ert to hyperthyroidism when the KI is discontinued. Over the years some effort was made to refine the calculation. Account was taken of uptake, half-life of the radioisotope in the thyroid, concentration per gram, and so on, but it is evident that the result in a given instance depends on factors that cannot be estimated precisely [46, 47]. One factor must be the tendency of the thyroid to return to normal if a dose of radiation is given that is large enough to make the gland approach, for a time, a normal functional state.In many patients, “cure” is associated with partial or total thyroid ablation.
Although we, and many endocrinologists, attempt to scale the dose to the particular patient, some therapists believe it is futile, advocate giving up this attempt, and provide a standard dose giving up to 10000 rads to the thyroid(47.1). Leslie et al reported a comparison of fixed dose treatment and treatment adjusted for 24 hour RAIU, using low or high doses, and found no difference in outcome in either rate of control or induction of hypothyroidism on comparison of the methods. They favor the use of a fixed dose treatment with a single high or low dose (47.2). Many attempts have been made to improve the therapeutic program by giving the RAI in smaller doses. Reinwein et al . studied 334 patients several years after they had been treated with serial doses of less than 50 uCi 131-I per gram of estimated thyroid weight. One-third of these patients had increased levels of TSH, although they were clinically euthyroid. Only 3% were reported to be clinically hypothyroid.
Dosage adjustmentsmade to induce euthyroidism usually include a factor for gland size, a standard dose in microCuries per gram, and a correction to account for 131-I uptake .
A“Low Dose Protocol” was designed to compensate for the apparent radiosensitivity of small glands and resistance of larger glands . Using this approach, after one year, 10% of patients were hypothyroid, 60% are euthyroid, and 30% remained intrinsically toxic , although euthyroid by virtue 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 dete