Practice Essentials
Amiodarone is a potent antiarrhythmic drug that is used to treat ventricular and supraventricular tachyarrhythmias. It is a benzofuran-derived, iodine-rich compound with some structural similarity to thyroxine (T4). Amiodarone contains approximately 37% iodine by weight. Each 200-mg tablet is estimated to contain about 75 mg of organic iodide, 8-17% of which is released as free iodide. Standard maintenance therapy with 200-mg amiodarone can provide more than 100 times the daily iodine requirement. It is highly lipid-soluble and is concentrated in the adipose tissue, muscle, liver, lung, and thyroid gland. [1]
The elimination half-life of amiodarone is highly variable, ranging from 50-100 days; total body iodine stores remain increased for up to 9 months after discontinuation of the drug. Thyroid abnormalities have been noted in up to 14-18% of patients receiving long-term amiodarone therapy. However, a meta-analysis suggested that with the lower doses of amiodarone (150-330 mg) incidence of thyroid dysfunction is 3.7%. The effects range from abnormal thyroid function test findings to overt thyroid dysfunction, which may be either amiodarone-induced thyrotoxicosis (AIT) or amiodarone-induced hypothyroidism (AIH). [2, 3, 4, 5] Both can develop in apparently normal thyroid glands or in glands with preexisting abnormalities.
Signs and symptoms of thyrotoxicosis and hypothyroidism induced by amiodarone
Signs and symptoms of AIT include the following:
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Unexplained weight loss
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Heat intolerance or increased perspiration
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Profound muscle weakness
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Unexplained fatigue
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Emotional lability
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Frequent stools
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Oligomenorrhea
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Anxiety, nervousness, or palpitations
Signs and symptoms of AIH include the following:
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Fatigue
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Lethargy
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Cold intolerance
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Mental sluggishness
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Weakness
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Constipation
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Menorrhagia
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Dry skin
Workup in thyrotoxicosis and hypothyroidism induced by amiodarone
Lab findings for AIH are similar to those for spontaneous hypothyroidism and include decreased levels of serum free T4 and increased levels of serum thyroid-stimulating hormone (TSH). Serum thyroglobulin levels are often increased, probably because of TSH-enhanced thyroid stimulation.
Lab findings for AIT include elevated levels of serum total and serum free T4 and T3, and undetectable levels of TSH. Low TSH levels and elevated free T4 levels are also commonly seen in the early phases of amiodarone therapy and in patients with severe nonthyroidal illness who have euthyroidism and are treated with amiodarone. Therefore, the measurement of free T3 levels may be helpful in differentiating conditions, because free T3 levels are increased in hyperthyroidism, while they are decreased in early phases of treatment with amiodarone.
Although lab studies can confirm a diagnosis of thyrotoxicosis, further studies are necessary to recognize the correct type of AIT. [6] An ultrasonogram of the thyroid that shows abnormalities such as hypoechoic or nodular patterns or increased gland size is more indicative of type 1.
Management of thyrotoxicosis and hypothyroidism induced by amiodarone
The initial management of AIT involves deciding whether to discontinue amiodarone therapy. This depends on the patient's cardiac condition, the availability of alternate therapies, and the type of AIT present in the patient.
Mild AIT subsides spontaneously in up to 20% of cases upon discontinuation of amiodarone therapy.
Type 1 thyrotoxicosis is treated with high doses of thionamides (eg, methimazole [40-60 mg/d] or propylthiouracil [600-800 mg/d]) to block thyroid hormone synthesis. Adding potassium perchlorate may block iodide uptake by the thyroid and deplete intrathyroidal iodine stores. Because potassium perchlorate is a drug that potentially causes aplastic anemia, limit it to patients whose condition cannot be controlled by methimazole alone.
Type 2 thyrotoxicosis is treated with a relatively long course of glucocorticoids.
If thyrotoxicosis is exacerbated after initial control, it is usually treated with steroids. In type 1 AIT, this exacerbation may be due to mixed forms, which respond to the addition of steroids. In type 2 AIT, relapse can occur after discontinuation of corticosteroid treatment, and steroid treatment may need to be restarted.
Hypothyroidism in patients with no preexisting thyroid disease often resolves after discontinuation of amiodarone therapy. However, hypothyroidism may persist after discontinuation of treatment in patients with underlying chronic autoimmune thyroiditis and high titers of anti-thyroid peroxidase (anti-TPO) antibodies. In this case, the patient may require permanent T4 replacement therapy.
Total or near-total thyroidectomy is performed in cases of AIT that fail to respond to combination therapy with thionamides, perchlorate, and corticosteroids. Thyroidectomy is also performed in patients who need amiodarone therapy but whose resulting hyperthyroidism does not respond to medical treatment. In addition, it is carried out for immediate control of a thyrotoxic state (eg, during thyroid storm), as well as in patients with intractable arrhythmias.
Pathophysiology
Amiodarone causes a wide spectrum of effects on the thyroid.
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Amiodarone inhibits type 1 5'-deiodinase enzyme activity, thereby decreasing the peripheral conversion of T4 to triiodothyronine (T3) and reducing the clearance of both T4 and reverse T3 (rT3). Consequently, the serum levels of T4 and rT3 increase and the serum levels of T3 decrease by 20-25%.
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Amiodarone inhibits entry of T4 and T3 into the peripheral tissue. Serum T4 levels increase by an average of 40% above pretreatment levels after 1-4 months of treatment with amiodarone. This, in itself, does not constitute evidence of hyperthyroidism (thyrotoxicosis).
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Inhibition of type 2 5'-deiodinase enzyme activity in the pituitary due to feedback regulation is seen in the first 1-3 months and leads to an increase in thyroid-stimulating hormone (TSH) levels. This is not an indication for T4 replacement in these patients. Serum TSH levels return to normal in 2-3 months as T4 concentrations rise sufficiently to overcome the partial block in T3 production. The response of TSH to thyroid-releasing hormone (TRH) may be reduced.
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Amiodarone and its metabolites may have a direct cytotoxic effect on the thyroid follicular cells, which causes a destructive thyroiditis.
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Amiodarone and its metabolite desethylamiodarone can act as a competitive antagonist of T3 at the cardiac cellular level.
In summary, serum T4 levels rise by 20-40% during the first month of therapy and then gradually fall toward high normal. Serum T3 levels decrease by up to 30% within the first few weeks of therapy and remain slightly decreased or low normal. Serum rT3 levels increase by 20% soon afterward and remain increased. Serum thyrotropin (TSH) levels usually rise after the start of therapy but return to normal in 2-3 months.
Two forms of AIT have been described. Type 1 usually affects patients with latent or preexisting thyroid disorders and is more common in areas of low iodine intake. Type 1 is caused by iodine-induced excess thyroid hormone synthesis and release (Jod-Basedow phenomenon). Type 2 occurs in patients with a previously normal thyroid gland and is caused by a destructive thyroiditis that leads to the release of preformed thyroid hormones from the damaged thyroid follicular cells. However, mixed forms of AIT may occur in an abnormal thyroid gland, with features of destructive processes and iodine excess.
The most likely mechanisms of AIH are an enhanced susceptibility to the inhibitory effect of iodine on thyroid hormone synthesis and the inability of the thyroid gland to escape from the Wolff-Chaikoff effect after an iodine load in patients with preexisting Hashimoto thyroiditis. In addition, iodine-induced damage to the thyroid follicles may accelerate the natural trend of Hashimoto thyroiditis toward hypothyroidism. Patients without underlying thyroid abnormalities are postulated to have subtle defects in iodine organification that lead to decreased thyroid hormone synthesis, peripheral down regulation of thyroid hormone receptors, and subsequent hypothyroidism.
Epidemiology
Frequency
United States
The prevalence of AIT in the United States is 3%; the prevalence of AIH is 22%. The relative prevalence of the 2 forms of AIT is unknown.
International
Some studies indicate that the incidence varies with the dietary iodine intake in the population. AIT occurs more frequently in geographical areas with low iodine intake, whereas AIH is more frequent in iodine-replete areas. However, in a Dutch study of persons with euthyroidism living in an area with moderately sufficient iodine intake, the incidence of AIT was twice that of AIH.
Mortality/Morbidity
Although amiodarone-associated thyroid dysfunction is usually a mild clinical condition, it can be severe, life threatening, and even lethal. Fatal cases of thyroid storm and myxedema coma have been reported despite various aggressive therapies.
Race
No well-described racial differences exist.
Sex
AIH is more frequent in females, with a female-to-male ratio of 1.5:1. AIT, however, is more frequent in males, with a male-to-female ratio of 3:1.
Age
The risk of AIH is higher in elderly persons, [7] probably because of the higher prevalence of underlying thyroid abnormality.
Prognosis
The prognosis for AIT may be very poor even though a wide range of antithyroid therapy is available. This prognosis emphasizes the need for careful monitoring of patients receiving amiodarone treatment.
The long-term prognosis for AIH is usually good.
A randomized, double-blind study by Diederichsen et al indicated that in patients without previous thyroid dysfunction, short-term amiodarone use can be safe. The study looked at the effects of 8 weeks of either amiodarone or placebo therapy in 212 patients with atrial fibrillation undergoing catheter ablation. Although the amiodarone patients had higher levels of TSH, T4, and free T4, as well as lower levels of T3 and free T3, than did the placebo group, thyroid dysfunction peaked at 1 month, was declining at 3 months, and returned to baseline levels by 6 months. [8]
A study by Wang et al indicated that in patients with paroxysmal atrial fibrillation and AIT, early catheter ablation is safe and effective, although the rate of atrial tachyarrhythmia recurrence is higher than in controls for as long as 3 months after pulmonary vein isolation. [9]
Patient Education
Instruct patients about the adverse effects of amiodarone therapy. Give them a list of potential symptom manifestations. Because the development of thyrotoxicosis is sudden and explosive, instruct patients to watch for symptoms and to seek treatment promptly.
Patients should also be aware of the potential side effects of antithyroid medications. Instruct patients to watch for signs such as fever, sore throat, jaundice, or oral ulcers.
