I swallowed a capsule this morning and am currently a
walking source of gamma radiation. I must stay away from people for three days,
from children, pregnant women, and small animals for five. I laid in a supply
of groceries and figure I’ll have a few quiet days.
Last spring, when I had my annual physical (thank you,
Obamacare!), my doctor ordered routine blood tests. He called me when he got
the results: definitely too much thyroid hormone, which had made my pituitary
gland shut down. He gave me an order for a thyroid scan and uptake and referred
me to an endocrinologist.
The thyroid gland makes a couple of hormones called
triiodothyronine and thyroxine, both of which contain iodine, which isn’t
processed in other places in the body. These hormones regulate a lot of things,
including how fast your body operates. More hormone means faster heart rate,
faster digestion, along with affecting bone growth and probably a lot of other
things. That can damage your heart and cause other problems. I have a mild tremor,
which may be a result of the hyperthyroidism. I suggested to the
endocrinologist that I would be happy to have the weight-loss symptom, but she
said it doesn’t always work like that.
The specificity of iodine to the thyroid gland suggested to
the people investigating radionuclides for medicine sixty or so years ago, that
iodine isotopes might be useful in imaging the thyroid and dealing with its
maladies. Iodine, conveniently, has 37 isotopes to
choose from, emitting different kinds of radiation with different half-lives.
The stable isotope, which is put into iodized salt to prevent iodine
deficiency, is I-127. The radioactive iodine isotopes are formed as fission
products in nuclear reactors; their mass (like that 127) is about half that of
uranium or plutonium. They can also be formed in cyclotrons and other
accelerators.
How to measure the speed with which the thyroid is working?
Feed someone an iodine isotope, and then scan the thyroid, located conveniently
just under the skin at the base of the neck, with a radiation counter. That’s
the scan and uptake that the doctor ordered.
For the scan, I took a standard-size capsule containing 100
microcuries of I-123, in the form of sodium iodide, NaI, which is very much like
table salt. Someone with literary leanings might find it amusingly
self-referential that some radiation detectors use NaI. NaI to detect NaI.
I didn’t feel anything; didn’t expect to. I went home for
six hours to allow my body to move the iodide from my stomach to my bloodstream
to my thyroid, where it would stick for a while. When I went back, I lay down
in the detector apparatus, and the technician positioned the detector in
several places around my thyroid. I went back two days later for more counting.
The endocrinologist got the scan and uptake; the scan shows
where the radioiodine accumulates, and the uptake says how fast. She then did
an ultrasound that confirmed that the whole left side of my thyroid had turned
into an excited nodule. She wanted more detailed blood tests to make sure. Then
we met to talk about the way forward.
The hyperactivity could be stopped with medication,
radiation, or surgery. I had researched the processes and have a friend who had
her thyroid removed, although her surgery was several decades ago. I’m not fond
of fussing with medication, would rather have a real solution to a problem. The
doctor wasn’t wild about it either. I looked at both options closely. It was
possible that the radiation would kill only the nodule, leaving some thyroid
function, perhaps close to normal. Surgery would remove the entire thyroid. I’m
not fond of general anaesthetic, either. I’ve looked at the chemistry of that,
and my personal experiences have not been pleasant. Take a pill instead? Not a
hard decision for me.
There are downsides to radiation: the radiation could kill
the whole thyroid, but that would not be different from the outcome of surgery.
Either way, I would have to take thyroid hormone the rest of my life, which is
not that big a deal.
So I swallowed a pill this morning containing 12 millicuries
(milli- where the scan was micro-; a thousand times as much) of I-131, the
isotope everyone worried about from Fukushima. Didn’t feel anything this time,
either. The staff in Radiology were very helpful and answered all my questions.
Thanks particularly to Rick and David. They seemed to be pleased to have a
patient who understood the science. It’s a fair bit of radiation: 160 millisievert
whole-body exposure, and 350 sievert to my thyroid. Again, a factor of a
thousand in the units.
Comparing those measurements to xkcd’s graphic on radiation exposure shows
that my whole-body dose is about equivalent to the worst exposure of workers at
Fukushima. It probably adds a percent or two to my chance of getting cancer,
but cancer may take decades to develop, and I am getting older. But my thyroid
is getting zapped.
The beta radiation, which is doing the work, is pretty much
confined to my thyroid, but there is also gamma radiation that gets past my
skin. That’s why I’m not supposed to be around people. The biological
half-life, the time it takes for my body to eliminate the I-131, is much
shorter than the isotope half-life. I will pee out 80% of it during the next
two days. Some will be eliminated in my saliva and sweat. So keeping that away
from other people is a good idea, too.
After ten half-lives, a radioisotope is down to a millionth
of its original amount, and I take that as being gone for all practical
purposes. So whatever I-131 I may spread around the house, a small amount now,
will be gone for sure in three months (8-day half-life times 10 half-lives,
rounded up).
I will have blood tests every four weeks and see the
endocrinologist again in six months. The effects of radioiodine thyroid therapy
are slow.
I wrote the above yesterday, Tuesday, June 17. It seems to
me on the morning of June 18 that my tremor is already less, but I do not
discount the possibility of psychosomatic improvement. I’ve also noticed that,
as I read sources I might link in this post, that I’ve had many of the symptoms
of hyperthyroidism. They weren’t severe; the tremor was the most noticeable,
and I tend to take a broad definition of normal when considering how my body
functions.
The graphic is from a commercial site, so I won’t link. My
capsule was transparent, and the contents were grayish.
Cross-posted at Nuclear Diner.
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