With regard to radioiodine therapy,[7] therapy with iodine 131 is indicated for most patients with distant
metastatic disease. It may also be useful in the therapy for locally invasive neck disease, as well as for
some cervical nodal metastases, particularly those that are not amenable to surgery. The routine
radioiodine ablation of remnant thyroid tissue in patients with low-risk cancer has not been shown to
enhance the survival rate in most studies, and its effect on recurrences is controversial.[8,9] [10] Although
many experts recommend ablation of remnant thyroid tissue for almost all patients after a total
thyroidectomy, the decision to use radioiodine should be individualized and based on clinical experience.
Quantitative radioiodine scanning may be indicated in some patients before therapy with radioactive
iodine. If metastatic disease is present, a large dose (generally 100-200 mCi) of iodine 131 is
administered. For ablation of remnant thyroid tissue, a dose of 29.9 to 100 mCi of iodine 131 is usually
employed. Doses of 30 mCi or greater require hospitalization because of the need for patient isolation, so
that outpatient doses of 29.9 mCi are now commonly used, especially for patients with low-risk cancer.
Successful ablation is lower with doses of 29.9 mCi compared with higher doses, especially with larger
thyroid remnants, but the need for hospitalization is eliminated. Outpatient doses of 29.9 mCi may be
repeated, however. Thus, no firm recommendation can be made with regard to the optimal radioiodine
dose.

Prior to radioiodine body scanning and therapy, levothyroxine must be withheld or withdrawn for
approximately 4 to 6 weeks to maximize thyrotropin stimulation of the remaining thyroid tissue. The
resulting hypothyroidism is tolerated poorly by some patients, and it may be attenuated by the
administration of liothyronine sodium, for the first 3 to 4 weeks after levothyroxine therapy is
discontinued, ensuring a shorter duration of hypothyroidism. Approximately 2 weeks after treatment with
liothyronine is withdrawn, a serum thyrotropin level is obtained to confirm hypothyroidism; a thyrotropin
value of greater than 30 mU/L prior to radioiodine therapy is optimal for scanning and therapy. Some
clinicians recommend a low-iodine diet for at least 2 weeks prior to radioiodine administration.
Pregnancy must be excluded in women of childbearing age prior to radioiodine administration.