Iodine pills, Japan and Science

In the midst of the tragedy in Japan, experts are concerned with the possibility of multiple nuclear meltdowns which could exacerbate the situation tremendously. In particular the Fukushima Daini plant has been a major concern due to the inability of its cooling system to keep fuel rods exposed. To prevent radiation sickness in the event of a meltdown, Japanese officials are preemptively distributing potassium-iodine pills to those in the immediate area. As a medical student interested in radiobiology, I was immediately curious why iodine was the pill of choice and not some other salt.

Turns out there is an interesting physiological rationale behind this:

In the event of a nuclear meltdown, the most dangerous radioactive material to be expelled from a reactor is Iodine-131. I-131 is released as a gas, making it easy to enter our bodies and preferentially be taken up by the thyroid. In addition I-131 has a very short half-life (~1 week), meaning it can cause some serious damage very quickly (i.e. thyroid cancer).

Once in our body, I-131 is shuttled to our thyroid due to a special receptor present on follicular cells of the thyroid gland (follicular cells = work horse cell of the thyroid). These special receptors are known as Sodium/Iodine(Na/I) symporters or NIS.

Essentially, it's tough for iodine to get into thyroid cells due to issues with its concentration gradient, charge (its present as I- in the body, aka iodide) and lack of dedicated I- transporters. Luckily, our body can couple I- transport to the transport of sodium, making it energetically efficient to transport I- into follicular cells. This form of transport is called 'symport' and is the driving force for the majority of ion transport in the body. NIS mediated iodine transport facilitates this symport and is a major site of regulation with regards to iodine uptake.

NIS can take up most if not all forms of iodine, including I-131. Physiologically, this presents a serious problem: if I-131 is the most readily available form of iodine in the body (as it would be after a meltdown), our thyroid would unknowingly take it up and cause glandular suicide!

Schematic illustration of a thyroid follicular cell showing the key aspects of thyroid iodine transport and thyroid hormone synthesis (courtesy of International Journal of Endocrinology and Metabolism)

That's where iodine pills come in. Iodine pills create a way for iodide (I-) to saturate the NIS co-transporter, effectively making it impossible for other forms of iodide to enter the thyroid. So, if one were to take KI pills before a meltdown, there would be no way for I-131 to enter the thyroid in the event of a meltdown. Accordingly, I-131 would be filtered by the kidney and excreted into the urine, effectively saving the thyroid from damage.