Other considerations in thyrotoxicosis and hyperthyroidism

Thyrotoxicosis can increase drug clearance, leading to increased dose requirements of many drugs. If a patient is taking other drugs, review the doses after starting treatment to control thyrotoxicosis; doses may need to be reduced as the patient becomes euthyroid. Paradoxically, sensitivity to warfarin can be increased in thyrotoxicosis, as catabolism of vitamin K–dependent clotting factors is accelerated—a lower than normal dose of warfarin may be required. At the time of writing, the effect of hyperthyroidism on direct-acting oral anticoagulants (DOACs) has not been studied, but based on pharmacodynamics, a similar increase in sensitivity could be expected.

Thyrotoxicosis can cause atrial fibrillation. Do not use amiodarone to manage atrial fibrillation caused by thyrotoxicosis because it contains iodine, which impairs the response to antithyroid drugs, and also precludes use of radioiodine for several months because iodine uptake into the thyroid is blocked. See also Amiodarone-induced thyrotoxicosis.

Iodine exposure (particularly from radiological contrast media) can exacerbate hyperthyroidism, especially in nodular thyroid disease. Iodine-induced hyperthyroidism can be self-limiting, so don’t start treatment with an antithyroid drug unless thyrotoxicosis is persistent.