Time interval between the last dose of propylthiouracil and I-131 therapy influences cure rates in hyperthyroidism caused by Graves' disease

Transient Hypothyroidism after Radioiodine for Graves' Disease: Challenges in Interpreting Thyroid Function Tests

Graves' disease is the most common cause of hyperthyroidism and is often managed with radioactive iodine (RAI) therapy. With current dosing schemes, the vast majority of patients develop permanent post-RAI hypothyroidism and are placed on life-long levothyroxine therapy. This hypothyroidism typically occurs within the first 3 to 6 months after RAI therapy is administered. Indeed, patients are typically told to expect life-long thyroid hormone replacement therapy to be required within this timeframe and many providers expect this post-RAI hypothyroidism to be complete and permanent. There is, however, a small subset of patients in whom a transient post-RAI hypothyroidism develops which, initially, presents exactly as the typical permanent hypothyroidism. In some cases the transient hypothyroidism leads to a period of euthyroidism of variable duration eventually progressing to permanent hypothyroidism. In others, persistent hyperthyroidism requires a second dose of RAI. Failure to appreciate and recognize the possibility of transient post-RAI hypothyroidism can delay optimal and appropriate treatment of the patient. We herein describe five cases of transient post-RAI hypothyroidism which highlight this unusual sequence of events. Increased awareness of this possible outcome after RAI for Graves' disease will help in the timely management of patients.

Radioiodine therapy in benign thyroid diseases: effects, side effects, and factors affecting therapeutic outcome

Radioiodine ((131)I) therapy of benign thyroid diseases was introduced 70 yr ago, and the patients treated since then are probably numbered in the millions. Fifty to 90% of hyperthyroid patients are cured within 1 yr after (131)I therapy. With longer follow-up, permanent hypothyroidism seems inevitable in Graves' disease, whereas this risk is much lower when treating toxic nodular goiter. The side effect causing most concern is the potential induction of ophthalmopathy in predisposed individuals. The response to (131)I therapy is to some extent related to the radiation dose. However, calculation of an exact thyroid dose is error-prone due to imprecise measurement of the (131)I biokinetics, and the importance of internal dosimetric factors, such as the thyroid follicle size, is probably underestimated. Besides these obstacles, several potential confounders interfere with the efficacy of (131)I therapy, and they may even interact mutually and counteract each other. Numerous studies have evaluated the effect of (131)I therapy, but results have been conflicting due to differences in design, sample size, patient selection, and dose calculation. It seems clear that no single factor reliably predicts the outcome from (131)I therapy. The individual radiosensitivity, still poorly defined and impossible to quantify, may be a major determinant of the outcome from (131)I therapy. Above all, the impact of (131)I therapy relies on the iodine-concentrating ability of the thyroid gland. The thyroid (131)I uptake (or retention) can be stimulated in several ways, including dietary iodine restriction and use of lithium. In particular, recombinant human thyrotropin has gained interest because this compound significantly amplifies the effect of (131)I therapy in patients with nontoxic nodular goiter.

Influence of Propylthiouracil and Methimazole Pre-treatment on the Outcome of Iodine-131 Therapy in Hyperthyroid Patients with Graves' Disease

There is ongoing controversy about potential differences in the influence of the anti-thyroid drugs propylthiouracil (PTU) and methimazole (MMI) on radioiodine treatment of Graves' hyperthyroidism. This retrospective study investigated the influence of PTU and MMI pre-treatment, individually or sequentially, on the outcome of iodine-131 (131I) therapy in 199 patients with Graves' disease who had been treated with 131I for the first time and followed up at 3 and 6 months after treatment. Pre-treatment with PTU, or sequential PTU and MMI pre-treatment, increased the failure rate of 131I therapy and reduced the rate of hypothyroidism. MMI pre-treatment alone had no significant influence on the results of 131I therapy. Logistic regression analyses indicated that PTU pre-treatment and having a thyroid gland of > 60 g were both significantly related to 131I therapy failure.

Propylthiouracil reduces the effectiveness of radioiodine treatment in hyperthyroid patients with Graves' disease

In order to assess the effect of propylthiouracil (PTU) or methimazole (MMI) pretreatment on patient outcome after radioiodine therapy, we examined 100 patients with Graves' disease 3, 6, 9, and 12 months after administration of a 10-mCi standard single dose of 131I. They were assigned to one of three groups: no drug (ND) treatment (30 cases); MMI (45 cases); and PTU (25 cases). Antithyroid drugs (ATD) were withdrawn 15 days before radioiodine administration. The groups were similar concerning age, gender, ATD pretreatment duration, goiter size, and initial serum triiodothyronine (T3), thyroxine (T4), free thyroxine (FT4), antithyroid autoantibody levels, 24-hour radioiodine uptake and 131I dose administered per gram of thyroid tissue. ND and MMI groups presented a similar rate of cure of 73.3% and 77.8% respectively (p = NS). In contrast, the PTU group showed a rate of cure of only 32% (p < 0.05). Logistic regression analysis indicated that PTU administration (p = 0.003) and thyroid size (p = 0.02) were the variables related to radioiodine therapy failure. Our data demonstrate that the chance of 131I treatment failure is higher in individuals using PTU than in patients using MMI or not using any ATD before radioiodine (odds ratio [OR] 5.84; 95% confidence interval [CI] 1.82-18.76) suggesting that PTU should be avoided in the treatment of patients with Graves' disease.

Use of cold iodine in patients with Graves' disease: observations from a clinical practice

OBJECTIVE

To determine the efficacy and safety of cold iodine after radioactive iodine therapy (RAI) and the effects of prior antithyroid drug therapy in patients with Graves' disease.

METHODS

We undertook a review of medical records of 280 consecutive patients with Graves' disease who were treated with RAI followed by cold iodine. Logistic regression methods were used to model treatment failure.

RESULTS

Of the 280 patients, 246 (88%) were successfully treated with a single dose of RAI, whereas 34 patients (12%) required further therapy. The mean duration of time to achieve euthyroidism or hypothyroidism in successfully treated patients was 68 days. Of the 209 patients who were never given antithyroid drugs (ATDs), 190 (91%) were successfully treated with a single dose of RAI. No differences were found in patient age, thyroid gland size, or initial free thyroxine index between the successfully treated group and the treatment failure group among those who received RAI alone. Of the 71 patients who had been treated with ATDs before RAI administration, 56 (79%) were successfully treated with a single dose of RAI. The treatment failure rate of 21% in patients who had received ATDs was significantly higher than the 9% failure rate observed in those who were never given ATDs (P = 0.01). Multivariate analysis showed that larger gland size increased the likelihood of treatment failure [c2(2) = 11.76)] and prior treatment with ATDs doubled the risk of treatment failure after adjusting for gland size. No serious adverse events were noted after RAI or cold iodine therapy.

CONCLUSION

Treatment of Graves' disease with RAI followed by cold iodine was safe and effective. The use of ATDs before 131I ablation resulted in a 2.3-fold higher treatment failure rate. ATDs need be used only in high-risk persons, and if such therapy is undertaken, higher doses of 131I should be administered to reduce the incidence of treatment failure.