What is the further management for a patient with hyperthyroidism taking Neo Mercazole (Carbimazole) who develops acute thyroiditis, throat pain, fever, and neck swelling after a upper respiratory infection (URI)?

Immediate Management: Suspect Agranulocytosis and Discontinue Neo Mercazole

This patient presenting with fever, throat pain, and neck swelling while on carbimazole (Neo Mercazole) most likely has drug-induced agranulocytosis, not acute thyroiditis—immediate discontinuation of carbimazole and urgent complete blood count are mandatory. 1, 2, 3

Critical First Steps

Immediate Actions Required

• Stop Neo Mercazole immediately without waiting for laboratory confirmation, as agranulocytosis is a life-threatening emergency that typically occurs within the first 3 months of therapy but can develop at any time 1, 2, 3
• Obtain urgent complete blood count with differential to assess absolute neutrophil count (ANC), as severe neutropenia (ANC <500/mm³) requires immediate intervention 4, 2
• Check liver function tests, as carbimazole can cause concurrent hepatotoxicity presenting with fever and malaise 1, 2
• Measure thyroid function tests (TSH, free T4, free T3) to assess current thyroid status 4, 5

Clinical Differentiation: Agranulocytosis vs Thyroiditis

• Agranulocytosis presents with: fever, sore throat, malaise, and occurs in 3 per 10,000 patients on antithyroid drugs, typically within the first 3 months 1, 2, 3
• Drug-induced thyroiditis presents with: neck pain, thyrotoxic symptoms (palpitations, tremor), and typically follows a hyperthyroid-to-hypothyroid course 4, 5
• The combination of URI followed by fever and throat pain strongly suggests agranulocytosis with secondary infection rather than thyroiditis alone 1, 2

Management Algorithm Based on Laboratory Results

If Agranulocytosis Confirmed (ANC <500/mm³)

• Admit to hospital immediately for isolation and monitoring 2
• Administer granulocyte colony-stimulating factor (G-CSF) to accelerate neutrophil recovery, which provides longer protection against infection 4, 2
• Start broad-spectrum antibiotics if infection is suspected or confirmed, as patients are at high risk for sepsis 4, 2
• Never restart carbimazole or switch to propylthiouracil, as cross-reactivity occurs and both drugs can cause agranulocytosis 1, 3

If Severe Neutropenia (ANC 500-750/mm³)

• Consider dose reduction if carbimazole were to be continued, but given the clinical presentation with fever and throat pain, complete discontinuation is safer 4
• Monitor ANC every 2-3 days until recovery to >1,000/mm³ 4

If Hepatotoxicity Present (ALT >3x Upper Normal)

• Discontinue carbimazole permanently, as hepatotoxicity can be severe and even lethal with cholestatic or hepatocellular patterns 1, 2
• Provide supportive care and monitor liver enzymes weekly until normalization 2

Definitive Management of Hyperthyroidism

Immediate Thyroid Control

• Start beta-blockers (propranolol 40-80 mg three times daily or atenolol 25-50 mg once daily) for symptomatic control of hyperthyroidism while planning definitive therapy 5, 6
• Beta-blockers provide rapid symptom relief without affecting thyroid hormone production 5

Definitive Treatment Options (After Agranulocytosis Resolves)

• Radioactive iodine (¹³¹I) ablation is the preferred definitive treatment after carbimazole-induced agranulocytosis, as antithyroid drugs cannot be safely restarted 7, 3
• Thyroidectomy should be considered if radioactive iodine is contraindicated or patient preference, particularly given the severity of the adverse reaction 3
• Monitor for post-ablation complications including acute neck edema and thyroiditis, which can occur 3 days after ¹³¹I treatment and may require corticosteroids 7

Critical Pitfalls to Avoid

Common Errors in Management

• Do not attribute fever and throat pain to "thyroiditis" alone without checking complete blood count, as this delays recognition of life-threatening agranulocytosis 1, 2
• Never switch from carbimazole to propylthiouracil after agranulocytosis, as cross-reactivity occurs in up to 50% of cases 1
• Do not restart carbimazole even at lower doses after agranulocytosis, as recurrence is likely and potentially fatal 1, 3
• Avoid attributing neck swelling solely to ultrasound findings of "thyroiditis" without considering drug-induced complications 2, 7

Monitoring Requirements Post-Recovery

• If radioactive iodine is chosen, warn patient about potential acute neck edema occurring 2-3 days post-treatment, which may require corticosteroids 7
• After definitive treatment (ablation or surgery), monitor for hypothyroidism development and initiate levothyroxine when TSH >10 mIU/L or if symptomatic with any TSH elevation 8
• For elderly patients or those with cardiac disease, start levothyroxine at lower doses (25-50 mcg daily) if hypothyroidism develops post-treatment 8, 5

Special Considerations

If Patient Has Concurrent Adrenal Insufficiency

• Start corticosteroids before initiating thyroid hormone replacement to prevent adrenal crisis 5

Pregnancy Considerations

• If patient is pregnant or planning pregnancy, thyroidectomy is preferred over radioactive iodine, and propylthiouracil cannot be used due to agranulocytosis history 1