OG12.9 | Thyroid Disorders in Pregnancy — Summary & Reflection

KEY TAKEAWAYS

Thyroid disorders in pregnancy must be diagnosed using trimester-specific TSH reference ranges (T1: 0.1–2.5; T2: 0.2–3.0; T3: 0.3–3.0 mIU/L), not non-pregnant ranges. Hypothyroidism (most commonly Hashimoto's) requires levothyroxine, with a ~30% dose increase on confirmation of pregnancy; monitor TSH every 4–6 weeks in T1. The fetal brain depends entirely on maternal T4 for the first 12 weeks — untreated hypothyroidism impairs neurodevelopment. Hyperthyroidism (Graves' most common): antithyroid drugs are needed. PTU is preferred in the first trimester (carbimazole causes aplasia cutis/embryopathy in organogenesis); switch to carbimazole in T2 (PTU hepatotoxicity risk). Target FT4 in upper normal range — overtreatment causes fetal hypothyroidism. Gestational transient thyrotoxicosis (hCG-mediated, peaks T1, no TRAb, associated with hyperemesis) — no ATD, supportive only. Thyroid storm = ICU emergency: PTU + Lugol's iodine (1 h after PTU) + propranolol + dexamethasone. High maternal TRAb at 36 weeks → neonatal Graves' surveillance. Postpartum thyroiditis (anti-TPO positive) = hyper → hypo sequence; 20–30% permanent hypothyroidism.

REFLECT

A woman with known hypothyroidism on levothyroxine has just had a positive pregnancy test at home. She calls your clinic at 7 weeks. Applying Kolb's cycle — what prior experience from this module do you bring to this call? What abstract conceptualisation (the mechanism of TBG increase and the fetal brain's T1 dependence) makes 'increase your dose now' more than a rote instruction? What concrete active experiment (dose change + TSH in 4 weeks) will you prescribe? Reflect: she is already 7 weeks, meaning she has already missed 7 weeks of the critical developmental window at her old dose. Does that change your urgency? And think forward — what counselling would you give her before her NEXT pregnancy to prevent this gap entirely? This is the difference between a pharmacist-level 'take this drug' and a physician-level 'here is why, and here is how we protect your baby's brain'.