A 65-year-old man is evaluated in the clinic for progressively worsening fatigue, constipa… — USMLE Step 3

A $65$ -year-old man is evaluated in the clinic for progressively worsening fatigue, constipation, and an unintended $4\text{-kg}$ ( $8.8\text{-lb}$ ) weight gain over the past three months. His medical history is notable for ischemic cardiomyopathy and refractory atrial fibrillation, for which a new antiarrhythmic medication was initiated six months ago. Vital signs show a heart rate of $55/\text{min}$ and blood pressure of $110/70\text{ mm Hg}$ . Physical examination reveals delayed relaxation of the deep tendon reflexes and dry, coarse skin. Laboratory testing shows a serum thyroid-stimulating hormone (TSH) level of $18.4\text{ mIU/L}$ (reference: $0.5-5.0\text{ mIU/L}$ ) and a free thyroxine (T4) level of $0.5\text{ ng/dL}$ (reference: $0.9-1.7\text{ ng/dL}$ ). Which of the following is the most likely primary cellular mechanism responsible for this patient's current endocrine laboratory abnormalities?

Inhibition of thyroid hormone synthesis due to excessive intrathyroidal iodine concentrationAnswer
B
Decreased peripheral conversion of thyroxine to active triiodothyronine
C
Autoimmune-mediated lymphocytic infiltration and destruction of thyroid follicles
D
Direct cytotoxic disruption of thyroid follicular cells with release of preformed hormone
E
Suppression of thyrotropin-releasing hormone from the hypothalamus

Answer

Inhibition of thyroid hormone synthesis due to excessive intrathyroidal iodine concentration

The correct answer identifies the Wolff-Chaikoff effect. Amiodarone is highly lipid-soluble and composed of

37\%

iodine by weight. Normal daily iodine requirement is roughly

150\text{ \mu g}

, but a standard maintenance dose of amiodarone delivers several milligrams of free iodine daily. This massive iodine load transiently inhibits thyroid peroxidase, blocking the organification of iodine and subsequent synthesis of thyroid hormones (T3 and T4). Most patients experience an 'escape' from this effect within a few weeks; however, some patients (particularly those with underlying autoimmune thyroid disease) fail to escape, leading to overt primary hypothyroidism characterized by an elevated TSH and low free T4. This is the pathophysiology underlying Amiodarone-Induced Hypothyroidism (AIH).

Step-by-Step Solution

Analyze the patient's clinical presentation and recent medication history.

The patient has symptoms of hypothyroidism (fatigue, constipation, weight gain, bradycardia, delayed reflexes) occurring 6 months after starting a new antiarrhythmic drug for atrial fibrillation (likely amiodarone).

Identifying the clinical syndrome and temporal correlation with a high-risk medication is the first step in diagnosing drug-induced endocrine dysfunction.

Interpret the laboratory findings.

TSH is significantly elevated (

18.4\text{ mIU/L}

) and free T4 is low (

0.5\text{ ng/dL}

), confirming a diagnosis of primary hypothyroidism.

The combination of high TSH and low T4 localizes the defect to the thyroid gland itself (primary failure).

Correlate the drug exposure (amiodarone) with the primary thyroid failure to determine the underlying cellular mechanism.

Amiodarone is

37\%

iodine by weight. The massive iodine load inhibits thyroid peroxidase, preventing iodine organification and thyroid hormone synthesis (the Wolff-Chaikoff effect).

Understanding the pharmacologic mechanisms of amiodarone on the thyroid axis is essential for distinguishing between its various endocrine side effects (hypothyroidism vs. thyrotoxicosis).

Key Concept

Amiodarone-induced hypothyroidism is primarily caused by the Wolff-Chaikoff effect, where a massive iodine load inhibits thyroid peroxidase and blocks the organification of iodine, leading to decreased thyroid hormone synthesis.

Estimated Time:1m 30s

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