What should be done for a patient on Carbimazole (methimazole) with elevated Free Thyroxine (FT4) and suppressed Thyroid-Stimulating Hormone (TSH) levels?

Increase Carbimazole Dose Immediately

Your patient remains hyperthyroid on Carbimazole 10mg daily—the FT4 is elevated at 12.67 pmol/L (reference 4.87-11.72) and TSH remains suppressed at 0.06 mIU/L (reference 0.4001-4.049), indicating inadequate control of thyrotoxicosis that requires dose escalation to prevent cardiovascular complications, atrial fibrillation, and bone loss. 1, 2

Current Thyroid Status Assessment

• The suppressed TSH (<0.1 mIU/L) combined with elevated FT4 definitively confirms persistent hyperthyroidism despite treatment 3
• This biochemical profile indicates the current 10mg daily dose is insufficient to control thyroid hormone synthesis 1, 2
• Prolonged uncontrolled hyperthyroidism significantly increases risk for atrial fibrillation (5-fold in patients ≥45 years), osteoporosis, fractures, and cardiovascular mortality 3

Immediate Dose Adjustment Strategy

Increase Carbimazole to 30mg once daily to achieve biochemical control within 6-12 weeks, as single daily dosing of 30mg has proven equally effective as divided doses for treating thyrotoxicosis 1, 2

• The 30mg once-daily regimen achieves euthyroidism in most patients within 1-3 months, with 77-100% achieving normal thyroid hormone levels 1, 2
• Single daily dosing is as effective as divided doses (10mg three times daily) based on carbimazole's long intrathyroidal half-life 1
• Continue propranolol 20mg three times daily for the first 4 weeks after dose increase to control adrenergic symptoms (palpitations, tremor, anxiety) while awaiting biochemical response 1, 4

Monitoring Protocol After Dose Increase

• Recheck TSH and free T4 in 6 weeks to evaluate response to the increased dose 3, 1
• Target TSH within reference range (0.4-4.0 mIU/L) with FT4 in the low-normal range to confirm adequate control 2
• Monitor complete blood count and liver enzymes at 6 weeks, as carbimazole carries risk of agranulocytosis (rare but serious) and hepatotoxicity 1, 5

Critical Warning Signs Requiring Immediate Action

• Instruct the patient to stop carbimazole immediately and seek urgent medical attention if fever, sore throat, mouth ulcers, or signs of infection develop—these may indicate agranulocytosis 5
• Monitor for signs of hepatotoxicity (jaundice, dark urine, right upper quadrant pain), which would necessitate immediate discontinuation 6
• If the patient develops severe hepatotoxicity or agranulocytosis, lithium 300mg daily (maintaining levels 0.4-0.6 mEq/L) can serve as an alternative while awaiting definitive radioiodine ablation 6

Expected Timeline and Outcomes

• Clinical euthyroidism typically occurs within 1-3 months of starting 30mg daily dosing 2
• Patients with larger goitres or elevated alkaline phosphatase may require longer treatment duration (up to 41 weeks) 2
• Once TSH normalizes and FT4 reaches low-normal range, consider reducing to maintenance dose of 5-15mg daily 2

Common Pitfalls to Avoid

• Never continue inadequate dosing—persistent hyperthyroidism causes irreversible cardiovascular damage, bone loss, and increased mortality risk 3
• Do not wait longer than 6 weeks to reassess thyroid function, as prolonged uncontrolled hyperthyroidism significantly increases morbidity 1, 2
• Avoid stopping propranolol prematurely before achieving biochemical euthyroidism, as adrenergic symptoms will persist 1, 4

Alternative Considerations if Carbimazole Fails

• If the patient remains hyperthyroid after 3 months on 30mg daily, consider definitive therapy with radioiodine ablation or thyroidectomy 2, 5
• Percutaneous ethanol injection may be considered for toxic adenomas if radioiodine is contraindicated or refused, achieving complete cure in 77.9% of cases 4