What is the assessment and management plan for a 27‑year‑old woman with hyperthyroidism on carbimazole (Neomerdin) who developed a diffuse pruritic rash after starting the medication?

Assessment and Management of Carbimazole-Induced Pruritic Rash

This patient has developed a drug-induced hypersensitivity reaction to carbimazole (Neomerdin), and the medication must be discontinued immediately. 1

Assessment

Primary Diagnosis: Drug-induced pruritic rash secondary to carbimazole (Neomerdin)

Key Clinical Features Supporting This Diagnosis:

• Temporal relationship: Rash onset occurred days after starting carbimazole, which is the classic timeframe for drug hypersensitivity reactions 1
• Distribution pattern: Pruritus starting from nape and spreading to back, arms, and legs is consistent with systemic drug reaction 1
• Partial response to antihistamines: Cetirizine provided only partial relief, suggesting ongoing drug exposure as the trigger 2
• No systemic complications: Patient remains hemodynamically stable without fever, hepatotoxicity, or hematologic abnormalities at this time 3, 1

Critical Differential Considerations:

• Rule out severe complications: While this appears to be a mild cutaneous reaction, carbimazole can cause life-threatening adverse effects including agranulocytosis, severe neutropenia, and hepatotoxicity 3, 1
• Thyroid disease as pruritus cause: Unlikely to be the primary etiology given the temporal relationship with medication initiation and the fact that only 27% of thyroid disease patients develop pruritus 4

Immediate Management Plan

1. Discontinue Carbimazole Immediately

Stop Neomerdin (carbimazole) now. 1 This is non-negotiable given the drug-induced hypersensitivity reaction. Continuing the medication risks progression to more severe reactions including Stevens-Johnson syndrome, hepatotoxicity, or agranulocytosis 3, 1.

2. Obtain Baseline Laboratory Studies Before Switching Therapy

Order the following tests urgently:

• Complete blood count with differential: To rule out neutropenia, leukopenia, or agranulocytosis 3, 1
• Comprehensive metabolic panel with liver function tests: To exclude hepatotoxicity (elevated AST, ALT, alkaline phosphatase, bilirubin) 1
• Thyroid function tests (TSH, free T4, free T3): To assess current hyperthyroid status before switching medications 5

3. Symptomatic Treatment of Pruritic Rash

For this Grade 2 pruritus (intense, widespread, limiting activities):

Topical Therapy:

• Apply mometasone furoate 0.1% ointment or betamethasone valerate 0.1% ointment to affected areas twice daily 2, 6
• Apply emollients liberally and frequently to restore skin barrier function 6
• Avoid face application of high-potency steroids 6

Systemic Antihistamine Therapy:

Daytime regimen:

• Loratadine 10 mg once daily (first-line choice per American College of Dermatology) 2
• Alternative: Cetirizine 10 mg daily or fexofenadine 180 mg daily 2

Nighttime regimen (for sleep disruption):

• Hydroxyzine 25-50 mg at bedtime OR diphenhydramine 25-50 mg at bedtime 2, 6

Reassessment Timeline:

• Reassess in 2 weeks: If rash worsens or does not improve despite carbimazole discontinuation, consider dermatology consultation 5, 6
• Expected improvement: Rash should begin resolving within 7-14 days after carbimazole discontinuation 1

4. Alternative Antithyroid Therapy

Once laboratory results confirm no neutropenia or hepatotoxicity, switch to propylthiouracil (PTU):

• PTU 100-150 mg three times daily (adjust based on thyroid function and clinical response) 7, 1
• PTU has been successfully used as an alternative in patients with carbimazole hypersensitivity reactions without cross-reactivity in most cases 1
• Continue propranolol for symptomatic control of hyperthyroid symptoms 5

Critical monitoring after PTU initiation:

• Monitor CBC and liver function tests every 2 weeks for the first 3 months, then monthly 3, 1
• Watch for similar hypersensitivity reactions, though cross-reactivity is uncommon 1
• If PTU also causes adverse reactions, definitive therapy with radioactive iodine ablation or thyroidectomy should be considered 7

5. Patient Education and Safety Counseling

Instruct the patient to:

• Immediately report fever, sore throat, mouth ulcers, or worsening rash (signs of agranulocytosis) 3
• Immediately report jaundice, dark urine, or right upper quadrant pain (signs of hepatotoxicity) 1
• Avoid harsh soaps, wool clothing, and skin irritants 6
• Apply emollients after bathing while skin is still damp 6

6. Follow-Up Schedule

• 2-3 days: Review laboratory results and confirm PTU initiation if safe
• 2 weeks: Reassess rash resolution and thyroid function
• Monthly: Monitor thyroid function, CBC, and liver enzymes while on PTU 1

Important Clinical Pitfalls to Avoid

• Do not rechallenge with carbimazole: Once hypersensitivity develops, rechallenge risks severe reactions 1
• Do not delay laboratory testing: Carbimazole-induced agranulocytosis can be life-threatening and requires immediate detection 3
• Do not use high-potency topical steroids continuously beyond 4 weeks without dermatology supervision due to risk of skin atrophy and HPA axis suppression 6
• Do not assume thyroid disease is causing the pruritus: The temporal relationship with carbimazole initiation makes drug reaction the primary diagnosis 4, 1