What is iodine?
Iodine (I) is a mineral that is essential to human health, which we use to make the thyroid hormones thyroxine (T3) and triiodothyronine (T4). These hormones regulate our metabolism, synthesis of protein, enzymatic activity, and other biochemical reactions. They are also essential to unborn children’s and infants’ development of skeletal and nervous systems. Iodine deficiencies cause goiter; an enlargement of the gland. In addition to thyroid functions, iodine has a role in immunity, including breast health. 
How do we absorb iodine?
The element iodine is a gas, but we ingest it as a solid salt and that compound is known as iodide. For example, the potassium salt form of iodine is known as potassium iodide (KI). Iodine readily absorbs (about 96%) in the upper intestine (stomach and duodenum). 
How much iodine should we normally have?
• 100–199 mcg/L in children and adults
• 150–249 mcg/L in pregnant women
• Over 100 mcg/L in lactating women
Values lower than 100 mcg/L in children and non-pregnant adults indicate insufficient iodine intake, and urinary iodine levels lower than 20 mcg/L are considered severe deficiencies.
The US RDA for iodine is:
Table 1: Recommended Dietary Allowances (RDAs) for Iodine 
- Birth to 6 months 110 mcg*
- 7-12 months 130 mcg*
- 1-3 years 90 mcg
- 4-8 years 90 mcg
- 9-13 years 120 mcg
- 14-18 years 150 mcg
- 14-18 years Pregnant 220 mcg
- 14-18 years Lactation 290 mcg
- 19+ years 150 mcg
- 19+ years Pregnant 220 mcg
- 19+ years Lactation 290 mcg
However, international health agencies recommend that pregnant women get 250 mcg daily.
Typically, mountainous areas and areas subject to flooding have lower soil levels of iodine. Cruciferous vegetables are goitrogenic (inhibit the thyroid’s absorption of iodine), as are deficiencies of vitamin A or iron.
It takes numerous grams (one gram = 1,000 mg or 1,000,000 mcg) of iodine to cause overt toxicity, though side effects are possible at lower amounts. As with some other vitamins and minerals, symptoms of excess mimic those of deficiency. For iodine, these include goiter, elevated thyroid stimulating hormone (TSH) from the pituitary gland, and hypothyroidism (low thyroid function).
This table from the Institute of Medicine lists maximum recommended levels of supplemental iodine. These levels reflect a comfortable safety margin and few people would have side effects from taking iodine up to the amounts shown. In the event of a radiation emergency, higher amounts may be recommended by public health authorities.
Table 3: Tolerable Upper Intake Levels (ULs) for Iodine 
- Birth to 6 months Not possible to establish*
- 7–12 months Not possible to establish*
- 1–3 years 200 mcg
- 4–8 years 300 mcg
- 9–13 years 600 mcg
- 14–18 years 900 mcg
- 19+ years 1,100 mcg
Additionally, potassium iodide supplements may interact with some drugs including anti-thyroid medications used to fight overactive thyroid (hyperthyroidism), certain blood pressure medicines (angiotensin-converting enzyme (ACE) inhibitors), and potassium-sparing diuretics. 
What are the causes of possible exposure to radioactive iodine?
Radioactive iodine-129 (I-129) and iodine-131 (I-131) are produced by the fission of uranium atoms during the operation of nuclear reactors. Iodine-129 is also formed as a result of nuclear weapon explosions, the source of most of the radioactive iodine in our environment. 
How is radioactive iodine dangerous to human and animal health?
The body uses both normal (“stable”) and radioactive iodine, not being able to distinguish between them. Radioactive iodine can cause thyroid problems, yet it is also used to help diagnose and treat certain thyroid problems. Long-term exposure to radioactive iodine can cause nodules or cancer of the thyroid, but treatment with high doses of I-131 (the rapidly decaying one) may be used to treat thyroid cancer. Doctors also use lower doses of I-131 to treat overactive thyroids. 
How long is radioactive iodine dangerous?
While iodine-129 persists in the environment for millions of years, iodine-131 has a half-life (the time it takes for it to decay to 50% of the original amount) of only 8 days and will decay completely within months. Radioactive iodine is water soluble and can be ingested by drinking it in contaminated water, inhaling it as an airborne gas, eating produce that has it on the surface, eating fish that have absorbed it from the water, or from consuming milk or meat from animals that have eaten plants exposed to radioactive iodine. While iodine mainly collects in the thyroid gland, where it has a half-life of about 100 days, there are iodine receptors in other parts of the body - such as bone, kidney, spleen, and reproductive organs – where it has much shorter half-lives. 
What do public health authorities recommend to limit absorption of radioactive iodine?
In emergencies large doses of stable iodine are used to flood the bloodstream to help prevent the thyroid gland from taking up radioactive iodine, since raising the concentration of stable iodine in circulation would make it far less likely that the gland will absorb much radioactive iodine. These very high doses of stable iodine are not intended for daily use and may be hazardous for some people. 
What are good sources of stable iodine?
High dose potassium iodide in pills or capsules is the form given when public health authorities distribute iodine to populations at risk of radiation exposure.
Seafood, including fish and seaweed, and dairy products (due to fortified feed and iodine-based sanitation products used in milking, but the use of both of these are reportedly on the decline) provide significant amounts of stable iodine in the diet, but not enough to block the entry of radioactive iodine into the thyroid gland when an emergency is present.
Seaweeds have a flaw in that they typically contain less that 1% iodine by dry weight and also may contain undesirable heavy metals that are considered safe at recommended levels. In other words, don’t rely on kelp or dulse for much more than the RDA, or at most the UL (see above), because the levels of arsenic or other metals might also rise enough to become potential health hazards.
Iodized salt also provides essential iodine that can compete with radioactive iodine; a 1.5 gram serving (about ¼ teaspoon) provides about 71 mcg of iodine. Processed foods rarely use iodized salt, which must be declared on the label. However, there is only enough iodine in iodized salt to nourish the thyroid gland, not enough to exclude radioactive iodine from entering and causing harm. [1, 3]
Large doses of stable iodine reportedly protect the thyroid from accumulating radioactive iodine by competitively excluding that form because the thyroid is already full of stable iodine.
How much iodine should I take, and when?
“According to the FDA, the following doses are appropriate to take after internal contamination with (or likely internal contamination with) radioactive iodine:
•Adults should take 130 mg (one 130 mg tablet OR two 65 mg tablets OR two mL of solution).
•Women who are breastfeeding should take the adult dose of 130 mg.
•Children between 3 and 18 years of age should take 65 mg (one 65 mg tablet OR 1 mL of solution). Children who are adult size (greater than or equal to 150 pounds) should take the full adult dose, regardless of their age.
•Infants and children between 1 month and 3 years of age should take 32 mg (½ of a 65 mg tablet OR ½ mL of solution). This dose is for both nursing and non-nursing infants and children.
•Newborns from birth to 1 month of age should be given 16 mg (¼ of a 65 mg tablet or ¼ mL of solution). This dose is for both nursing and non-nursing newborn infants.
A single dose of KI protects the thyroid gland for 24 hours. A one-time dose at the levels recommended in this fact sheet is usually all that is needed to protect the thyroid gland. In some cases, radioactive iodine might be in the environment for more than 24 hours. If that happens, local emergency management or public health officials may tell you to take one dose of KI every 24 hours for a few days. You should do this only on the advice of emergency management officials, public health officials, or your doctor. Avoid repeat dosing with KI for pregnant and breastfeeding women and newborn infants. Those individuals may need to be evacuated until levels of radioactive iodine in the environment fall.” 
Taking a higher dose of KI, or taking KI more often than recommended, does not offer more protection and can cause severe illness or death.
When public health or emergency management officials tell the public to take KI following a radiologic or nuclear event, the benefits of taking this drug outweigh the risks. This is true for all age groups. Some general side effects caused by KI may include intestinal upset, allergic reactions (possibly severe), rashes, and inflammation of the salivary glands.
When taken as recommended, KI causes only rare adverse health effects that specifically involve the thyroid gland. In general, you are more likely to have an adverse health effect involving the thyroid gland if you
•take a higher than recommended dose of KI,
•take the drug for several days, or
•have pre-existing thyroid disease.
Newborn infants (less than 1 month old) who receive more than one dose of KI are at particular risk for developing a condition known as hypothyroidism (thyroid hormone levels that are too low). If not treated, hypothyroidism can cause brain damage. Infants who receive KI should have their thyroid hormone levels checked and monitored by a doctor. Avoid repeat dosing of KI to newborns.
Limitations of large dose stable iodine 
• Stable iodine may protect the thyroid if taken at the right time and at a high dose. However, it is not proven to protect other parts of the body and stable iodine does not block the radioactive form from the body.
• That limited protection for the thyroid from taking high dose stable iodine only lasts for about 24 hours.
• Stable iodine does not protect the body from any other radioactive materials that may be in the environment.
People in other countries, such as the United States, will not experience the amount of radiation that those in the immediate vicinity of the nuclear plants could. Since the high-dose potassium iodide (KI) works to protect the thyroid gland only for about 24 hours, it is probably wise to wait for a specific event to actually take that pill. Meanwhile, a reasonable daily dose of iodine within tolerable upper limit (UL) levels (between 200-1,100 mcg; see above) is probably wise to maintain some level of protection against potential low level increases in airborne radioactive iodine and to assure adequate levels in the thyroid so it’s not so anxious to take in whatever form is circulating in the body and there is plenty of the stable form available. High-dose seaweeds like kelp and dulse may also provide heavy metals in excess of safety recommendations and should only be taken at levels closer to the RDA.