Hyperthyroidism adrenoceptor antagonists

Q8 The drug treatment of thyrotoxicosis involves using antithyroid drugs car-bimazole (which is converted to the active compound methimazole) and propylthiouracil inhibit the synthesis of thyroid hormone. Propylthiouracil also inhibits peripheral conversion of T4 to T3. Many of the symptoms of hyperthyroidism can also be alleviated by fl-adrenoceptor antagonists. Iodine... 

The effect of beta-adrenoceptor antagonists on thyroid hormone metabolism is unlikely to play a significant role in their use in hyperthyroidism. Since D-propranolol has similar effects on thyroxine metabolism to those seen with the racemic mixture, membrane-stabilizing activity may be involved (176). 

Beta-adrenoceptor antagonists are used in the treatment of hyperthyroidism to alleviate symptoms but are not considered to influence thyroid hormone production nor the effect of the hormone on cell function. A study of the effect of beta-blockers on lymphocyte metabolism was done with the purpose to clarify their mechanism of action in hyperthyroidism [68]. When hyperthyroid subjects were treated with beta-blockers, lymphocyte heat production was found to be within normal limits, thus showing that beta-blockers prevent the expected increased of cell metabolism under stimulation of thyroid hormone. These results seem to indicate that increased thermogenesis in hyperthyroidism is mediated via adrenergic receptors, rather than via nuclear thyroid hormone receptors. 

As previously mentioned (see Section 4), non-selective (propranolol) and selective (metoprolol) beta-adrenoceptor antagonists could preclude an increase of lymphocyte metabolism, otherwise present in hyperthyroid patients. 

Excessive catecholamine action is an important aspect of the pathophysiology of hyperthyroidism, especially in relation to the heart (see Chapter 38). The 13 antagonists are beneficial in this condition. The effects presumably relate to blockade of adrenoceptors and perhaps in part to the inhibition of peripheral conversion of thyroxine to triiodothyronine. The latter action may vary from one 13 antagonist to another. Propranolol has been used extensively in patients with thyroid storm (severe hyperthyroidism) it is used cautiously in patients with this condition to control supraventricular tachycardias that often precipitate heart failure. 

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