Makindo Medical Notes

Related Subjects: |Thyrotoxicosis and Hyperthyroidism |Thyroid Storm - Thyrotoxic crisis |Graves Disease (Thyrotoxicosis) |Amiodarone and Thyroid disease |Thyroid Surgery (Thyroidectomy) |Hypothyroidism |Hashimoto's thyroiditis |DeQuervain's thyroiditis |Subacute Thyroiditis |Thyroid nodule |Congenital Hypothyroidism |Thyroid Function Tests and antibodies |Post partum thyroiditis |Sick Euthyroid Syndrome |Thyroid Exam (OSCE) |Thyroid Gland anatomy and Physiology |Thyroid Cancer

💡 Note: Amiodarone has a very long half-life (40–100 days) and contains a high iodine load. ⚠️ This iodine content can cause both hypothyroidism and hyperthyroidism, requiring close monitoring.

📖 About

• Amiodarone use requires joint monitoring by cardiology and endocrinology specialists.
• Iodine excess may suppress thyroid function acutely (Wolff–Chaikoff effect), but over time it can trigger thyroid dysfunction.
• Both hypothyroidism and thyrotoxicosis (Type 1 or Type 2) are recognised complications.

🧬 Aetiology

• Type 1 Amiodarone-induced thyrotoxicosis (AIT): Occurs in abnormal thyroid (nodular goitre, latent Graves’). Excess iodine → increased hormone synthesis. 🔍 Pathophysiology: "Jod-Basedow" phenomenon.
• Type 2 AIT: Occurs in previously normal thyroid. A destructive thyroiditis due to amiodarone toxicity → thyroid hormone release. 🔍 Pathophysiology: inflammatory damage, not hormone overproduction.
• Other drug-induced thyroid dysfunction: Lithium, interferon-α, HAART (HIV therapy).

🩺 Clinical Features

• Typical symptoms of thyrotoxicosis: weight loss, palpitations, tremor, heat intolerance, anxiety, irritability.
• Older patients may present with "apathetic thyrotoxicosis" (fatigue, depression, cardiac arrhythmias).
• Amiodarone also directly causes bradycardia & conduction abnormalities, which can mask classic hyperthyroid tachycardia.

🔎 Differential Diagnosis

• Graves’ disease.
• Toxic multinodular goitre.
• Thyroiditis (subacute, painless, postpartum).

🧪 Investigations

• 📊 TFTs: Hyperthyroidism → ↑ free T4, ↓ TSH. Sometimes ↑ T4 with normal TSH (monitor, repeat in 6–12 weeks).
• 🔬 Thyroid antibodies: TSH receptor antibodies (TRAb) may support Graves’ (Type 1 AIT).
• 🧲 Doppler US: ↑ vascularity in Type 1, ↓ in Type 2.
• 🧪 Interleukin-6: Elevated in Type 2 (inflammatory process).
• Routine baseline TFTs before starting amiodarone, then every 6 months.

⚕️ Management

• Type 1 AIT: Carbimazole (or methimazole). Consider stopping amiodarone if clinically safe. Potassium perchlorate sometimes used (specialist-only).
• Type 2 AIT: Steroids (e.g. prednisolone) → reduce inflammation and hormone release.
• Mixed / Uncertain type: Combination of carbimazole + steroids.
• Hypothyroidism: Start levothyroxine (25 mcg OD, titrate according to TSH). Amiodarone may be continued if essential for cardiac rhythm.
• Close monitoring required – joint decision-making between endocrinology and cardiology is essential.

📚 References

• NICE CKS: Thyroid dysfunction.
• European Thyroid Association guidelines on amiodarone-induced thyroid dysfunction.