Benefits
Iodine Deficiency Prevention/Treatment
Iodine deficiency is the leading preventable cause of intellectual disability worldwide, and an estimated 2 billion people still have insufficient iodine intake, mostly in regions without effective salt iodization. Universal salt iodization (using KI or potassium iodate) has dramatically reduced cretinism and goiter, and has made most of the developed world iodine sufficient, so people already getting enough from iodized salt, dairy and seafood have little to gain from adding more. Potassium iodide is one of the standard forms for population-level iodine fortification. One caution comes from the same literature: Zimmermann and Boelaert report that a population adapted to mild deficiency keeps normal thyroid function but at a price, a higher rate of autonomous thyroid nodules, and that raising iodine intake in such a population causes a transient rise in hyperthyroidism.
Medical use, not a supplement benefit: nuclear emergency thyroid blocking
This is a medical use rather than something a supplement does. KI tablets are FDA-approved for thyroid protection during radioactive iodine release events such as nuclear power plant accidents. The adult emergency dose is 130 mg of KI, about 100 mg of iodine, which is more than 600 times the 150 microgram daily nutritional requirement, and it is taken only when public health authorities direct it, once a day and only for as long as the risk of exposure lasts. Protection is specific to radioactive iodine and to nothing else, and taking iodine every day does not confer it. Mechanism: saturating the thyroid with stable iodine prevents radioactive iodine-131 uptake. Distributed by federal/state emergency management programs near nuclear facilities.
Medical use, not a supplement benefit: pre-operative thyroid preparation
This is a surgical protocol rather than something a supplement does. Lugol's solution (potassium iodide plus iodine) is given under medical supervision for a short period before thyroid surgery in hyperthyroidism, because high-dose iodine reduces thyroid gland vascularity and decreases hormone release (Wolff-Chaikoff effect). Nobody should take Lugol's solution or high-dose iodine on their own to try to calm an overactive thyroid, and the same iodine load can push a susceptible thyroid the other way.
Standardized Pharmaceutical
Unlike kelp-derived iodine, whose iodine content varies widely and which has been found to carry arsenic (Amster 2007 measured arsenic above the FDA tolerance in eight of nine kelp supplements tested, and reported a case of arsenic toxicosis traced to one), potassium iodide has a precisely known iodine content and consistent dosing. Pharmaceutical-grade quality assurance. Preferred form for clinical applications.
Medical use, not a supplement benefit: sporotrichosis
This is a prescribing fact rather than something a supplement does, and an iodine supplement is not a treatment for any infection. A saturated solution of potassium iodide (SSKI), prescribed and monitored by a doctor at doses far above nutritional intake, has long been used for cutaneous sporotrichosis, a fungal infection picked up from rose thorns or sphagnum moss. It is still used in some settings, though itraconazole is now first-line.
Mechanism of action
Thyroid Iodine Uptake
Iodine is concentrated by the thyroid gland (sodium-iodide symporter / NIS) for thyroid hormone synthesis. T3 and T4 contain 3 and 4 iodine atoms respectively. Iodine deficiency impairs thyroid hormone production → goiter (compensatory thyroid enlargement) and hypothyroidism.
Wolff-Chaikoff Effect (High-Dose Iodine)
Acute high-dose iodine (>1 mg/day) transiently inhibits thyroid hormone synthesis — basis for hyperthyroidism pre-op preparation. Most patients escape this effect within 1-2 weeks (Plummer effect); chronic high-dose iodine can cause iodine-induced hyperthyroidism in susceptible individuals (Jod-Basedow phenomenon).
Radioactive Iodine Blocking
Stable potassium iodide saturates thyroid iodine binding sites, preventing uptake of radioactive I-131 (released in nuclear accidents). Most effective if taken within 4-6 hours of exposure; effectiveness diminishes after 12-24 hours.
Mucolytic and Antifungal Properties
High-dose iodine has mild antifungal activity (basis for sporotrichosis use) and historical mucolytic use (chronic bronchitis); displaced by modern pharmaceuticals. Both effects belong to prescription-scale doses and have nothing to do with nutritional iodine intake.
Clinical trials
Program experience rather than a controlled comparison. After Chernobyl in 1986 Poland distributed KI within days to the great majority of its children, while distribution across the most contaminated parts of Belarus and Ukraine was late and patchy. (Becker and Zanzonico 1997, Thyroid)
Pediatric populations near Chernobyl.
What was actually measured in Poland was radiation dose, not cancer. Becker and Zanzonico report that about 16 million people received a single administration of KI with only rare side effects and a probable 40 percent reduction in projected thyroid radiation dose. Poland did not see the surge in childhood thyroid cancer that followed in Belarus and Ukraine, but Poland also received far less fallout and restricted contaminated milk and local produce, so the lower cancer rate cannot be credited to KI alone. On chronology, the FDA had already concluded in 1978 that KI was safe and effective for blocking radioiodine uptake, eight years before Chernobyl; the Chernobyl data informed the later dosing guidance rather than the original approval. None of this bears on everyday iodine supplementation.
WHO global iodine deficiency disorder elimination programs using iodized salt (potassium iodide or potassium iodate fortification). These are public health programs measured at population level, not tests of an iodine supplement in individuals.
Iodine-deficient populations globally.
Universal salt iodization has dramatically reduced goiter prevalence and cretinism in formerly iodine-deficient regions, and it is one of the most cost-effective public health interventions there is. Potassium iodide and potassium iodate are equivalent for fortification purposes. What this does not show is that an iodine supplement helps someone who already gets enough. Where iodine supplements have been tested by randomized trial, the result has not always been positive: a placebo-controlled trial of 200 micrograms of iodine a day in 832 mildly iodine-deficient pregnant women in India and Thailand (Gowachirapant 2017) found no effect on the children's verbal IQ, performance IQ or executive function at ages 5 to 6.