One organ, one element
The thyroid needs iodine to make thyroxine, and it is the only organ in the body that concentrates the element. It pulls iodine from the blood and stores it.
The gland has no way to distinguish iodine 131 from the stable iodine 127 it normally uses. They are chemically identical. So a release of iodine 131 is delivered, by the body’s own machinery, to a target weighing about twenty grams.
⚠️ This is what makes an eight-day isotope dangerous. Spread evenly through a 70-kilogram person the dose would be negligible. Concentrated in the thyroid it is not.
The same logic runs in reverse. Because uptake is so specific, a measured dose can be used to destroy thyroid tissue and leave everything else alone.
The tablets
Potassium iodide works by saturation. Flooding the thyroid with stable iodine fills its capacity, so radioactive iodine arriving afterwards is not taken up and is excreted instead.
Timing decides whether it works. Taken a few hours before exposure it blocks roughly 90 % of uptake. Taken at the same time, still most of it. Taken a day later, almost nothing — the gland has already absorbed what it was going to.
⚠️ The tablets protect the thyroid and nothing else. They do nothing against caesium 137, strontium 90, or any other component of a release. Stockpiling them against a general nuclear threat mistakes what they are for.
Countries with reactors distribute them in advance to households within a set radius, precisely because the window is too short to organise a handout after an accident.
What happened after Chernobyl
The route was milk.
Iodine 131 fell on pasture across Belarus, Ukraine and western Russia. Cows ate the grass and it passed into their milk within a day. Children drank the milk.
A child’s thyroid is smaller than an adult’s and its cells divide faster, so the same intake produces a much larger dose and a much higher cancer risk. Soviet authorities were slow to restrict milk and slow to distribute iodine tablets; in many areas neither happened.
UNSCEAR attributes around 5,000 thyroid cancers to the accident among people who were children or adolescents at the time. Thyroid cancer is usually treatable and mortality has been low, but the number is the clearest single health consequence of the accident.
⚠️ Poland, which distributed iodine tablets to over ten million children within days, recorded no comparable rise.
The oldest targeted therapy
Radioactive iodine has been used to treat thyroid disease since 1941, which makes it one of the first targeted cancer treatments of any kind.
For an overactive thyroid the patient swallows a capsule. The gland absorbs the iodine, the beta radiation destroys part of the tissue over weeks, and hormone production falls to normal — or below it, in which case the patient takes thyroxine for life, which is a manageable outcome.
For thyroid cancer a larger dose is given after surgery to destroy any remaining thyroid cells, including any that have spread. Because only thyroid tissue takes up iodine, metastases anywhere in the body are reached by a treatment that ignores everything else.
⚠️ Patients are radioactive afterwards. Depending on the dose they may be kept in a shielded room for a few days and advised to keep their distance from children and pregnant women for a week or two.
Eight days, and what that means
The half-life of 8.02 days sets everything about how iodine 131 is handled.
It is short enough that a release is over quickly: after three months essentially none remains, which is why the exclusion zones around Chernobyl and Fukushima are defined by caesium rather than iodine.
It is short enough that the isotope cannot be stockpiled. Hospitals order it for a specific patient on a specific day; a delivery delayed by a week has lost half its activity.
⚠️ And it is long enough to travel. Eight days is time for a plume to cross a continent and for contaminated milk to reach the shops, which is why food restrictions after an accident come first and fastest for dairy.