Incidence and predictability of amiodarone-induced thyrotoxicosis and hypothyroidism.

OBJECTIVES: To determine the incidence and predictability of amiodarone-induced thyrotoxicosis (AIT) and hypothyroidism (AIH) in patients with cardiomyopathy.

PATIENTS AND METHODS: A total of 72 patients (mean age 69 +/- 11 years) living in an area previously endemic for thyroid disease but with currently sufficient iodine intake were enrolled in this prospective study. All participants were treated with amiodarone for the first time. The course of thyroid function in patients with normal thyroid morphology and in those with goiter was monitored over a median follow-up period of eight months in 71 (98.6%) patients.

RESULTS: Of 72 participants, 18 (25.0%) had a morphologically normal thyroid gland as evidenced by sonography. The prevalence of thyroid dysfunction before initiation of amiodarone was 37.6% (27 of 72) with almost equal distribution between hypothyroidism and hyperthyroidism (14 and 13 patients). After treatment with amiodarone, thyroid dysfunction was diagnosed in 56.8% (25 of 44) of the patients without preexisting dysfunction. Of these 25 patients, nine (36%) developed either subclinical or overt AIH and 16 (64.0%) developed either subclinical or overt AIT. Although 61.1% (44 of 72) had normal thyroid function before initiation of amiodarone, this number decreased to 26.7% (19 of 71, P < 0.001) after treatment. Factors such as (99m)Tc-pertechnetate scan uptake, thyroid autoimmunity, age, thyroid autonomy or abnormal thyroid morphology were not significantly associated with the development of thyroid dysfunction.

CONCLUSIONS: Prevalence of thyroid dysfunction was high in elderly patients treated with amiodarone. Cases of AIT and AIH occurred in patients with and without preexisting thyroid disorders. Because of the high incidence of amiodarone-induced thyroid dysfunction, regular testing of thyroid function is mandatory during and following amiodarone treatment.