IodineMineral

• Making thyroid hormonesLikely helps

  Not really a claim so much as the reason iodine is a nutrient at all. It is built into T4 and T3, which regulate metabolic rate, protein synthesis and enzymatic activity [1].
  
  It is also required for fetal and infant skeletal and central nervous system development [1]. Everything else on this page follows from that one function. ^[1]

• Cognitive development in iodine-deficient childrenLikely helps

  A 2004 Cochrane Review concluded iodine supplementation in iodine-deficient children improves physical and mental development and reduces mortality, with only minor and transient adverse effects [1].
  
  A 2009 New Zealand randomized placebo-controlled trial gave 184 mildly deficient children 150 mcg/day for 28 weeks, lifting median urinary iodine from 63 to 145 mcg/L [1].
  
  They scored significantly better than placebo on perceptual reasoning and overall cognitive measures. Note the population: this is correcting a deficiency, not enhancement in children who already have enough. ^[1]

• Child IQ when the mother's status is lowPossibly helps

  ⚠ Observation and intervention disagree here, and the NIH review presents both without resolving it.
  
  A meta-analysis of 6,180 mother-child pairs across three European birth cohorts associated lower first-trimester maternal iodine status with lower verbal IQ in children aged 1.5–8 [1].
  
  But two similarly-designed randomized trials giving 150 or 200 mcg/day from early pregnancy to delivery found no benefit to child cognitive, language or motor scores at 1.5–2 years [1]. One followed through to age 5–6 and still found none.
  
  The American Thyroid Association and American Academy of Pediatrics nonetheless recommend 150 mcg/day supplemental iodine, as potassium iodide, for women pregnant, planning pregnancy or lactating [1]. ^[1]

• Fibrocystic breast diseasePossibly helps

  In a double-blind trial, 56 women taking 70–90 mcg of iodine as I2 per kg of body weight daily for 6 months reported decreased breast pain in 65% of cases, against 33% on placebo [1].
  
  A later double-blind trial in 111 women aged 18–50 compared 0, 1,500, 3,000 and 6,000 mcg/day of molecular iodine over 5 months [1]. The 3,000 and 6,000 mcg groups had significantly less pain, tenderness and nodularity, dose-dependently, with no major adverse events or thyroid function changes reported.
  
  ⚠ Read the numbers against the ceiling. Those doses are several times the adult UL of 1,100 mcg/day, and belong under physician supervision only [1]. ^[1]

• Thyroid blocking in a radiation emergencyLikely helps

  Iodine-deficient thyroids take up more radioactive iodine, which is why the FDA has approved potassium iodide at 16–130 mg, depending on age, as a thyroid-blocking agent during nuclear radiation emergencies [1].
  
  Poland used it widely after the 1986 Chernobyl accident and saw no substantial rise in childhood thyroid cancer [1]. Belarus and Ukraine, where iodide prophylaxis was not used and many children were mildly deficient, saw a sharp rise.
  
  This is emergency medicine at milligram doses, not daily supplementation at microgram doses. The two are not the same activity. ^[1]

• Nascent iodine marketing claimsNot shown to help

  Claims attached to "nascent" or "monatomic" iodine drops — clearing heavy metals from the body, electromagnetic recharging of cells — have no clinical evidence behind them [10].
  
  The FDA flagged unsupported health claims for this category of product as far back as 1929 [10]. ^[10]

• Goiter — the thyroid enlarging as it tries to trap more circulating iodine, usually the earliest clinical sign, driven by rising TSH once intake falls below roughly 100 mcg/day
• Hypothyroidism, frequently alongside goiter, once intake drops to about 10–20 mcg/day
• Fatigue, weight gain, cold intolerance, dry skin, constipation and depression — the downstream hypothyroid symptoms in adults
• Impaired mental function and reduced work productivity, in adults with mild-to-moderate deficiency
• Neurologic cretinism after severe deficiency in early gestation: intellectual disability, abnormal gait, deaf-mutism, without goiter or hypothyroidism in the child
• Myxedematous cretinism after fetal or neonatal deficiency later on: intellectual disability, short stature, goiter and hypothyroidism
• Neither form of cretinism responds to iodine supplementation once established — adequate maternal intake is what stops it arising
• An average IQ reduction of roughly 12–13.5 points, linked in meta-analysis to chronic moderate-to-severe deficiency, especially in children
• Increased risk of ADHD in children, associated with mild-to-moderate maternal deficiency^[1]

• Potassium iodide (KI)Predictable dosing. You get close to the number on the label, which is not true of every form here

  Elemental content

  The most common supplement and multivitamin form, typically 150 mcg per dose, alongside sodium iodide

  Absorption

  A small study measured 96.4% bioavailability — near-complete absorption [1][5].
  
  It is the form the American Thyroid Association recommends for pregnant and lactating women, and the form the FDA approved as a thyroid-blocking agent [1].

  Best for

  Predictable dosing. You get close to the number on the label, which is not true of every form here [1]

• Potassium iodateIodized salt in hot and humid climates, where plain iodide degrades

  Elemental content

  Reduced to iodide in the gastrointestinal tract before absorption, so it behaves much like KI in practice

  Absorption

  The WHO recommends iodate over iodide for salt iodization specifically [1].
  
  The reason is stability rather than absorption: it holds up better in warm, humid or tropical conditions.

  Best for

  Iodized salt in hot and humid climates, where plain iodide degrades [1]

• Kelp and seaweed-derived iodineLittle, if predictable intake matters — this is the main route by which people accidentally overshoot the UL

  Elemental content

  ⚠ Wildly variable. Commercially available whole and sheet seaweeds run from about 16 to 2,984 mcg of iodine per gram [1]

  Absorption

  Lower than KI in every study reviewed, though the size of the gap moves around.
  
  A UK trial in iodine-insufficient women found only 33% of the iodine in an Ascophyllum nodosum capsule was bioavailable [7]. A 2024 Norwegian crossover trial in 20 healthy women measured 75% from a sushi and wakame meal against 97% from a KI reference [6].
  
  A Chinese study of kombu found 23.7–29.5%, against 63.7–83.5% for KI [9]. A rat study of farmed sugar kelp found 73–81% urinary excretion against 94–95% for KI [8].
  
  The seaweed's polysaccharide matrix traps iodide, and organically-bound iodine absorbs worse than inorganic iodide, which is the mechanism behind the gap [6][8].

  Best for

  Little, if predictable intake matters — this is the main route by which people accidentally overshoot the UL [1]

• Molecular iodine (I2)Nothing you would pick off a shelf.

  Elemental content

  Elemental iodine, studied specifically at 1,500–6,000 mcg/day in the fibrocystic breast disease trials

  Absorption

  Not the form found in most standard multivitamin or single-iodine supplements [1].
  
  The doses it was studied at sit well above the adult UL, which makes it a supervised clinical use rather than a shelf product.

  Best for

  Nothing you would pick off a shelf. It appears here because the breast-pain trials used it [1]

• "Nascent" or monatomic iodineNothing that potassium iodide doesn't do more cheaply and more predictably

  Elemental content

  A marketing term, not a recognised chemical classification — the liquid-drop product traces to a 1931 Edgar Cayce formula

  Absorption

  No peer-reviewed evidence establishes it as chemically or clinically distinct from, or better than, standard potassium iodide [10].
  
  In solution, iodine of any labelled form converts to ordinary iodide anyway.

  Best for

  Nothing that potassium iodide doesn't do more cheaply and more predictably [10]

• Common side effects and iodism

  More commonly reported: diarrhea, nausea and vomiting, rash, swollen or tender salivary glands, and stomach discomfort [17].
  
  With regular high-dose use, iodism can develop — sore gums or teeth, bad headaches, red or watery eyes, swollen eyelids, blurry vision, a runny nose or excess saliva [17]. ^[17]

• Excess iodine causes the same damage as deficiency

  ⚠ The single most important thing on this page. High intakes can produce goiter, elevated TSH and hypothyroidism — the exact endpoints deficiency produces — because excess iodine can itself inhibit thyroid hormone synthesis in susceptible people [1].
  
  Iodine-induced hyperthyroidism, thyroiditis and papillary thyroid cancer have also been reported with excessive intake [1].
  
  Acute poisoning, usually from doses of many grams, causes burning of the mouth, throat and stomach, fever, abdominal pain, nausea, vomiting, diarrhea, weak pulse and coma [1]. ^[1]

• Hashimoto's and other autoimmune thyroid disease

  People with Hashimoto's may be especially sensitive to iodine's harmful effects, per NIDDK [11].
  
  Eating large amounts of kelp, dulse or other seaweed, or taking iodine supplements, can trigger or worsen hypothyroidism in this group.
  
  ⚠ The relationship is not a simple line, though. Chronic excess intake has been associated with a higher population prevalence of Hashimoto thyroiditis, via more immunogenic highly-iodinated thyroglobulin, while mild deficiency has been associated with a lower prevalence [14].
  
  Significant deficiency separately raises thyroid autoimmunity risk. So neither "more" nor "less" is a safe default — this is a conversation for a care team [11][14]. ^[11]

• The Wolff-Chaikoff effect

  Excess iodine triggers a rapid, normally self-limiting autoregulatory block of thyroid hormone synthesis [15].
  
  Most healthy people escape that block within days. Failure to escape — more common in older adults and in people with underlying autoimmune thyroid disease — can cause prolonged hypothyroidism. ^[15]

• Topical iodine is not a supplementation route

  Only small amounts of povidone-iodine antiseptic cross intact skin, and it is not a recognised way to meet dietary needs [18][19].
  
  Absorption climbs substantially through wounds, burns and mucosal surfaces, or with large-area and repeated application. Surgical hand-scrubbing more than 20 times a day was flagged as a scenario where cumulative absorption matters [20].
  
  An ex-vivo study measured 11.59 mcg/cm² permeating human skin over 24 hours, a flux of about 0.73 mcg/cm² per hour [20]. In those contexts it can be enough to cause systemic effects, including thyroid dysfunction in susceptible people. ^[20]

• Lithium

  Lithium and iodine can act synergistically to impair thyroid function [12][13].
  
  Case literature dating to the 1970s documents patients on lithium developing or worsening hypothyroidism after starting potassium iodide, resolving when the iodine or the lithium was stopped.
  
  Researchers advise against combining the two without close monitoring. ^[12]

• Antithyroid medication

  Methimazole and similar drugs used for hyperthyroidism already suppress thyroid hormone production [1].
  
  High-dose iodine on top can be additive, and could tip a person into hypothyroidism. ^[1]

• ACE inhibitors

  Benazepril, lisinopril, fosinopril and the rest of the class raise the risk of hyperkalemia when combined with potassium iodide [1].
  
  The issue is the potassium, not the iodide. ^[1]

• Potassium-sparing diuretics

  Spironolactone, amiloride and similar drugs carry the same hyperkalemia risk alongside potassium iodide [1].
  
  Same mechanism as the ACE inhibitors: elevated blood potassium. ^[1]

Seaweed — kelp, nori, kombu, wakame — is the richest food source, and also the least predictable. Content ranges from about 16 to 2,984 mcg per gram across commercially available species [1].

From the NIH's food composition table, per serving [1]:

- White or whole-wheat bread made with an iodate dough conditioner: 296 and 273 mcg per 2 slices, though only about 20% of US bread products use one

- Baked cod: 146 mcg per 3 oz

- Dried nori: 116 mcg per 2 tbsp flaked

- Cooked oysters: 93 mcg per 3 oz

- Plain nonfat Greek yogurt: 87 mcg per ¾ cup

- Nonfat milk: 84 mcg per cup

- Iodized table salt: 78 mcg per ¼ teaspoon

- Fish sticks: 57 mcg per 3 oz; hard-boiled egg: 31 mcg; pasta boiled in iodized-salted water: 30 mcg per cup

- Chocolate ice cream: 28 mcg per ⅔ cup; cheddar cheese: 14 mcg per oz; beef liver: 14 mcg per 3 oz; shrimp: 13 mcg per 3 oz

Most fruits, vegetables, non-iodized sea salt and plain rice, corn and beans contain essentially none [1].

Dairy content depends on whether cows got iodine feed supplements and whether iodine-based sanitizers were used at milking. Plant-based milk substitutes such as soy and almond carry relatively little [1].

Iodized salt is the primary global public-health measure against deficiency [1]. Specialty salts — sea salt, kosher salt, Himalayan pink, fleur de sel — are typically not iodized and supply virtually none.

⚠ Cooking losses are genuinely disputed. An Indian study of iodized-salt dishes found boiling lost the most at 40.2%, then microwaving at 27.1%, roasting at 10.6% and deep frying at 10.4% [21].

Total losses across all methods tested spanned 6.6–51.1%.

Other studies report boiling losses as low as 14.3% or as high as 66%, and one review says boiling, baking and canning cause losses of 10% or less [21]. The literature does not converge on a number.

The direction is at least consistent: wetter, hotter, longer cooking loses more than dry, quick methods. In fish specifically, boiling significantly reduced iodine in most species tested, while pan-frying and oven-baking did not [22].

⚠ Tap water is a regional question, not a universal one. A 2023 Norwegian analysis found roughly 0.08–0.11 mcg per 100 mL — negligible against a 150 mcg/day target [24].

But in regions with naturally iodine-rich groundwater, including parts of China, Brazil and Denmark, drinking water can supply over half of total intake [25]. There it is a risk factor for excess, not shortfall.

No controlled trial reviewed here compares morning against evening, or with food against empty stomach, for any clinical outcome.

What is established is that iodide is absorbed quickly and almost completely in the stomach and duodenum, with iodate converted to iodide and absorbed similarly [1]. Timing does not appear to be an absorption variable.

For medicinal "strong iodine" or Lugol's-type oral solution, Mayo Clinic notes it can be taken after meals, or with food or milk, if it upsets the stomach [27]. Taking it in a full glass of water, juice, milk or broth improves the taste.

That is a tolerability adjustment, not an absorption penalty. For standard supplements, consumer guidance is simply to follow the product label [17].

One separate note for levothyroxine users: mineral supplements are generally spaced several hours from thyroid hormone replacement, because levothyroxine needs an empty stomach. That is a levothyroxine-absorption issue, and no trial of iodine-specific spacing was located here.