Radioactive Isotopes Reveal Physiology
icki L. arrived early at the nuclear medi-
cine department of the health center. As
she sat in an isolated cubicle, a doctor in
full sterile dress approached with a small metal
canister marked with warnings. The doctor care-
fully unscrewed the top, inserted a straw, and
watched as the young woman sipped the fluid
within. It tasted like stale water but was a solution
containing a radioactive isotope, iodine-131.
Vicki’s thyroid gland had been removed
three months earlier, and this test was to deter-
mine whether any active thyroid tissue remained.
The thyroid is the only part of the body to metab-
olize iodine, so if Vicki’s body retained any of the
radioactive drink, it would mean that some of
her cancerous thyroid gland remained. By using
a radioactive isotope, her physicians could detect
iodide uptake using a scanning device called a
scintillation counter (F
g. 2A). ±igure 2B illustrates
iodine-131 uptake in a complete thyroid gland.
The next day, Vicki returned for the scan,
which showed that a small amount of thyroid tis-
sue was left and functioning. Another treatment
would be necessary. Vicki would drink enough
radioactive iodide to destroy the remaining tis-
sue. This time, she drank the solution in an iso-
lation room lined with paper to keep her from
contaminating the ²
oor, walls, and furniture. The
same physician administered the radioactive
iodide. Vicki’s physician had this job because his
thyroid had been removed many years earlier,
and therefore, the radiation couldn’t harm him.
After two days in isolation, Vicki went home
with a list of odd instructions. She was to stay
away from her children and pets, wash her cloth-
ing separately, use disposable utensils and plates,
and ²
ush the toilet three times each time she used
it. These precautions would minimize her contami-
nating her family—mom was radioactive!
Iodine-131 is a medically useful radioactive
isotope because it has a short half-life, a measure-
ment of the time it takes for half of an amount
of an isotope to decay to a nonradioactive form.
The half-life of iodine-131 is 8.1 days. With the
amount of radiation in Vicki’s body dissipating
by half every 8.1 days, after three months hardly
any would be left. If all went well, any remaining
cancer cells would leave her body along with the
radioactive iodine.
Isotopes of other elements have different
half-lives. The half-life of iron-59 is 45.1 days; that
of phosphorus-32 is 14.3 days; that of cobalt-60 is
5.26 years; and that of radium-226 is 1,620 years.
A form of thallium-201 with a half-life of 73.5
hours is commonly used to detect disorders in
the blood vessels supplying the heart muscle or
to locate regions of damaged heart tissue after a
heart attack.
Gallium-67, with a half-life of 78 hours, is
used to detect and monitor the progress of cer-
tain cancers and inflammatory illnesses. These
medical procedures inject the isotope into the
blood and follow its path using detectors that
record images on paper or F
Radioactive isotopes are also used to assess
kidney function, estimate the concentrations of
hormones in body ²
uids, measure blood volume,
and study changes in bone density. Cobalt-60 is
a radioactive isotope used to treat some cancers.
The cobalt emits radiation that damages cancer
cells more readily than it does healthy cells.
Scintillation counters detect radioactive isotopes.
) A scan of the thyroid gland
twenty-four hours after the patient receives
radioactive iodine. Note how closely the scan
in (
) resembles the shape of the thyroid
gland shown in (
previous page 84 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 86 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off