[3]. Its gravest effects fall on the developing fetus and young child: a lack of iodine during pregnancy can cause permanent brain damage, stunted growth, and the severe condition historically called cretinism ^[3]. For these reasons, universal salt iodization has been adopted in much of the world and has dramatically reduced severe deficiency, although insufficient intake persists in parts of Europe and the developing world, and pregnant women remain especially vulnerable because their iodine needs rise ^[4][1].

Although deficiency is the larger problem, excessive iodine intake can also cause harm ^[2]. High intakes, from over-iodized salt, certain seaweeds, supplements, or medications, are tolerated by most healthy people but can precipitate an overactive or underactive thyroid or autoimmune thyroid disease, particularly in those with preexisting thyroid conditions or a history of deficiency ^[2]. Beyond nutrition, iodine compounds are widely used in medicine as antiseptics, such as povidone-iodine for cleaning skin and wounds, and as contrast agents in X-ray and CT imaging ^[3].

At normal intakes iodine is safe and necessary. Problems come at the extremes: deficiency causes hypothyroidism and goiter, while excess can cause both hyperthyroidism and hypothyroidism, and can worsen autoimmune thyroid disease. People with Hashimoto's or Graves' should be especially careful with supplemental iodine. High acute doses can cause GI upset and a metallic taste.

Most of iodine's interactions run through the thyroid. Lithium blocks thyroid hormone release and is goitrogenic in its own right, so an iodine load on top of lithium raises the risk of hypothyroidism and goitre beyond either alone. The same additive effect applies to methimazole and propylthiouracil, where supplemental iodine compounds the intended block. Amiodarone is roughly 37% iodine by weight and releases large quantities of it, so extra iodine adds to an already heavy load and to the risk of both iodine induced hypothyroidism and thyrotoxicosis.

Potassium iodide also carries a potassium load, and combining it with potassium sparing diuretics, ACE inhibitors or angiotensin receptor blockers can push serum potassium into a dangerous range.

An iodine load saturates the sodium iodide symporter, which blunts radioiodine uptake. Recent iodine exposure, whether from supplements, iodinated contrast media or povidone iodine applied to mucosa or broken skin, can invalidate a thyroid scan or weaken radioiodine therapy for weeks afterwards.

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