Amiodarone-Induced Thyroid Dysfunction

A 68 year old male with a history of atrial fibrillation presents with progressive weight loss, palpitations, heat intolerance, fatigue, and worsening episodes of tachyarrhythmia over the past 2 months. He has been taking amiodarone for the past year. Laboratory studies show suppressed TSH and elevated free T4. Thyroid ultrasound demonstrates increased vascularity. Diagnosis?

Diagnosis is Amiodarone-Induced Thyroid Dysfunction (most consistent with Amiodarone-Induced Thyrotoxicosis Type 1).

1. Definition

Amiodarone-Induced Thyroid Dysfunction is a spectrum of thyroid abnormalities caused by amiodarone therapy, resulting in either:

1. Amiodarone-Induced Hypothyroidism (AIH)
2. Amiodarone-Induced Thyrotoxicosis (AIT)

It occurs because amiodarone delivers a large iodine load and directly alters thyroid hormone synthesis, metabolism, and action.

2. Etiology

1. Caused by amiodarone exposure
2. Amiodarone characteristics:
• Contains approximately 37% iodine by weight
• A daily dose of 200 mg releases approximately 6 mg of free iodine/day, greatly exceeding normal iodine requirements
3. Mechanisms:
• Excess iodine load
• Direct drug and metabolite (desethylamiodarone) effects
• Altered thyroid hormone metabolism
• Altered thyroid hormone action

3. Pathophysiology

3.1 Wolff–Chaikoff Effect

Excess iodine inhibits:

• Iodide organification
• Thyroid hormone synthesis

Normally:

• Temporary suppression
• Escape phenomenon restores thyroid function

Failure to escape leads to:

• Amiodarone-Induced Hypothyroidism (AIH)

3.2 Jod-Basedow Phenomenon

Occurs in:

• Autonomous thyroid tissue
• Multinodular goiter
• Latent Graves disease

Results in:

• Iodine-induced increased hormone synthesis

Causes:

• Type 1 Amiodarone-Induced Thyrotoxicosis

3.3 Deiodinase Inhibition

Amiodarone and desethylamiodarone inhibit:

• Type 1 deiodinase
• Type 2 deiodinase

Results:

• ↓ Peripheral conversion of T4 → T3
• ↑ Reverse T3
• Transient ↑ TSH
• Altered thyroid hormone profile

4. Effects on Thyroid Function Tests

Early Therapy

1. ↑ TSH (transient)
2. ↑ T4 / FT4
3. ↑ Reverse T3
4. ↓ T3

Chronic Euthyroid Amiodarone Effect

1. FT4 mildly elevated or upper-normal
2. T3 low-normal or mildly low
3. TSH returns toward normal after approximately 2–3 months

5. Clinical Features

5.1 Amiodarone-Induced Hypothyroidism (AIH)

1. Fatigue
2. Weight gain
3. Cold intolerance
4. Bradycardia
5. Constipation

Typically develops:

• 6–12 months after therapy

Risk factors:

• Iodine-sufficient regions
• Women
• Hashimoto thyroiditis
• Anti-TPO positivity

5.2 Amiodarone-Induced Thyrotoxicosis (AIT)

1. Weight loss
2. Heat intolerance
3. Fatigue
4. Muscle weakness
5. Palpitations
6. Recurrence or worsening of arrhythmias

More common:

• In iodine-deficient regions
• In men

Can occur:

• During therapy
• Months after discontinuation

6. Diagnosis

6.1 AIH

Subclinical

• Elevated TSH
• Normal FT4

Overt

• Elevated TSH (often >10 mU/L)
• Low FT4

6.2 AIT

Laboratory findings:

• ↓ TSH
• ↑ FT4
• T3 normal or elevated

Differentiation requires:

• Thyroid ultrasound with Doppler
• Radioiodine uptake
• Clinical context

Type 1 vs Type 2 AIT

Feature	Type 1	Type 2
Underlying thyroid disease	Present	Absent
Doppler flow	Increased	Reduced
Radioiodine uptake	Normal/low-normal	Very low
IL-6	Variable	May be elevated
Mechanism	Increased synthesis	Destructive release

7. Management

7.1 Amiodarone-Induced Hypothyroidism (AIH)

Overt AIH

1. Levothyroxine
2. Amiodarone is usually continued
3. Start with a low dose and titrate
4. Higher doses may be required

Subclinical AIH

1. May be monitored or treated individually
2. Decision depends on:
• Symptoms
• Degree of TSH elevation
• Anti-thyroid antibody status
• Cardiac status

7.2 Amiodarone-Induced Thyrotoxicosis (AIT)

Type 1

1. Thionamides
2. ± Sodium perchlorate
3. Consider discontinuing amiodarone in coordination with cardiology because benefit may be limited by the drug’s long half-life and underlying arrhythmia indication

Definitive therapy:

• Thyroidectomy
• Radioiodine if uptake is sufficient or recovers

Type 2

1. Glucocorticoids
2. Spontaneous resolution may occur but can be prolonged
3. Surgery if severe

Mixed

1. Combination therapy may be required

8. Monitoring

Before Therapy

1. TSH
2. FT4

Additional tests may be considered:

• FT3
• Anti-TPO if thyroid disease is suspected

Routine baseline thyroid ultrasound is not universally required.

During Therapy

1. TSH and FT4 every 6 months

9. Key Clinical Insight

Amiodarone can cause both hypothyroidism and thyrotoxicosis because of iodine excess combined with direct effects on thyroid hormone synthesis and metabolism

10. Exam Level Pearls

1. AIH → impaired synthesis due to failure of Wolff–Chaikoff escape
2. AIT Type 1 → increased hormone synthesis
3. AIT Type 2 → destructive thyroiditis
4. Treat overt AIH with levothyroxine
5. Treat AIT Type 1 with thionamides
6. Treat AIT Type 2 with glucocorticoids
7. Monitor thyroid function regularly during amiodarone therapy