Evaluation of Thyroid Stunning From a Diagnostic Dose of I-123

This study plans to learn more about a medical problem called thyroid stunning. Thyroid
stunning is a temporary decrease in the function of thyroid cells after small doses of
radioiodine, a radioactive drug that is commonly used to treat thyroid cancer. Thyroid
stunning can be an important medical problem in patients who have thyroid cancer that has
spread beyond the thyroid gland (metastases). In patients with thyroid cancer metastases,
some researchers believe it is good to give the maximum safe dose of radioiodine in order to
have the greatest chance of destroying the cancer. The maximum safe dose is the highest dose
of radioiodine that a patient can safely receive, and this dose is unique to each person.
This special personalized dose is determined by first doing a dosimetry study. Dosimetry
study is a planning study that calculates the safest dose that the patient can receive.
Currently, it is not known whether the small dose of radioiodine that is used for the
dosimetry study may stun the cancer cells and make the thyroid cancer treatment less
effective.

There are two important types of radioiodine. I-123 is a form of radioiodine that is used to
take pictures of the thyroid gland. I-131 is a form of radioiodine that is used to treat
thyroid cancer. It is hoped that this study will produce important information that may be
the first step in resolving the thyroid stunning question. Investigators will use I-123, a
form of iodine that is currently not known to cause thyroid stunning, before thyroid cancer
treatment with I-131. Investigators will try to prove that I-123 does not cause thyroid
stunning and does not make the thyroid cancer treatment less effective.

Patients are being asked to be in this research study because they have differentiated
thyroid cancer, and they have no evidence of thyroid cancer that has spread to the other
parts of their body. As a result, those patients thyroid cancer treatment with radioiodine
will be a smaller amount than what is needed to treat patients with thyroid cancer that has
spread to other parts of the body. Thyroid stunning is not a medical problem for patients
like these. However, by participating in this study, the information investigators gather
from those patients treatment of thyroid cancer may help those patients with thyroid cancer
metastases.

Inclusion Criteria:

- Patient must be 21-years-old or greater.

- Patient must be status post near total thyroidectomy for differentiated thyroid cancer
without known distant metastases and who are planning to undergo routine remnant
thyroid tissue ablation with I-131.

- Patients must qualify for thyroid ablation with I-131.

Exclusion Criteria:

- Women who are pregnant or breastfeeding.

- Prior bovine TSH use.

- Known metastatic thyroid cancer.

- History of cardiovascular disease that may adversely affect patient participation at
the discretion of the primary investigator.

- Patients on hemodialysis.

- Patients with acute serious illnesses at the discretion of the primary investigator.