IM12.12-14 | Thyroid Pharmacotherapy and Definitive Therapy — Summary & Reflection

KEY TAKEAWAYS

Thyroid pharmacotherapy requires precision in drug selection, dose, sequencing, monitoring, and patient communication.

Levothyroxine (hypothyroidism):
- Healthy adult: 1.6 µg/kg/day ideal body weight (typically 100–125 µg/day)
- Elderly / cardiac disease: start 25–50 µg/day, titrate 25 µg every 6–8 weeks
- Pregnancy: dose increase of 25–50 µg upon pregnancy confirmation; target TSH <2.5 mIU/L (1st trimester), <3.0 mIU/L (2nd/3rd trimester)
- Secondary hypothyroidism: cortisol first; titrate to FT4, not TSH
- Monitoring: TSH at 4–6 weeks after each dose change; annually once stable
- Interactions: food, calcium, iron, antacids reduce absorption — take 30–60 min before food; separate supplements by 4 h

Thionamides (hyperthyroidism):
- Carbimazole: 20–40 mg/day initially; reduces to 5–15 mg/day maintenance
- PTU: 100–200 mg every 8h; also inhibits peripheral T4→T3 conversion
- First trimester pregnancy: PTU preferred (carbimazole embryopathy)
- Adverse effects: rash (common), agranulocytosis (0.1–0.5% — fever+sore throat+mouth ulcers = stop drug + urgent CBC), PTU hepatotoxicity (rare)
- Graves disease remission on drugs: ~30–50%; relapse 50–70% → definitive therapy

Definitive therapy:
- Radioiodine: Graves relapse (no active severe ophthalmopathy), toxic adenoma, TMNG; contraindicated in pregnancy/breastfeeding; stop carbimazole 3–7 days before
- Surgery: large goitre with compression, active severe ophthalmopathy, suspected malignancy, pregnancy (2nd trimester, if drugs fail); pre-op euthyroid with carbimazole + Lugol's iodine 10–14 days

Thyroid storm sequence: PTU → Lugol's iodine (≥1h after PTU) → propranolol → hydrocortisone → supportive care; Burch-Wartofsky score ≥45 = storm

Myxoedema coma sequence: Airway → hydrocortisone IV → IV levothyroxine 200–500 µg loading → active rewarming → treat precipitant

REFLECT

Return to the three patients in the opening hook. Patient 1 — the pregnant woman at 10 weeks — needed PTU (not carbimazole) because of the embryopathy risk; the correct drug required knowledge of the first-trimester rule. Patient 2 — the 58-year-old man with TMNG — cannot stop carbimazole and expect remission, because TMNG never remits; he needs to understand that definitive therapy (radioiodine or surgery) is the plan. Patient 3 — the 44-year-old woman — needs a calculated levothyroxine dose (1.6 µg/kg × 68 kg ≈ 100 µg), explained simply and with the five practical points about how to take it. Each patient needed not just the correct drug but the correct communication. Think about this: how would you explain to Patient 2 — a man who has taken carbimazole for 18 months and 'feels fine' — why he cannot simply stay on the medication indefinitely? What language would you use to explain relapse risk, antithyroid drug adverse effects, and the rationale for a definitive procedure that will make him permanently hypothyroid? The ability to translate pharmacological reasoning into patient-understandable language is the completion of the prescribing skill — and it is what the NMC IM12.13 competency ultimately measures.