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Difficulty: HardEndocrinology and Metabolism

A 6868-year-old man is evaluated for a 33-week history of unexplained weight loss, heat intolerance, and resting tremor. He has a history of refractory paroxysmal atrial fibrillation, for which he has been receiving amiodarone and apixaban therapy over the last 33 years. His temperature is 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F}), blood pressure is 142/84 mmHg142/84\text{ mmHg}, and pulse is 114/min114/\text{min} and irregularly irregular. Physical examination is notable for a fine tremor in the outstretched hands. The thyroid gland is not enlarged, and no nodules are palpated. Laboratory tests are obtained:

TestResultReference Range
TSH<0.01μU/mL<0.01 \mu\text{U/mL}0.4−4.2μU/mL0.4-4.2 \mu\text{U/mL}
Free T44.1 ng/dL4.1\text{ ng/dL}0.8−1.8 ng/dL0.8-1.8\text{ ng/dL}
Free T36.8 pg/mL6.8\text{ pg/mL}2.3−4.2 pg/mL2.3-4.2\text{ pg/mL}

A color Doppler ultrasonography of the neck shows a structurally normal thyroid gland with markedly decreased vascular flow. Which of the following is the most appropriate targeted therapy for this patient's thyroid abnormality?

  1. Oral prednisoneAnswer
  2. B
    Oral methimazole
  3. C
    Potassium iodide
  4. D
    Radioactive iodine ablation
  5. E
    Oral levothyroxine

Answer

Oral prednisone
This patient presents with primary thyrotoxicosis (suppressed TSH, elevated free T4 and free T3) in the setting of chronic amiodarone therapy. Amiodarone is an iodine-rich class III antiarrhythmic drug that can cause two distinct types of amiodarone-induced thyrotoxicosis (AIT). Type 1 AIT occurs in patients with underlying thyroid pathology (such as a multinodular goiter or latent Graves disease); the massive iodine load leads to excess thyroid hormone synthesis. On color Doppler ultrasonography, Type 1 AIT shows increased vascular flow. Type 2 AIT, conversely, is a destructive thyroiditis caused by amiodarone's direct toxic effect on thyroid follicular cells, leading to the release of preformed thyroid hormone. Because the gland is not actively synthesizing new hormone, color Doppler ultrasonography demonstrates markedly decreased or absent vascular flow. This patient's normal-sized thyroid and decreased vascular flow confirm Type 2 AIT. The treatment of choice for Type 2 AIT is oral glucocorticoids (e.g., prednisone), which provide a potent anti-inflammatory effect and also inhibit the peripheral conversion of T4 to active T3.

Step-by-Step Solution

1
Interpret the laboratory findings.
The patient has a suppressed TSH with elevated free T4 and free T3.
This establishes a diagnosis of primary thyrotoxicosis.
2
Identify the etiology of the thyrotoxicosis based on history and imaging.
The patient has been on amiodarone, indicating amiodarone-induced thyrotoxicosis (AIT). The color Doppler ultrasound shows markedly decreased vascular flow.
Amiodarone can cause Type 1 AIT (increased vascularity, underlying thyroid pathology) or Type 2 AIT (decreased vascularity, destructive thyroiditis). The Doppler findings confirm Type 2 AIT.
3
Determine the appropriate management for Type 2 AIT.
Oral glucocorticoids (prednisone) are the treatment of choice.
Glucocorticoids reduce the destructive inflammation in the thyroid gland and decrease the peripheral conversion of T4 to T3.

Key Concept

Amiodarone-induced thyrotoxicosis (AIT) Type 2 is a destructive thyroiditis characterized by decreased vascular flow on Doppler ultrasound and is treated with glucocorticoids.
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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