Table 6.8-3. Differential diagnosis and treatmenta of type I and type II amiodarone-induced thyrotoxicosis

Type I

Type II

Previous history of thyroid disease

Multinodular goiter or Graves disease (usually undiagnosed)

None

Pathomechanism

Excess iodine causing increased synthesis of thyroid hormones (3 mg of inorganic iodine per 100 mg of amiodarone with typical diet including <0.5 mg of iodine/d)

Toxic effect of amiodarone (inflammation) causing damage of thyroid cells and release of thyroid hormones

Iodine uptake

>5%

<2%

Color flow thyroid Doppler ultrasonography

Thyroid gland frequently enlarged, nodules may be present; increased blood flow

Normal appearance of thyroid gland; significantly decreased/absent blood flow

TRAb

Increased in Graves disease

Negative

Pharmacologic treatmenta

Eg, methimazole 40-60 mg/d + sodium perchlorate (<4 weeks) 200-250 mg qid (inhibits iodine accumulation in thyroid gland); consider radical treatment

For instance, prednisone 40-60 mg/d for 1-3 months, then taper off the dose over another 2 months

a When differential diagnosis of these types is impossible and thyroid status from before amiodarone use is unknown, combined treatment can be used: start with methimazole; add glucocorticoids if there is no improvement.

qid, 4 times a day; TRAb, thyroid-stimulating hormone receptor antibodies.