Patients greater than or equal to 18 years of age, with well-differentiated, non-medullary thyroid cancer, including papillary (including follicular-variant), follicular, and Hurthle cell.

Patients who recently have undergone a total or near-total thyroidectomy and are scheduled to undergo initial 131I diagnostic studies and ablation.

Patients must have evidence of residual thyroid tissue after thyroid surgery confirmed by ultrasound or other imaging technique (e.g., a technetium [Tc 99] pertechnetate or thallium scan).

Patients who have undergone thyroidectomy or other thyroid surgery at least 6 weeks prior to enrollment.

All patients who have been sustained on maintenance THST for at least 4 weeks, but not longer than 12 weeks after thyroidectomy.

All patients who have confirmed serum TSH levels less than or equal to 0.5 mU per liter within 7 days prior to the first Thyrogen dose, and prior to randomization.

Female patients of childbearing age must have a negative serum human chorionic gonadotropin (HCG) pregnancy test prior to entering the study and within 5 days of any 131I administration and must be following an approved method of contraception.

Patients who are committed to following the protocol requirements as evidenced by providing written informed consent.

Patients for whom a 131I scan is currently contraindicated because withdrawal from THST is not an option due to pituitary dysfunction or other compelling medical reasons are excluded.

Patients should not have a concurrent major medical disorder (e.g., documented cardiac disease, debilitating cardiopulmonary disease, advanced renal failure, advanced liver disease or advanced pulmonary disease) who may be too ill to adequately comply with the requirements of this study.

Patients with non-thyroidal conditions known to effect 131I uptake (e.g., congestive heart failure, renal failure) are excluded.

Patients should not have undergone any intravenous water soluble radiographic contrast administration within the previous 4 weeks.

Patients should not have received intrathecal or cholecystographic iodinated contrast agent administration within 3 months prior to enrollment.

Patients should not be taking drugs that affect thyroid or renal function (e.g., renal drugs, lithium, or corticosteroids).

Patients should not be participating in another investigational drug study or in such a study within 30 days of their enrollment in this study.

No patients with a recent history of alcoholism or drug abuse, severe emotional behavioral or psychiatric problems who, in the opinion of the investigator, would not be able to comply with the requirements of this study.