Wasn't iodine used for treating radiation poisoning

The short version: yes, stable iodine (potassium iodide) blocks radioactive iodine from entering the thyroid, but it is a narrow countermeasure—it only protects the thyroid, does not treat whole-body radiation poisoning, and timing and dosing are critical.

You are right that iodine has been used for radiation exposure. The mechanism is straightforward: the thyroid cannot distinguish between stable iodine (I-127) and radioactive iodine (I-131). If you flood the gland with stable iodine beforehand, it becomes saturated and refuses the radioactive version. A 2016 study in healthy Japanese adults showed that a single low dose of inorganic iodine suppressed thyroidal uptake of radioactive iodine from a median of 13% down to 3%, a drop of about 77% [2]. A 1997 study in pregnant chimpanzees confirmed the same principle works in pregnancy, reducing fetal thyroid uptake to negligible levels without harming maternal or fetal thyroid function [1]. These are real mechanistic studies with measured outcomes, not theoretical models.

But the limits matter and are almost never explained to the public. First, timing: potassium iodide must be taken within hours of exposure—ideally before or immediately after—to be effective. After the radioactive iodine has already entered the thyroid, taking more iodine can actually worsen the problem by providing more substrate for hormone synthesis in an already-contaminated gland. Second, it only protects the thyroid gland. Radioactive iodine is concentrated there, but the rest of the body still absorbs the radiation; potassium iodide does nothing for cesium-137, strontium-90, or any other isotope released in a nuclear accident. A 2012 study of the Fukushima fallout estimated that countermeasures including iodine distribution reduced thyroid doses for infants from about 67 mSv to about 12 mSv, a meaningful reduction, but those estimates also assumed rapid action and did not address non-thyroid cancers [23].

The risk is that the public sees "iodine protects against radiation" and reaches for high-dose supplements days or weeks after an event, when it is too late, or takes doses far above the recommended 130 mg for adults. That can cause its own harm: iodine-induced thyroiditis, hyperthyroidism in predisposed individuals, and suppression of thyroid function in infants exposed through breast milk from mothers taking high doses. The 2017 study of lactating mothers on 50 mg per day of potassium iodide found that infants had urinary iodine levels of 15 mg/L, about 100 times normal, with some showing elevated TSH [22]. Too much stable iodine is not harmless.

My call: potassium iodide works for its specific narrow indication—blocking thyroid uptake of radioactive iodine if taken promptly—but it is not a general "radiation treatment," dosing matters, and taking it without actual exposure causes real endocrine risks. Confidence: high on the mechanism and the human data showing uptake suppression; moderate on the narrow scope being communicated honestly (it almost never is).