Preparation for Ablative 131I Therapy

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The multidisciplinary team has to decide whether or not ablative 131I therapy will be given, and if so, at which time. If there is only a low risk of cancer-specific mortality and a low risk of relapse according to the prognostic score, there may be no indication. However, ablative 131I therapy may still be favored in that it improves the diagnostic accuracy of serum thyroglobulin and 131I total body scan for remaining tumor tissue after total ablation of the thyroid gland [22]. The internist-endocrinologist can play an important role in the preparation for 131I treatment. For the efficacy of 131I ablation depends on how much of the administered 131I dose is taken up by remaining thyroid tissue, which is greatly stimulated by high serum TSH and low serum inorganic iodide concentrations. The endocri-nologist should see to it that at the time of 131I ablation the serum TSH is elevated (arbitrarily up to at least 25-30 mU/L) and the serum iodide is definitely not high but preferably low. After (near) total thyroidectomy simply withholding T4 treatment will ensure high TSH levels after 4-6 weeks, the usual time for 131I ablation. If for any reason the interval between surgery and planned 131I treatment is longer, it is prudent to start T4 treatment in order to avoid severe hypothyroidism in the patient. In such cases, T4 treatment should be discontinued 4 weeks before the planned 131I therapy. Alternatively, after discontinuation of T4 treatment one may administer T3 for 3 weeks (25 |mg daily in week 1, 2 x 25 |mg daily in week 2,3 x 25 |mg daily in week 3), then stop T3 for 2 weeks and administer the therapeutic dose of 131I in week 6. The advantage of this alternative scheme is that the patient may suffer less from hypothyroid symptoms and signs. A recent study, however, questions the benefits from using T3 in preparing patients for 131I therapy [23]. Serum TSH concentrations of more than 30mU/L were reached 18 days after thyroidectomy and 22 days after withdrawal of suppressive thyroxine treatment in more than 95% of patients, with minimal symptoms of hypothyroidism. These authors recommend serum TSH measurements twice weekly, starting 10 days after thyroidectomy or T4 withdrawal.

With regard to iodine intake, patients are usually instructed to avoid iodine-containing medications and iodine-rich foods for 10 days prior to 131I therapy. The value of a stringent low iodine diet has been questioned as similar ablation rates were observed in patients on a low iodine diet and on a regular diet [24]. However, a recent study reports a significantly higher successful ablation rate after a low iodine diet than in controls (65% versus 48%) [25].A 2-week low iodine diet will also induce iodine deficiency in patients who continue their levothyroxine medication [26].

The patient should be informed of possible damage of 131I therapy to radiation-sensitive tissues. Usually this is done by the nuclear medicine physician, but the endocrinologist may play a role as well. Many patients in their reproductive years are concerned with gonadal damage. The standard advice is to refrain from pregnancy and not father a child during the first 4 to 6 months after 131I therapy. About 20-30% of women experience transient amenorrhea or menstrual irregularities in the first year after treatment [27,28]. Apart from a greater miscarriage rate in the first year 131I therapy seems to have no effect on fertility or the outcome of subsequent pregnancies, but menopause occurs on average 1.5 year earlier [29]. In men, spermatogenesis can be transiently suppressed associated with a rise of serum FSH, related to the total amount of 131I [30]. The risk of permanent male infertility is very low; in men likely to receive a cumulative dose of >17 GBq sperm banking may be considered.

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