Follow-Up of a Patient with Hyperthyroidism on Carbimazole

1. Initial Assessment & Baseline

• Baseline TFTs: TSH, free T4, free T3
• FBC (baseline WBC/neutrophil count) — not for routine serial monitoring, but useful at start
• LFTs (especially if PTU considered; less critical for carbimazole)
• TRAb (TSH receptor antibodies) — guides prognosis and duration of treatment
• Clinical symptoms review: palpitations, weight loss, heat intolerance, tremor, diarrhea

2. Starting Dose & Titration

• Carbimazole initial dose: 10–20 mg every 12 hours (i.e., 20–40 mg/day)
• Once euthyroid is restored, can switch to once-daily dosing
• Two main regimens:

3. Monitoring Schedule

Thyroid Function Tests (TFTs)

• First review: 4–6 weeks after starting treatment
• Monitor free T4 (unbound T4) as the primary indicator of response at first
• TSH remains suppressed for several months even after euthyroidism is achieved — do NOT use TSH alone to guide early dose adjustments
• Once TSH suppression resolves, TSH can be used alongside free T4 to monitor therapy
• Most patients do not achieve euthyroidism until 6–8 weeks after treatment

• Every 4–6 weeks during initial dose adjustment
• Every 3 months once stable/euthyroid
• At least annually in remission

TRAb Levels

• Recheck at 12–18 months (at end of planned treatment course)
• Undetectable TRAb predicts higher remission rates; persistent TRAb predicts relapse
• TRAb testing before stopping treatment helps counsel the patient on relapse risk

4. Adverse Effects — What to Monitor Clinically

A. Agranulocytosis (most serious — ~0.1–0.5% incidence)

• Risk highest in first 90 days, especially with doses >40 mg/day methimazole equivalent
• Do NOT do routine serial FBC — onset is idiosyncratic and abrupt, so prospective monitoring is not useful
• Instead: Patient education is the most critical intervention
  • Give written instructions: stop carbimazole immediately and seek urgent medical attention if any of these occur:
    • Sore throat
    • Fever
    • Mouth ulcers
  • Arrange urgent FBC if these symptoms occur; if neutrophils <0.5 × 10⁹/L → agranulocytosis confirmed → stop drug permanently
  • Do NOT restart; cross-sensitivity between carbimazole/methimazole and PTU is ~50%

B. Minor side effects (1–5%)

• Rash, urticaria, pruritus, fever, arthralgia
• May resolve spontaneously or with antihistamine
• Can try switching to PTU (with caution re hepatotoxicity) for minor reactions
• Stop drug permanently for major reactions

C. Cholestasis/hepatotoxicity

• Cholestatic jaundice is associated with carbimazole/methimazole (less severe than PTU hepatitis)
• Check LFTs if the patient develops jaundice or abdominal pain

5. Adjunctive Therapy

• Beta-blockers (propranolol 20–40 mg q6h, or atenolol/metoprolol): Useful in early weeks to control adrenergic symptoms (palpitations, tremor, anxiety) while waiting for antithyroid effect
• Atrial fibrillation: Discuss anticoagulation (CHA₂DS₂-VASc score); most revert to sinus rhythm once euthyroid — long-term anticoagulation not usually needed
• Warfarin dosing: Reduced doses needed when thyrotoxic (increased clearance of clotting factors); adjust as patient becomes euthyroid

6. Duration of Treatment & Remission

• Standard course: 12–18 months
• Remission rates: 30–60% at 18 months for the titration regimen
• Higher remission if TRAb becomes undetectable
• Predictors of relapse (high-risk features — consider ablative therapy):
  • Young age
  • Male sex
  • Smoker
  • Large goiter
  • Severe initial hyperthyroidism
  • Persistent TRAb at end of treatment

7. Follow-Up After Stopping Treatment

• Monitor closely in year 1 for relapse — TFTs every 3 months
• At least annually thereafter
• If relapse occurs: can offer prolonged low-dose antithyroid drugs, radioiodine (¹³¹I), or thyroidectomy

8. Special Situations

1. Use free T4 (not TSH) to guide early dose adjustments — TSH stays suppressed for months.
2. Do not routinely check FBC — instead, educate the patient about agranulocytosis warning symptoms (sore throat, fever, mouth ulcers).
3. Aim for euthyroidism by 6–8 weeks; plan a 12–18 month course.
4. Check TRAb at end of treatment to predict relapse risk.
5. All patients need annual follow-up after stopping carbimazole, as late relapse and also late hypothyroidism can occur.

Carbimazole Dose Reduction Based on Free T4 — Titration Regimen

The Core Principle

Step-by-Step Dose Reduction Protocol

Starting dose

At 4–6 weeks (First Review)

Example: Patient starts at 40 mg/day. At 6 weeks, baseline fT4 was 50 pmol/L and is now 22 pmol/L (>50% fall, still mildly elevated) → reduce to 20 mg/day. At next check it is 14 pmol/L (normal) → reduce to 10 mg/day.

At 10–12 weeks (Second Review)

• If fT4 now normal → reduce to maintenance dose: 5–10 mg/day
• If fT4 still mildly elevated → keep at 20 mg, recheck in 4–6 weeks
• If fT4 low → reduce dose or pause briefly; check for symptoms of hypothyroidism

Maintenance Phase (once stable)

• Target: fT4 in the normal range, TSH ideally normalised
• Usual maintenance dose: 5–10 mg/day (once daily)
• Once stable for ≥3 months: extend TFT monitoring to every 3–6 months
• TSH becomes useful as a monitoring tool only once it has normalised (usually after 3–4 months of treatment)

Practical Decision Guide

fT4 HIGH (>ULN)
  └─ Very high or unchanged      → maintain or increase dose
  └─ Falling but still elevated  → reduce by 50% if fallen >50% from baseline

fT4 NORMAL (within reference range)
  └─ Reduce to maintenance 5–10 mg/day
  └─ If also TSH now normal → consider once-daily dosing

fT4 LOW (<LLN)  ← iatrogenic hypothyroidism
  └─ Reduce dose immediately to 2.5–5 mg/day
  └─ Check for symptoms (fatigue, cold intolerance, bradycardia)
  └─ Or add levothyroxine (block-replace approach)
  └─ A raised TSH above normal at this stage = over-treated → reduce dose

Key Rules to Remember

1. Use fT4, not TSH, for the first 3–4 months. TSH stays suppressed even after fT4 normalises — using TSH early leads to over-treatment.
2. TSH rising above normal = iatrogenic hypothyroidism → dose is too high, reduce immediately.
3. Aim for smallest effective dose: 5–10 mg/day at maintenance — high doses long-term increase agranulocytosis risk.
4. The goal is to keep fT4 in the normal range (typically 9–22 pmol/L depending on local lab) for the entire 12–18 month course.
5. Once the maintenance dose is stable and fT4 normal for ≥3 months, check TFTs every 3–6 months rather than 4–6 weekly.

Summary Timeline