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Original Studies |
Endocrine Division, Hospital de Clínicas de Porto Alegre, Universidade Federal do Rio Grande do Sul, 90035–003 Porto Alegre, RS, Brazil
Address all correspondence and requests for reprints to: Ana Luiza Maia, M.D., Ph.D., Serviço de Endocrinologia, Hospital de Clínicas de Porto Alegre, Rua Ramiro Barcelos 2350, 90035–003 Porto Alegre, RS, Brazil. E-mail: almaia{at}vortex.ufrgs.br
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Abstract |
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In patients receiving 131I alone, mean serum T4 levels did not change after therapy. Mean serum FT4 and T3 levels decreased significantly 5 days after 131I administration (15% and 18%, respectively). Serum T3 reached its lowest level on day 30 (38%). With pretreatment, mean serum T4, FT4, and T3 levels increased (38%, 39%, and 70%, respectively) after methimazole discontinuation and before 131I administration. After 131I, serum T4 levels peaked on day 7 (23% vs. treatment day; 70% vs. baseline); FT4 levels peaked on day 14 (53% vs. treatment day; 107% vs. baseline). The serum T3 concentration increased 9% on day 2 (85% vs. baseline) and decreased from day 14 (15%) to day 30 (21%). We conclude that interruption of antithyroid drugs causes a short term increase in serum thyroid hormone levels in patients with Graves’ hyperthyroidism receiving 131I. Thyroid hormone levels stabilize or decrease during the first 30 days after 131I therapy.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Few studies have focused on short term serum thyroid hormone changes after radioiodine administration. The accumulated data show conflicting results. Some studies report short term elevation in thyroid hormone levels (5, 6), whereas others report no changes (7, 8) or a decrease in thyroid hormone levels after radioiodine therapy (9, 10). None of those studies has examined the relative effects of pretreatment with antithyroid drugs on the short term biochemical course after radioiodine administration.
The role of pretreatment with antithyroid drugs was recently addressed by Burch et al. (11), who reported a transient increase in thyroid hormone levels after interruption of drug treatment for radioiodine therapy, with no further increase in hormone levels after radioiodine administration. Those researchers have also shown that serum thyroid hormone levels decrease after radioiodine therapy without pretreatment with antithyroid drugs. However, that study did not follow a randomized control design, and only four patients were assigned to radioiodine therapy without pretreatment with antithyroid drugs.
Radioactive iodine therapy without pretreatment with antithyroid drugs may be a safe, effective, and less expensive alternative to treat Graves’ hyperthyroidism. Therefore, the present randomized, prospective study was designed to evaluate the effect of pretreatment with antithyroid drugs on short term thyroid hormone levels after radioiodine therapy in Graves’ hyperthyroidism.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The study was carried out between February 1997 and August 1998. Consecutive patients with a recent diagnosis of Graves’ disease attending the Endocrine Division at Hospital de Clínicas de Porto Alegre were eligible. Graves’ hyperthyroidism was diagnosed on the basis of suppressed TSH levels by sensitive assay, elevated serum thyroid hormone levels and 24-h radioiodine uptake, and detectable levels of antibodies against the TSH receptor. Exclusion criteria were previous treatment with radioiodine or thyroidectomy, signs of moderate or severe ophthalmopathy (proptosis >22 mm, ophthalmoplegia, chemosis, or lagophthalmos), severe heart disease (symptomatic coronary heart disease, class III heart failure, New York Heart Association criteria), debilitating conditions, and large and compressive goiters (>150 g). Patients previously treated with antithyroid drugs whose treatment had been interrupted at least 3 months before the study were included.
Sixty patients were enrolled. Five patients were lost to follow-up; 1 was excluded because of pregnancy, and 3 patients did not achieve biochemical euthyroidism after treatment with methimazole. Thus, 51 patients participated in the study.
During the enrollment period the patients underwent a complete physical examination, including ocular examination and electrocardiogram. Data about duration of the disease, previous antithyroid drug therapy, and history of smoking were recorded, and thyroid volume was assessed by ultrasound. The study protocol was approved by the ethics committee at the Hospital, and all patients gave their informed written consent.
Treatment protocol and serial evaluation
Patients were randomly assigned to receive radioiodine therapy alone (28 patients) or pretreatment with antithyroid drugs plus radioiodine therapy (23 patients). In the first group, on the day of treatment, patients received a dose of radioiodine (200 µCi/g thyroid tissue as estimated by ultrasound, corrected by 24-h radioiodine uptake). A clinical and laboratory assessment was performed on the day of treatment and on days 2, 5, 7, 14, 20, and 30 after treatment.
In the second group, patients were treated with methimazole (30 mg daily) until biochemical euthyroidism was achieved. Patients were considered to have reached euthyroidism when serum thyroid hormone levels were within the laboratory reference range. Antithyroid therapy was interrupted 4 days before patients were given radioiodine (radioiodine dose was calculated in the same way as for the first group, based on a second 24-h radioiodine uptake performed on the day of treatment). A clinical and laboratory assessment was carried out on days 4 and 1 before radioiodine therapy, on the day of treatment, and on days 2, 5, 7, 14, 20, and 30 after radioiodine therapy.
Clinical evaluations were always performed by the same physician, who
did not know whether the patient had received methimazole. To evaluate
the degree of thyrotoxicosis, Wayne’s questionnaire was employed
(euthyroidism, 
10; suspicion of hyperthyroidism, 11–19;
hyperthyroidism, 
20) (12); After radioiodine administration, symptoms
and signs of thyrotoxicosis were graded according to Wayne’s therapy
index (12). Serum levels of T4, free
T4 (FT4), and
T3 were measured every morning on days scheduled
for clinical and laboratory assessment, as described above. None of the
patients received antithyroid drug therapy during the 30 days after
radioiodine therapy. The ß-adrenergic blocking agent propanolol
(40–120 mg/day) was given to patients when tachycardia was more
than120 beats/min.
Serum hormone measurements
Assays were performed on batched serum samples (duplicates) that
had been stored at -20 C pending study completion. Serum
T4 and T3 levels were
measured using RIA (Diagnostic Products, Los Angeles, CA;
Immunotech, Marseille, France), and serum
FT4 was measured using Coat-a-Count assay
(Immunotech, Marseille, France). Intraassay coefficients
of variation were as follows: T4, 3–8%;
T3, 7–10%; and FT4,
3–6%. For values in the hyperthyroid ranges, intraassay coefficients
of variation were the following: T4, 6–9%;
T3, 6–12%; and FT4,
4–8%. Interassay coefficients of variation were as follows:
T4, 10%; T3, 12%; and
FT4, 7%. TSH was measured with a double
antibody-sensitive assay (Immulite, Diagnostic Products).
Plasma levels of TSH antibodies were determined by radioreceptor assay
(CIS-Bio International, Cardiff, France). Reference
ranges for each of these assays are shown in Table 1
.
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Baseline clinical and laboratory characteristics of the two
groups of patients were compared using the 
2
test or Fisher’s exact test for qualitative variables or using
Student’s t test or Mann-Whitney’s U test for quantitative
variables. In each group, the variation of Wayne’s therapy index and
thyroid hormone levels over time was assessed by ANOVA by repeated
measures (Friedman’s test), followed by Dunnett’s test. The
correlation between changes in therapy index and serum thyroid hormone
levels over time was assessed using Spearman’s rank correlation
procedure. P < 0.05 was considered statistically
significant. The Statistical Package for Social Science 7.5
professional software (SPSS, Inc., Chicago, IL) was used
for statistical analysis.
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The characteristics of the 51 patients with Graves’
hyperthyroidism who were randomly assigned to receive radioactive
iodine alone or radioactive iodine plus treatment with antithyroid drug
are shown in Table 1
. There were no significant differences between the
2 groups with respect to any of the characteristics listed.
Patient follow-up
The median for the period of time required to achieve biochemical euthyroidism in the group of patients pretreated with antithyroid drugs was 12 weeks (2–48 weeks). The mean number of medical visits was significantly higher in this group (3 ± 1 vs. 12 ± 4; P < 0.001). The 24-h radioiodine uptake determined 4 days after stopping methimazole in pretreated group was not different from that before treatment (78% vs. 72%; P = 0.589). The mean given dose of radioiodine (10.4 ± 5.4 vs. 8.2 ± 3.9) and the number of patients using propranolol (three vs. two) and/or oral contraceptive (eight vs. eight) were similar in both groups. Methimazole was replaced by propylthiouracil (300 mg/day) in three patients who developed rash or pruritus.
In the group of patients treated with radioiodine alone, a significant
improvement was observed as early as 2 days after radioiodine therapy.
The group of patients pretreated with antithyroid drugs experienced
significant improvement of signs and symptoms after biochemical
euthyroidism was achieved, whereas no other changes were observed
either during preparation for radioiodine therapy or after therapy, as
assessed by the Wayne’s therapy index questionnaire (Fig. 1).
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In patients who did not receive pretreatment, we observed mean
increases of 5% and 9% in serum T4 and
FT4 levels, respectively, on day 2 (Fig. 2), but these were not statistically
significant upon post-hoc testing. A subsequent decrease in
mean serum T4 (25.4 to 22.5 µg/dL; 13%),
FT4 (4.5 to 3.8 ng/dL; 15%), and
T3 (533.9 to 435.6 ng/dL; 18%) levels occurred
as early as 5 days after radioiodine administration. The decrease in
serum T4 levels was not statistically significant
compared to day of treatment values; however, this decrease was
significant compared to day 2 values (P < 0.01). Serum
T4 and FT4 levels did not
change between days 5 and 30, whereas the mean serum
T3 levels continued to drop, reaching the lowest
value on day 30 (332.8 ng/dL; P < 0.01; an additional
decrease of 24%).
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). An
additional significant increase occurred in both hormone fractions
after radioiodine ablation; serum T4 levels
reached the highest value on day 7 (15.1 µg/dL; P <
0.01; an increase of 23% compared to day of treatment value and of
70% compared to baseline value). Serum FT4
levels reached the highest value on day 14 (2.9 ng/dL; an increase of
36% compared to day of treatment value and of 107% compared to
baseline value). Mean serum T3 levels increased
from 147.7 to 251.2 ng/dL (an increase of 70%) after antithyroid drug
discontinuation and before radioiodine therapy. An additional increase
of 9% in serum T3 levels, from 251.2 to 273.5
ng/dL, was observed on the second day after radioiodine administration
(P < 0.01; 85% compared to the baseline), followed by
a progressive decrease from day 14 (213.3 ng/dL; P <
0.01; decrease of 15%) to day 30 (197.9 ng/dL; P <
0.01; decrease of 21%). To determine whether changes in serum thyroid hormone levels after radioiodine administration would be different in patients with large goiters, we analyzed the results in seven patients with estimated thyroid size greater than 50 g (60.3 ± 12.2) in the nonpretreated group. In this subgroup, the variation in mean serum T4, FT4, and T3 levels after radioiodine therapy displayed an identical pattern to that in the whole group, although statistical significance was achieved only for the decrease in serum FT4 levels on day 30 (4.6 to 2.4 ng/dL; P < 0.05; decrease of 91%; results not shown).
For individual analysis of serum thyroid hormone changes, we considered that increases greater than 2 times the assay variation were significant. We identified a progressive increase in serum thyroid hormone levels after radioiodine dosing in two patients from the group receiving radioiodine alone (serum T4: day 0, 13.5 and 28.9 µg/dL; day 30, 19.5 and 46.9 µg/dL, respectively; serum T3: day 0, 184.9 and 370.4 ng/dL; day 30, 481.8 and 735 ng/dL, respectively). In the pretreated group, three patients coursed with steady increase in serum thyroid hormone levels after radioiodine administration, as follows: serum T4: day 0, 8.6, 13.3, and 9.9 µg/dL; day 30, 17.0, 18.6, and 18.8 µg/dL, respectively; serum T3: day 0, 127.6, 212.2, and 156.9 ng/dL; day 30, 254.6, 330.1, and 297.5 ng/dL, respectively. All patients present maximal serum thyroid hormone levels on day 30. We did not identify any special aspect of disease presentation in any of these patients. One of them, the subject with highest thyroid hormone levels, developed atrial fibrillation on day 30 that was promptly reversed by propanolol administration.
Correlational analysis showed that the changes in therapy index over time in patients receiving only radioiodine were positively correlated with the changes in mean serum FT4 (r = 0.823; P = 0.014) and T3 levels (r = 0.937; P < 0.000). The correlation between serum T4 levels and Wayne’s therapy index was not statistically significant (r = 0.577; P = 0.150).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The practice of prescribing antithyroid drugs before radioiodine therapy began soon after clinical observations of worsening of hyperthyroidism shortly after radioiodine treatment (13). In fact, Larsen (14) observed a reduction in glandular T4 concentration in patients treated with antithyroid drugs before surgery compared with that in nonpretreated patients. However, the exact frequency of clinically significant worsening of hyperthyroidism after radioiodine therapy is unknown, and it varies from series to series (3); similarly, studies dealing with changes in serum thyroid hormone after radioiodine treatment show conflicting results (5, 6, 7, 8, 9, 10).
Recently, Burch et al. (11) suggested that short term increases in thyroid hormone levels after radioiodine therapy could be caused by discontinuation of drug therapy, rather than by radioiodine treatment itself. Those researchers observed a deterioration in thyroid function in a group of 17 patients immediately after interruption of drug treatment and before radioiodine dosing. Because that study did not look for biochemical euthyroidism in all patients before iodine administration, one could speculate that the increase in thyroid hormone levels occurred mainly in patients whose hyperthyroidism was not controlled (15). The results of this study, in which biochemical euthyroidism was required before radioiodine therapy, confirm those described by Burch et al. (11) and clearly demonstrate that drug discontinuation caused a significant increase in serum thyroid hormone levels even in euthyroid patients. An interesting observation was that despite the additional increment in thyroid hormone levels after radioiodine treatment, stability or decrease occurred after 7–14 days of radioiodine ablation, indicating that in most patients it is not necessary to reintroduce drug therapy, at least during the first 30 days after radioiodine administration.
Another major issue is whether pretreatment with antithyroid drugs will alter thyroid hormone levels after radioactive iodine therapy. We found that pretreatment with methimazole has little or no influence on thyroid hormone levels after radioiodine therapy, except for the effects of drug discontinuation described previously. The major difference observed between the groups was in terms of the rate of decrease in T3 levels, which could be explained in part by the more rapid clearance in hyperthyroid patients. Radioiodine may cause follicular disruption followed by hormone leakage into the circulation (16), as shown by the slight increase in T4 and FT4 levels 2 days after radioiodine administration in nonpretreated patients. However, the subsequent decrease in hormone levels suggests either that the follicular cell is rapidly repaired or that the store of hormone is already depleted in a highly stimulated gland.
The greater magnitude of the acute decrease in circulating T3 in response to radioiodine compared with the decrease in T4 and FT4 could be due to an effect of radioiodine on thyroidal T3 production. The disproportionate increase in T3 production in Graves’ disease is well known, and it has been attributed to both the increase in the T3 to T4 ratio in Graves’ thyroglobulin (17) and to high type 1 deiodinase levels in the gland (18). The recent finding of large amounts of type 2 deiodinase in Graves’ thyroid suggests that the conversion of T4 into T3 may have an important role in T3 overproduction (19). As radioiodine therapy causes cell damage, a possible explanation for the acute decline in serum T3 levels could be loss of deiodinase activity in the thyroid gland. In this study, the T3 to T4 ratio in nonpretreated patients decreased from 20.6 on the treatment day to 15.2 on day 30. The faster decrease in serum T3 levels was also observed by Burch et al. (11), who suggested a direct effect on thyroid secretion and/or decreased T4 availability for peripheral conversion to T3.
Two previous studies have reported an increase in serum thyroid hormone levels after radioiodine therapy (5, 6). We do not have an explanation for the difference between those results and our results, except that the studies had a different design and included patients with different causes of hyperthyroidism. Furthermore, individual analysis of thyroid hormone changes is complicated, because it is difficult to determine whether hormone variation originates from the disease itself, whether it is a result of assay variation, or whether it is induced by treatment. In our study, we identified 2 patients in the nonpretreated group whose serum thyroid hormone levels increased after radioiodine administration. Several noteworthy inferences originate from this observation. As the increase in serum thyroid hormone was progressive, reaching the highest level after the 10- to 14-day interval in which radioiodine thyroiditis is believed to peak (20), it is possible that this increase reflects treatment failure rather than a direct radioiodine effect; in fact, these 2 patients were the only ones, among 28 patients, who did not present a decrease in serum T3 levels after radioiodine dosing. The speculation that patients with large goiter would be at higher risk for worsening of hyperthyroidism was not supported by the analysis of thyroid hormone changes in our subgroup of 7 nonpretreated patients with goiter greater than 50 g.
As expected, isolated radioiodine therapy did not control the hyperthyroid state during the 30-day observation period, indicating that pretreatment with antithyroid drugs should be considered when a faster biochemical euthyroidism is required.
In conclusion, we have demonstrated that short term increase in thyroid hormone levels in patients with Graves’ hyperthyroidism receiving radioiodine treatment occurs mainly as a result of discontinuing antithyroid drug therapy. Among patients who receive radioiodine therapy without pretreatment, serum thyroid hormone levels will not change or decrease in the 30-day interval after radioiodine administration. We postulate that radioactive iodine therapy without pretreatment with antithyroid drugs can be safely prescribed for most patients with hyperthyroidism associated with Graves’ disease.
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Acknowledgments |
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Footnotes |
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Received June 4, 1999.
Revised July 28, 1999.
Accepted August 10, 1999.
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References |
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,
P < 0.05 for the comparison with the value
on day 2 after radioiodine administration. §, P <
0.01 for the comparison with the value on the day of radioiodine
administration (day 0).