Control Of Antithyroid Drug Therapy

The aim of drug therapy is to control the hyperthyroidism until a natural remission takes place. The duration of therapy that minimises the relapse rate is controversial, and 12-18 months' total therapy before withdrawal as a routine is commonly advised. Longer (minimum 24 months) treatment is usual for young patients with large, vascular goitres, because of the higher risk of recurrence. Most patients enter remission, but some will relapse — usually during the first three months after withdrawal from treatment. Approximately 30—40% of patients remain euthyroid 10 years later. If hyperthyroidism recurs, there is little chance of a second course of thionamide achieving long-term remission.

The use of levothyroxine concurrently with an antithyroid drug ('block and replace regimen') facilitates of maintenance of a euthyroid state, and reduces the frequency of clinic visits. There is no good evidence that the choice of titration or block-replace regimen influences the relapse rate.

P-adrenergic blockade. There is increased tissue sensitivity to catecholamines in hyperthyroidism with an increase in either the number of (3-adrenoceptors or the second messenger response (i.e. intracellular cyclic AMP synthesis) to their stimulation. Therefore some of the unpleasant symptoms are adrenergic.

Quick relief can be obtained with a p-adrenoceptor blocking drug (judge dose by heart rate) though these do not block all the metabolic effects of the hormone, e.g. on the myocardium, and the basal metabolic rate is unchanged. For this reason they should not be used as sole therapy except in mild thyrotoxicosis in preparation for radioiodine treatment, and should be continued in these patients until the radioiodine has taken effect. They do not alter the course of the disease, nor biochemical tests of thyroid function. Any effect on thyroid hormonal action on peripheral tissues is clinically unimportant. It is desirable to choose a drug that is nonselective for pt and p2 receptors and lacks partial agonist effect (e.g. propranolol 20-80 mg 6-8-hourly, or timolol 5 mg once daily). Usual contraindications to (i-blockade (see p. 478) should be observed, especially asthma.

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