DNB EXAM NOTES: Thyroid Disorders in Pregnancy

1. PHYSIOLOGICAL CHANGES IN PREGNANCY ⭐

• Maternal thyroid demand increases by ~50% during pregnancy
• Raised oestrogen increases TBG - plateaus at 18-20 weeks
• First trimester hCG (weak TSH-like action) transiently raises fT4/fT3 and suppresses TSH
• Iodine requirement rises due to: increased synthesis + increased renal clearance + placental storage + fetal uptake
• Fetal thyroid takes up iodine from 10-12 weeks; produces hormones from 18-22 weeks
• Fetus entirely dependent on maternal T4 before 18-22 weeks and receives maternal T4 until birth

2. DEFINITIONS & PREVALENCE ⭐

3. TFT INTERPRETATION ⭐

• Always use trimester- and manufacturer-specific reference ranges [Grade B]
• Pragmatic upper TSH limit if no local range: 4.0 mU/L in first trimester
• Treatment target: TSH < 2.5 mU/L + fT4 within trimester-specific normal range

4. IODINE IN PREGNANCY ⭐

• Recommended intake: 200-250 mcg/day
• Supplementation: 150 mcg/day (as potassium iodide in prenatal vitamins) [Grade C]
• Upper safe limit: <500 mcg/day (above this causes serious harm)
• Ideally start before conception for maximum fetal neurodevelopmental benefit

5. HYPOTHYROIDISM ⭐⭐

Risks of untreated OH:

Preconception:

• OH / severe SCH (TSH >10): Titrate levothyroxine to TSH ≤ 2.5 mU/L [Grade B]
• Mild SCH (TSH between ULN and 10, TPOAb+): Consider levothyroxine [Grade C]

In pregnancy:

• Self-initiate dose increase immediately on positive pregnancy test: double the dose on 2 days per week (~29% increase) [Grade A]
• Monitor TSH + fT4 every 4-6 weeks until 20 weeks, then at 28 weeks [Grade A]
• Treatment: Levothyroxine (T4) ONLY - no desiccated thyroid or combined T4/T3 (insufficient fetal brain T4 transfer)
• Up to 40% need further dose adjustments during pregnancy
• Return to pre-pregnancy dose 2 weeks postpartum; confirm with TFT at 6 weeks

6. HYPERTHYROIDISM ⭐⭐

Gestational Transient Thyrotoxicosis (GTT) vs Graves':

Antithyroid Drug (ATD) Choice: ⭐⭐

• Before 10 weeks: PTU (propylthiouracil) preferred [Grade B/D]
• After 20 weeks: Switch to carbimazole (CMZ) - to avoid PTU hepatotoxicity
• Conceived on CMZ: Switch to PTU ASAP, before 10 weeks [Grade D]
• Dose conversion: 200 mg PTU = 10 mg CMZ (ratio 20:1)

Why PTU first?

ATD monitoring:

• First half of pregnancy: TFTs every 2-4 weeks
• After 20 weeks: TFTs every 4-8 weeks
• Target: fT4 in upper half of pregnancy reference range (NOT TSH)

ATD adverse effects:

• Minor (skin rash): 3-5%
• Agranulocytosis: 0.15%
• Liver failure: 0.1% (almost exclusively PTU)

7. THYROID ANTIBODIES (TPOAb) ⭐

• Routine TPOAb testing NOT recommended in euthyroid pregnant women [Grade B]
• Levothyroxine NOT recommended for TPOAb-positive euthyroid women [Grade A]
• If already known TPOAb-positive: offer TFTs at first trimester contact and at 20 weeks [Grade C]
• TPOAb positivity (even euthyroid) is associated with miscarriage, preterm birth, PPT

8. THYROID NODULES/GOITRE ⭐

• FNA: safe at any gestation if malignancy suspected [Grade B]
• Thyroid surgery: ideally 14-22 weeks of gestation [Grade C]
• Enlarged thyroid: alert anaesthetic team for airway assessment

9. POSTPARTUM THYROIDITIS (PPT) ⭐

• Incidence: 5-10%; classic triphasic pattern:
  1. Thyrotoxic phase (1-4 months) - destructive, ATDs NOT helpful; use beta-blockers if symptomatic
  2. Hypothyroid phase (4-8 months) - levothyroxine if symptomatic
  3. Recovery by 12 months (in most)
• ~50% develop permanent hypothyroidism
• Monitor TSH annually in all women with PPT history [Grade C]
• No proven prophylaxis (levothyroxine, iodine, selenium all fail to reliably prevent PPT)

KEY NUMBERS (must memorise) ⭐⭐