irtue of antithyroid drug treatment. At ten year follow-up, 40% were euthyroid and 60% hypothyroid. A problem with low-dose therapy is that about 25% of patients require a second treatment and 5% require a third. Although this approach reduces early hypothyroidism, it does so at a cost in time, money and patient convenience (Fig. 2). To answer these problems, patients can be re-treated, if need be, within six months, and propranolol and antithyroid drugs can be given between 131-I doses if needed. Unfortunately, experience shows that even low-dose 131-Itherapy is followed by a progressive development of hypothyroidism in up to 40 – 50% of patients ten years after therapy[ 54- 57].
|Table 6. LOW Dosage Schedule for 131-I Therapy|
Thyroid wt. in gms.
uCi retained/gm thyroid at 24h
Thyroid rads, avg.
Impressed by the need to retreat nearly a third of patients, a “Moderate Dose Protocol” was developed Table -6). This is a fairly conventional program with a mean dose of about 9 mCi. The 131-I dosage is related to gland weight and RAIU and is increased as gland weight increases. The calculation used is as follows: uCi given = (estimated thyroid weight in grams) X (uCi/g for appropriate weight from Table) / (fractional RAIU at 24 hours) (For readers who may find difficult the conversion of older units in Curies, rads, and rems to newer units of measurement, see Table -7.)
Table 6. MODERATE Dosage Schedule for 131-I Therapy
Thyroid wt. in gms.
uCi retained/gm thyroid at 24h
Thyroid rads, avg.
|Table 7. Conversion of International Units of Measurement|
|Becquerel (Bq)||2.7 x 10 -11Curies (1mCi=37MBq, 100mCi= 3.7GBq)|
|Gray (Gy)||100 rads ( 1 rad= 0.01Gy)|
|Sievert (Sv)||100 rems (1 rem = 0.01 Sv)|
Probably it is wise to do uptakes and treatment using either capsules or liquid isotope for both events. Rini et al have reported that RAIU done with isotope in a capsule appears to give significantly lower values (25 – 30% lower) than when the isotope is administered in liquid form, and this can significantly influence the determination of the dosage given for therapy(57.2). Berg et al report using a relatively similar protocol (absorbed doses of 100-120 Gy) and that 93% of their patients required replacement therapy after 1-5 years [57.1].
More recently two more studies have presented methods for more accurately delivering a specific radiation dose to the thyroid, and report curing up to 90% of patients, with low incidence of recurrence or hypothyroidism(57.3, 57.4). Franklyn and co-workers have recently analyzed their data on treatment of 813 hyperthyroid patients with radioactive iodide and corroborate many of the previously recognized factors involved in response. Lower dose (in this case 5 mCi), male gender, goiters of medium or large size and severe hyperthyroidism were factors that were associated with failure to cure after one treatment. They suggest using higher fixed initial doses of radioiodine for treating such patients (58.2), as do Leslie et al(58.4). Santos et al (58.4) compared fixed doses of 10 and 15mCi and found no difference in utcome at 12 months post treatment. These suggest a standard 10mCi dose, with the larger dose reserved for larger glands.
Planned thyroid partial or complete ablation-All attempts to induce euthyroidism by a calculated moderate dose protocol end up with some patients hypothyroid, and others with persistent hyperthyroidism requiring further treatment. At this time many physicians giving 131-I therapy make no attempt to achieve euthyroidism, and instead use a dose sufficient to largely destroy the thyroid, followed by T4 replacement therapy . For example, a dose is given that will result in 7-20 mCi retained at 24 hrs, which is intended to induce hypothyroidism, accepting that in some (or many) patients this will ablate the thyroid completely. They argue that this is realistic and preferable since it offers 1) near certainty of prompt control, 2) avoids any chance of persistent or recurrent disease, 3)there is no benefit in having residual thyroid tissue, and 4) hypothyroidism is inevitable in 