A 54-year-old woman comes to the emergency department with palpitations. She has experienced paroxysmal atrial fibrillation for the past 6 months. She has had extensive investigations for an underlying cause, including echocardiography and thyroid function testing, which were normal. The patient was started on amiodarone 6 weeks ago by her cardiologist and did well in the interim. She has had no change in her weight or intolerance to hot or cold temperatures. Her other medications include atenolol and low-dose aspirin. On physical examination, the patient appears comfortable. Her blood pressure is 132/82 mm Hg and heart rate is 112/min and irregular. Laboratory studies show a normal complete blood count and basic metabolic panel. ECG shows atrial fibrillation with rapid ventricular response. Thyroid function markers are as follows:
What is the most likely explanation for this patient's thyroid function tests?

Question

A 54-year-old woman comes to the emergency department with palpitations.  She has experienced paroxysmal atrial fibrillation for the past 6 months.  She has had extensive investigations for an underlying cause, including echocardiography and thyroid function testing, which were normal.  The patient was started on amiodarone 6 weeks ago by her cardiologist and did well in the interim.  She has had no change in her weight or intolerance to hot or cold temperatures.  Her other medications include atenolol and low-dose aspirin.  On physical examination, the patient appears comfortable.  Her blood pressure is 132/82 mm Hg and heart rate is 112/min and irregular.  Laboratory studies show a normal complete blood count and basic metabolic panel.  ECG shows atrial fibrillation with rapid ventricular response.  Thyroid function markers are as follows:  
What is the most likely explanation for this patient's thyroid function tests?

Quizplus · Accepted Answer

Explanation: 11ec2b69_bec9_a8d0_8f63_83ebd4c52d54_MD0005_00 Amiodarone may cause significant thyroid dysfunction due to its high iodine content, direct effects on thyroid follicular cells, and changes in thyroid hormone metabolism. There are 3 thyroid-related syndromes associated with amiodarone use: Amiodarone decreases the peripheral conversion of T4 to T3. As in this patient, this may result in higher T4, lower T3, and normal or mildly elevated TSH (<20 mU/L). The thyroid test abnormalities are most prominent in the first 1-3 months after starting amiodarone and may become less apparent afterward. These patients are clinically euthyroid. The high iodine content in amiodarone inhibits thyroid hormone synthesis, leading to primary hypothyroidism (Wolff-Chaikoff effect). Amiodarone can also induce thyrotoxicosis (amiodarone-induced thyrotoxicosis [AIT]) due to either increased thyroid hormone synthesis (AIT type 1) or destructive thyroiditis (AIT type 2). In both AIT subtypes, TSH will be low. (Choice B) Aspirin in high doses (>2 g/day) displaces thyroid hormone from thyroid hormone-binding proteins, leading to an increased thyroid hormone effect with subsequently decreased TSH, total T3, and total T4. Low-dose aspirin does not have a significant effect. (Choice C) Propranolol, a non-cardioselective beta blocker, decreases conversion of T4 to T3 and is therefore useful in thyrotoxic emergencies. However, atenolol, a cardioselective beta blocker, has no effect on thyroid function. (Choice D) Thyroid function abnormalities, referred to as euthyroid sick syndrome (ESS), can occur in acutely ill patients. Most patients with ESS will have a low T3; however, in severe cases, T4 and TSH can be low as well. Mildly elevated TSH may occur during the recovery phase of the illness. (Choice E) Hyperthyroid patients with Graves' disease have elevated circulating thyroid hormone and low TSH levels. Educational objective: Amiodarone decreases the peripheral conversion of T4 to T3, causing increased serum T4 and decreased serum T3 levels with normal to borderline elevated TSH. References: Amiodarone and the thyroid. (http://www.ncbi.nlm.nih.gov/pubmed/15989900)

A 54-Year-Old Woman Comes to the Emergency Department with Palpitations