|
131I is a safe and efficient procedure for controlling hyperthyroidism
(13). Generally, the cost of treatment is low as compared to the one based
on antithyroid drug therapy or surgery. In the case of hyperthyroidism,
no ideal form of therapy is available since we remain unable to treat directly
the cause of thyroid overactivity, whatever its etiology. Therefore, a
somewhat empirical attitude based on experience is to be adopted by the
practitioner in order to solve the patient's problem:
1. There are firstly important clinical considerations. If the state of hyperthyroidism is responsible for many patient's complaints or clinically severe, the antithyroid drugs come as a priority in the therapeutic strategy. Euthyroidism has to be reached as soon as possible. If there is only moderate hyperthyroidism, 131I represent an excellent choice, particularly if the thyroid is diffusely enlarged. Since radioiodine takes 6 to 8 weeks to produce its effects, ß adrenergic blocking agents will in the meantime efficiently control the patient's symptomatology. Normally, euthyroidism is to be expected to be reached 3-6 months after 131I administration. If not, the administration of a second dosis of 131I has to be considered. For obvious reasons, pregnancy and breast-feeding represent absolute contraindications to radioiodine administration. Age per se is not a difficulty in the decision - making procedure favouring radioiodine administration since there is neither any evidence of 131I-induced secondary cancers (including leukemia), nor any enhanced prevalence of genetic defects in the progeny of radioiodine-treated thyrotoxic patients (6-11). Important to be remembered: because of a longer life expectancy, the chances of late hypothyroidism are higher in a young patient treated with 131I. Depending on the radioactive iodine dose, up to 50% of the treated population may be hypothyroid after 15 years (13). Radioiodine treatment or surgery are advised if there are problems of patient's compliance with the antithyroid drugs. The choice will depend on the patient's preference, the excellency of the surgical team and the medical pressure to quickly solve the problem medically. From the literature, it is very difficult to assess whether or not, subtotal thyroidectomy is less often complicated by thyroid insufficiency than radioiodine. For obvious reasons, the practitioner and the surgeon will favour to protect the patient from any post-surgical thyrotoxic relapse and therefore, will be reluctant to leave too much thyroid tissue in the neck: thyroid insufficiency is then almost unavoidable. Nowadays again, thyroid substitution therapy is cost-effective and thyroid insufficiency is easily corrected. Although all doctors, especially the endocrinologist, dream of "the” ideal treatment leading to a full recovery of the euthyroid status such a goal is not met in the case of toxic goiter... We are left with a relatively empirical strategy when the antithyroid drugs have failed to solve the patient's problem. 2. Methodological criteria have to be met before administering radioiodine. A tracer uptake study will assess the avidity of the gland for iodine, and more particularly will exclude any significant iodine overload of the thyroid gland which would in turn, reduce the efficiency of the radioiodine treatment. Thyroid radioactive iodine uptake studies are recommended to be calculated in gray (rad), the radiation given to the gland. Because of the heterogeneous distribution of the radioisotope, a higher dose of 131I is given in multinodular goiter as compared to diffuse hyperplasia. Relatively higher dose of 131I are also needed in toxic adenoma, probably because of the biological properties of the autonomous thyroid tissue. 3. For more than 30 years, there has been a literature-consuming debate regarding the best definitive way to calculate the dosis of 131I to be administered for the treatment of hyperthyroidism. A dose calculated from thyroid size and 131I thyroid uptake parameters theoretically remains the ideal approach. Unfortunately, this remains an unsolved problem: important functional characteristics of the thyroid gland often remain unknown or poorly defined on an individual basis, particularly the exact thyroid volume of the gland and the thyroid cell radiosensitivity. In the literature, there are indications of a lower prevalence of hypothyroidism in patients treated with a calculated 131I dose as compared to those receiving an empirically chosen radioiodine dose, at least after the first 5 to 10 years of treatment. On a longtime run, there is actually no evidence that the calculated approach gives better results than the so-called standard for 131I administration. Different elements of uncertainty associated with the fact that hyperthyroidism is usually evaluated on an ambulatory basis, have thus practically led many thyroidologists to administer standard fixed doses between 5 to 15 mCi (185-555 MBq) of 131I depending on the goiter size and the severity of the disease. The results of a recent prospective, randomized and well conducted as a multicentered study on standard vs calculated 131I therapy by Peters et al. are worth mentioning (14). A total of 230 patients with Graves' disease were enrolled in this prospective study. One group of patients received a standard dose of 555 MBq (15 mCi) of 131I while the other received a calculated dose which was expected to integrate the thyroid volume and the radioactive iodine thyroid uptake parameters in order to deliver 100 Gy (10,000 rads) to the gland. The results clearly indicate that the therapeutic outcome remains dose-related and inversely correlated with the thyroid gland size. It is unfortunately too early to assess the incidence of hypothyroidism since such a study is based on a group of patients enrolled between 1987 and 1990. It is however already concluded that to obtain a 100% therapeutic success, the calculated target dose should average 200 Gy (20,000 rad). If this is true, one may expect that a high therapeutic success will be highly correlated with a high incidence of hypothyroidism. It is interesting to note that 25 years ago, the goal for the calculation of the 131I therapeutic dose was mainly to avoid late hypothyroidism, and now is to quickly and effectively control thyrotoxicosis. This last approach makes sense: a longstanding medical antithyroid treatment often keeps the patient in "an atmosphere of disease” and can be a source of personal and professional problems while the risk of an early thyroid insufficiency is easily controlled by thyroid substitution therapy. 4. Graves' ophtalmopathy has been reported to be worsened by radioiodine therapy, but this can be efficiently prevented by short-term glucocorticoid administration. Taking into account the fluctuations observed in the course of Graves' ophtalmopathy, there is no statistical convincing evidence that radioiodine per se aggravates the ophtalmopathy. There are even some authors who advocate ablative dose of 131I to cure Graves' ophtalmopathy (15). Prospective studies are badly needed even if they are difficult to perform. 5. 131I treatment does not enhance the risk of carcinogenesis or of any deleterious genetic defects (6). The dose to the gonads is roughly of 2 cGy (200 mrad/mCi) of administered radioiodine which is lower than many commonly performed radiologic procedures like a plain film X-ray for the lumbar spine, an intravenous urogram or a barium enema (5). The risks of 131I treatment on a longrun evaluation is truly negligible. So what, to correctly and efficiently treat my patient? An algorithm tries to approach this problem (Fig. 1) and can be commented as follows: 1. If one admits that 50% of the patients treated by antithyroid drugs relapse after stopping the medication, the other 50% benefits from a well conducted treatment with methimazole (thiamazole) or propylthiouracil and will not relapse. It is thus generally reasonable to start with the antithyroid drug treatment, except if there are problems like compliance, oversensitivity, allergy or mechanical problems. Generally, euthyroidism will be reached within 2-3 months. It is easy to maintain euthyroidism, even if in some cases, a small dose of thyroxine has to be combined with the antithyroid drugs. The therapy is maintained for 15-18months. In cases of Graves' disease, a disappearance of the circulating TSH-receptor antibodies is to be expected and is expected to be associated with a good prognosis. 2. In case of relapse after such a medical treatment, 131I is to be considered. However, if the patient has a large multinodular goiter, with severe compressive signs of the trachea or the esophagus, surgery is primarily to be advised, provided the patient can be referred to an experienced surgical team. 3. If the patient is in the category of those at risk of relapse (large diffuse goiter, multinodular goiter, persistence of high levels of TSH-receptor stimulating antibodies,...) or if there are problems of compliance, it is advised (a) to attempt to reach euthyroidism through a 4-6 months antithyroid drug treatment, followed (b) by the administration of a relatively large dose of 131I, after stopping the antithyroid medication. Surgery may be prefered, if there is severe tracheal compression. The patient is at risk of hypothyroidism and an appropriate follow-up of the patient is advised. In case of thyroid insufficiency (high TSH levels), thyroid substitution therapy is started. Fig. 1: Decision-making strategy in the treatment of thyrotoxicosis
|
