99mTc Sestamibi Thyroid Scan in Amiodarone-Induced Thyrotoxicosis Type I

Area of Science:

• Endocrinology
• Nuclear Medicine
• Cardiology

Background:

• Amiodarone is a widely used antiarrhythmic medication.
• Amiodarone contains a high iodine content, which can precipitate thyroid dysfunction.
• Amiodarone-induced thyrotoxicosis (AIT) is classified into two main types with distinct pathophysiologies and treatment strategies.

Observation:

• A 65-year-old male patient on amiodarone for atrial fibrillation presented with clinical hyperthyroidism.
• The specific type of AIT was initially unclear.
• A Technetium-99m sestamibi (Tc sestamibi) thyroid scan revealed diffusely increased uptake and retention in an enlarged thyroid gland.

Findings:

• The Tc sestamibi scan pattern was consistent with AIT type I, which is associated with excessive thyroidal iodine in latent Graves disease.
• Treatment with methimazole effectively controlled the patient's thyrotoxicosis.
• Subsequent iodine-131 (I iodide) thyroid scan and uptake study confirmed the diagnosis of Graves disease.

Implications:

• Tc sestamibi scintigraphy can be a valuable tool in differentiating between AIT type I and AIT type II.
• Accurate diagnosis is crucial for guiding appropriate therapeutic interventions in amiodarone-induced thyrotoxicosis.
• This case underscores the importance of considering underlying thyroid conditions, such as Graves disease, in patients developing amiodarone-induced thyrotoxicosis.