Thyroid disorders

Quick Overview

Thyroid disorders are common endocrine conditions requiring precise TFT interpretation and evidence-based management. NICE NG145 emphasizes systematic approach to subclinical vs overt disease, appropriate levothyroxine dosing (1.6 mcg/kg ideal body weight), and recognition of thyroid storm. Mastery of TFT patterns and treatment thresholds is essential for safe prescribing and complication prevention.

Core Facts & Concepts

TFT Interpretation Thresholds:

Key Numbers:

• Levothyroxine dosing: 1.6 mcg/kg ideal body weight (typically 100-125 mcg daily in adults)
• Dose adjustment: 25-50 mcg increments every 6-8 weeks
• Target TSH: 0.5-2.5 mU/L (pregnancy: trimester-specific targets)
• Carbimazole starting dose: 15-40 mg daily (titration regimen) OR 40 mg daily (block-and-replace)
• Thyroid storm mortality: 20-30% despite treatment

Thyroid Storm Criteria (≥45 points):

• Temperature >38.5°C (5-30 points)
• HR >130 bpm (10-25 points)
• Heart failure present (15 points)
• CNS effects: agitation/delirium/psychosis/coma (10-30 points)

Autoantibodies:

• TPO antibodies: Hashimoto's thyroiditis (95%), Graves' disease (75%)
• TSH receptor antibodies (TRAb): Graves' disease (diagnostic, 90% sensitivity)
• Thyroglobulin antibodies: Hashimoto's thyroiditis

Problem-Solving Approach

Step 1: Confirm Diagnosis

1. Repeat TFTs in 3 months for subclinical disease (exclude transient dysfunction)
2. Check TPO/TRAb antibodies to determine etiology
3. Thyroid ultrasound if nodules palpable or cancer suspected

Step 2: Decide Treatment Threshold

Hypothyroidism:

• Treat if: TSH >10 mU/L OR TSH 4.5-10 mU/L + symptoms/pregnancy/TPO+
• Levothyroxine: 1.6 mcg/kg IBW (reduce to 1.2 mcg/kg if >65 years or IHD)
• Check TFTs 6-8 weeks after initiation/dose change

Hyperthyroidism:

• Carbimazole regimens (NICE NG145):
  • Titration: 15-40 mg daily, reduce by 5 mg every 4-6 weeks (12-18 months total)
  • Block-and-replace: 40 mg carbimazole + 50-100 mcg levothyroxine (6 months)
• Propylthiouracil: Use in first trimester pregnancy (lower teratogenicity) or carbimazole allergy

Step 3: Monitor for Complications

🚩 Red Flags:

• Agranulocytosis (0.3% on carbimazole): Sore throat/fever → stop drug, urgent FBC
• Thyroid storm: ICU admission, propylthiouracil 600 mg loading + propranolol + hydrocortisone 100 mg QDS
• Atrial fibrillation: Present in 10-25% of hyperthyroidism (anticoagulate per CHA₂DS₂-VASc)

⚠️ Warning: Never start levothyroxine at full replacement dose in elderly/IHD patients-risk of MI. Start 25 mcg daily and titrate slowly.

Analysis Framework

Differential Diagnosis by TFT Pattern:

Discriminating Features: Causes of Hyperthyroidism

📌 Remember: The 3 T's of Thyroid Storm - Temperature >38.5°C, Tachycardia >130 bpm, Tremor/agitation (CNS dysfunction)

Visual Aid

Levothyroxine Dose Adjustment Guide:

Key Points Summary

✓ TFT interpretation: Subclinical = TSH abnormal but T4/T3 normal; treat subclinical hypothyroidism if TSH >10 OR TSH 4.5-10 + symptoms/pregnancy/TPO+

✓ Levothyroxine dosing: 1.6 mcg/kg ideal body weight (typically 100-125 mcg), reduce to 1.2 mcg/kg if >65 years or IHD; check TFTs 6-8 weeks after dose changes

✓ Carbimazole regimens: Titration (15-40 mg, reduce every 4-6 weeks, 12-18 months) OR block-and-replace (40 mg + levothyroxine, 6 months)

✓ Thyroid storm: Mortality 20-30%; treat with propylthiouracil 600 mg loading, propranolol, hydrocortisone 100 mg QDS, supportive ICU care

✓ Agranulocytosis risk: 0.3% on carbimazole-warn patients to stop drug and seek urgent FBC if sore throat/fever develops

✓ TRAb positive = Graves' disease (90% sensitivity); radioiodine uptake differentiates thyroiditis (low uptake) from Graves'/toxic nodule (high uptake)

✓ Common pitfall: Starting full-dose levothyroxine in elderly/IHD patients-always start 25 mcg and titrate slowly to avoid precipitating MI