Carbimazole Dose Adjustment for Improving Hyperthyroidism
Reduce carbimazole from 30mg to 15-20mg once daily immediately, as the patient's free T4 has normalized (12.50 pmol/L is within normal range) and continuing the current dose risks inducing iatrogenic hypothyroidism.
Clinical Context and Rationale
Your patient demonstrates excellent response to treatment with progressive normalization of thyroid function (FT4: 45 → 20.20 → 12.50 pmol/L), though TSH remains suppressed (<0.05 mIU/L). This pattern indicates:
- Biochemical improvement: The falling FT4 suggests effective thyroid suppression 1
- Lag in TSH recovery: TSH normalization typically occurs 6-12 months after achieving euthyroidism, significantly later than FT4 normalization 2
- Risk of over-treatment: Maintaining 30mg daily with normalized FT4 substantially increases risk of iatrogenic hypothyroidism 3
Specific Dose Adjustment Algorithm
Immediate Action (Now)
- Reduce carbimazole to 15-20mg once daily 3, 4
- Continue as single daily dose (equally effective as divided dosing) 5
- Recheck thyroid function tests in 2-4 weeks 1
Monitoring Schedule
- Weeks 2-4: Repeat TSH and FT4 to assess response 1
- If FT4 remains normal-low: Further reduce to 10mg daily 2
- If FT4 rises: Maintain current dose and recheck in 4 weeks 1
- Once stable: Extend monitoring to every 6-8 weeks 6
Target Maintenance Dose
- Aim for 5-10mg daily as maintenance dose 2
- Research demonstrates 66% of patients achieve remission with ≤5mg daily 2
- Lower maintenance doses do not compromise remission rates 2
Critical Monitoring Points
Watch for Over-Treatment Signs
- Rising TSH above normal range with normal/low FT4 indicates over-treatment 6
- Symptoms of hypothyroidism: fatigue, weight gain, cold intolerance, constipation 6
- If either develops: reduce dose further or discontinue temporarily, recheck in 2-3 weeks 6
Symptomatic Management
- Continue beta-blocker (propranolol 25-50mg or atenolol) if patient has residual tachycardia, tremor, or palpitations 1
- Beta-blockers can be tapered as symptoms resolve 1
Important Clinical Caveats
Pitfall to avoid: Do not wait for TSH normalization before dose reduction. TSH recovery lags significantly behind FT4 normalization (often 6-12 months), and maintaining high-dose carbimazole during this period causes unnecessary hypothyroidism 2.
Severe hyperthyroidism consideration: If initial FT4 was >260 nmol/L (approximately 45 pmol/L in your patient), higher doses are initially required, but dose reduction is still essential once FT4 normalizes 3.
Long-term planning: After achieving stable euthyroidism on low-dose maintenance therapy (typically 12-18 months total treatment), consider discontinuation with close monitoring, as 60% achieve sustained remission 2. However, 77% of relapses occur within the first year post-treatment, necessitating continued surveillance 2.
Serious adverse effects: Monitor for agranulocytosis, hepatitis, vasculitis, and thrombocytopenia throughout treatment, though these are rare 1. Instruct patient to report fever, sore throat, or unusual bleeding immediately.