Preparation of patients with thyroid hormone withdrawal and dietary iodine restriction is essential for optimal 131I imaging or therapy. For patients currently on thyroid hormone suppression therapy, the T4 is withdrawn for 4 to 6 weeks with a substitution of triiodothyronine (T3) for 2 to 3 weeks. Then T3 is discontinued and the patient is placed on a low-iodine diet (< 50 pg/d) for 1 to 2 weeks prior to the 131I uptake and scan study. This regimen will allow 90% of patients to achieve a serum TSH concentration > 30 mU/L to stimulate iodide uptake by residual thyroid tissue.
At the University of California-San Francisco, an outpatient dose of 30 to 50 mCi 131I is used to ablate residual thyroid tissue at 6 to 12 weeks after operation if focal uptake is detected in the thyroid bed on a 2 to 3 mCi 131I diagnostic scan. Conversely, patients with residual tumor or distant metastases are treated with 100 to 200 mCi 131I with radiation isolation either as outpatients or in the hospital. In either instance, a neck and body scan is obtained 1 week after the treatment dose of 131I to detect additional areas of uptake that may suggest metastatic disease.
It usually takes 6 to 12 months after initial therapy for 131I to achieve maximal effects. Serum TSH and Tg concentrations may be measured every 3 months during this period. The 131I diagnostic scan is repeated at the end of the year after adequate thyroid hormone withdrawal and dietary iodine restriction. If there is uptake in the neck or the body or if serum Tg is > 5 ng/mL when the patient is hypothyroid, a repeat treatment with 100 to 200 mCi of 131I should be given.
After the first negative 131I scan, the patient may be followed periodically with a recombinant human TSH (rhTSH) protocol rather than T4 withdrawal (see below).
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