Response to methimazole in Graves' disease

OBJECTIVE A variety of regimens continue to be used In the treatment of Graves' disease with antithyrold drugs. We have lnvestigated the factors which determine the initial response to methimazole (time until euthyroidism Is achieved) In Graves' disease. PATIENTS Five hundred and nine patients with Graves' disease in different European countries with normal and subnormal iodine supply. Patients were randomized to treatment with either 10 or 40mg of methimazole per day for one year, with levothyroxine supplementation as required to maintain euthyroidism. Investigations were carried out before treatment and at 3 and 6 weeks and 3, 6, 9 and 12 months. MEASUREMENTS Response was assessed by serial measurements of serum thyroid hormones. TSH receptor antibodies, thyroid autoantibodies and urinary Iodide excretion were measured centrally. Twenty-minute thyroid uptake was measured by standard techniques. Data were collected and analysed centrally. Standard techniques as well as a stepwise logistic regression model were used to examine the relations between methimazole dose, age, goitre size, presence of endocrine eye signs, thyroid hormone levels, urinary iodide excretion, thyroid uptake, Index of disease severity (Crooks), presence of TSH receptor antibodies and duration of the hyperthyroid phase. RESULTS Within 3 weeks, 40.2% of patients responded to 10mg of methimazole and 77.5% responded within 6 weeks. The corresponding figures for 40mg of methimazole were 64.6 and 92.6%. Significant associations were found between duration of hyperthyroldism and the following variables: goitre size, urinary iodide excretion, methimazole dose, presence of TSH receptor antibodies (TBIAb), Index of disease severity (Crooks) and pretreatment thyroid hormone levels. Response to methimazole was delayed In patients with large goitres, iodine excretion of ≧ 100μg/g creatinine, high pretreatment thyroid hormone levels, elevated levels of TBIAb and treatment with only 10 mg of methimazole. In the 10-mg group, 46% of patients were euthyrold within 3 weeks when urinary Iodide was <50μg/g of creatinine, and only 27% when iodide was above 100μg/g. By stepwise logistic regression, the main factors for the response to methimazole were dally dose, pretreatment T3 levels, and goitre size. CONCLUSION Methimazole dose, pretreatment serum T3 levels, and goitre size are the main determinants of the therapeutic response to methimazole In Graves' disease, at least In areas comprising low, subnormal and normal iodine supply.