Like chromium-51, iodine is a versatile tracer element. It is used to
determine blood volume, cardiac output, plasma volume, liver activity,
fat metabolism, thyroid cancer metastases, brain tumors, and the size,
shape, and activity of the thyroid gland.
[Illustration: _A linear photoscanner produced these pictures of (A) a
normal thyroid, (B) an enlarged thyroid, and (C) a cancerous thyroid._]
Because of its unique connection with the thyroid gland, iodine-131 is
most valuable in measurements connected with that organ. Thyroxin, an
iodine compound, is manufactured in the thyroid gland, and transferred
by the blood stream to the body tissues. The thyroxin helps to govern
the oxygen consumption of the body and therefore helps control its
metabolism. Proper production of thyroxin is essential to the proper
utilization of nutrients. Lowered metabolism means increased body
weight. Lowered thyroid activity may mean expansion of the gland,
causing one form of goiter.
Iodine-131 behaves in the body just as the natural non-radioactive
isotope, iodine-127, does, but the radioactivity permits observation
from outside the body with some form of radiation counter. Iodine can
exist in the body in many different chemical compounds, and the counter
can tell where it is but not in what form. Hence chemical manipulation
is necessary in applying this technique to different diagnostic
procedures.
The thyroid gland, which is located at the base of the neck, is very
efficient in trapping inorganic iodide from the blood stream,
concentrating and storing the iodine-containing material and gradually
releasing it to the blood stream in the form of protein-bound iodine
(PBI).
One of the common diagnostic procedures for determining thyroid
function, therefore, is to measure the percentage of an administered
dose of ¹³¹I that is taken up by the gland. Usually the patient is given
a very small dose of radioactive sodium iodide solution to drink, and
two hours later the amount of iodine in the gland is determined by
measuring the radiation coming from the neck area. In hyperthyroidism,
or high thyroid gland activity, the gland removes iodide ions from the
blood stream more rapidly than normal.
[Illustration: Screening test for Hyperthyroidism