Dose For Graves Disease

The Natural Thyroid Diet

The Natural Thyroid Diet

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The amount of RAI to be administered for treating hyperthyroidism related to GD may be selected empirically or determined by a dose calculation based on the assessments of thyroid mass and function (Table 1). Standard treatment usually involves a single administration of RAI. The administration of small amounts of activity (e.g., 2 mCi) at frequent intervals is not recommended, because it allows patients to remain hyperthyroid for longer periods of time, and has not been proven superior at preventing iatrogenic hypothyroidism.

To deliver a specific dose to the thyroid, it is necessary to know the gland size, maximal uptake, and effective half-life of iodine in the targeted thyroid tissue. It may be assumed that the effective biological half-life of RAI is four to six days in the majority of patients with GD. For dosimetry calculation,

Table 1 Calculation of Administered Activities for Treatment of Benign Thyroid Diseases

Use the following equation to correct for 24-hour RAI uptake in target thyroid tissue:

Administered activity = Thyroid tissue mass (g) x Activity per g tissue/RAI uptake at 24 hours with RAI uptake expressed as a fraction of 100% uptake. (e.g., 30% uptake is 0.30)

Typical activities per gram for various hyperthyroid diseases: Graves' disease: 2.96-7.4 Mbq Graves' disease with "rapid turnover": 5.55-7.4 MBq Solitary hyperfunctioning nodule: 7.4 MBq Toxic multinodular goiter: 3.7-7.4 MBq Euthyroid goiter: 3.7-4.625 MBq

Retreatments may require larger amounts of administered activity. Abbreviation: RAI, radioactive iodine.

most physicians use a formula prescribing an activity per estimated gram of thyroid, corrected for the 24-hour uptake. A simple, commonly used formula for treating GD prescribes 2.96-7.4 MBq (80-200 mCi) per estimated mass of thyroid tissue in grams (9):

Administered activity = Thyroid tissue mass (g) x Activity per g tissue/RAI uptake at 24 hours with RAI uptake expressed as a fraction of 100% uptake. (e.g., 30% uptake is 0.30)

Although it may be possible to estimate thyroid mass by palpation, it is difficult to estimate the degree of thyroid hyperfunction (i.e., thyroid uptake) on the basis of clinical findings alone. Thus, most methods for calculating administered activity will require thyroid RAI uptake measurements, typically at 24 hours. A more complicated, but potentially efficacious approach has been the use of late RAI uptake measurements to allow the estimation of physiologic half-life of RAI (10).

Most recommended administered activities range between 3.7 and 7.4 MBq (100-200 mCi) per gram, corrected for percentage thyroid uptake. Administration of 2.96-4.44 MBq (80-120 mCi) per gram will generally deliver doses of 50100 Gy to the thyroid (9). Lower doses may reduce the incidence of hypothyroid-ism following treatment, but will increase the likelihood that a second treatment will be needed. Administration of 5.5 MBq (150 mCi) per gram will yield a dose of approximately 120 Gy to the thyroid. Larger doses can increase the likelihood of developing hypothyroidism in the post-treatment period, but should reduce morbidity related to prolonged hyperthyroidism. If a high success rate is the primary goal, doses between 200 and 300 Gy may be used.

Occasionally, patients with GD may demonstrate RAI uptake more at four to six hours than at 24 hours. This condition of "rapid turnover" may necessitate the administration of larger amounts of radioactivity (5.5-7.4 MBq/g), owing to the shorter physiologic half-life of iodine in this situation. Larger amounts of radioactivity may also be reasonable in patients with relatively low iodine uptake (9).

Patients with persistent hyperthyroidism following a first treatment with RAI may benefit from additional treatments. Higher doses are often used for retreatments, which are typically given three to six months after the initial treatment. Previous RAI treatment failure does not lessen the chance of a successful retreatment (11).

Although treatments based on dose calculations appear efficacious, they have not proven superior to the use of empirically selected administered activities. The advantages of using a fixed administered activity for treating hyperthyr-oidism are its simplicity and successful outcome in an acceptable number of patients. The effectiveness of such an approach appears comparable to a dose calculation method. In one prospective trial, patients with GD, hyperfunctioning solitary nodules and multinodular goiters, were randomized to receive a fixed radioiodine dose (5, 10, or 15 mCi, based on palpable gland size), versus a calculated dose based on thyroid gland size and 24-hour radioiodine uptake (12). Comparable rates of euthyroidism, hypothyroidism, and persistent hyperthyroidism were seen in both groups. There were also comparable reductions in gland size. A number of other studies have also supported the equivalence of using several fixed administered activities versus a calculated activity for the treatment of GD or toxic multinodular goiter (13,14).

If a fixed amount of activity is chosen, it is still important to keep in mind that the effectiveness of therapy remains dependent on the total radiation dose to the target tissue. In a randomized trial comparing the administration of a standard activity (15 mCi) to an administered activity, calculated to give a target dose of 100 Gy, it was shown that the success rate of the treatment in either arm was dependent on both the thyroid volume and target dose (15). If the estimated dose to the thyroid exceeded 200 Gy in patients receiving a standardized treatment with 15 mCi, a success rate of 80% was obtained.

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