Thyroid Disorders in Pregnancy

Screening in Pregnancy

• Routine universal screening for thyroid dysfunction is not recommended

• High-Risk Factors (test TSH at first antenatal visit)

  • Personal history of thyroid disorder (treated or current)

  • Family history of thyroid disease

  • T1DM or other autoimmune disorders (e.g. coeliac disease, pernicious anaemia)

  • Goitre or nodules on clinical exam

  • Infertility, recurrent miscarriage, or history of preterm delivery

• Trimester-Specific TSH Ranges (approximate)

  • 1st trimester: 0.1–4.0 mU/L

  • 2nd trimester: 0.2–4.0 mU/L

  • 3rd trimester: 0.3–4.0 mU/L

• Additional Notes

  • Check TPO antibodies if TSH is elevated or borderline, as antibody positivity increases risk of hypothyroidism and adverse pregnancy outcomes

  • Mild maternal thyroid dysfunction can affect foetal neurodevelopment

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Management of Hypothyroidism

• Overt Hypothyroidism (elevated TSH, low free T4)

  • Start or adjust levothyroxine promptly to maintain TSH <2.5 mU/L (particularly in 1st trimester)

  • If already on levothyroxine pre-pregnancy, increase dose by ~30% upon confirmation of pregnancy (often +2 extra doses per week)

  • Monitor TSH (and free T4 if indicated) every 4–6 weeks in the 1st and 2nd trimesters, then at least once in the 3rd trimester

  • Untreated hypothyroidism increases risks of miscarriage, pre-eclampsia, low birth weight, and impaired foetal neurodevelopment

• Subclinical Hypothyroidism (elevated TSH, normal free T4)

  • Treat if TSH >10 mU/L or if TSH is above trimester-specific range with positive TPO antibodies or history of miscarriage

  • Start levothyroxine 50 mcg daily, adjust based on TSH

  • Aim for TSH below trimester-specific upper limit (ideally <2.5 mU/L in 1st trimester)

• Postpartum Follow-Up

  • Re-check TSH ~6 weeks post-delivery if changes to medication in pregnancy

  • Many women may reduce levothyroxine dose postpartum, but watch for persistent hypothyroidism

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Management of Hyperthyroidism

• Graves’ Disease

  • Use the lowest effective dose of antithyroid drugs to avoid foetal hypothyroidism

  • 1st trimester: Propylthiouracil (PTU) is preferred (though monitor for hepatotoxicity)

  • 2nd & 3rd trimesters: Switch to carbimazole (risk of teratogenicity is greatest in the 1st trimester)

  • Monitor TSH, free T4 every 4–6 weeks to adjust dosing

  • If there are significant side effects or inadequate control, consider endocrine referral

• Gestational Hyperthyroidism

  • Transient hyperthyroidism of hyperemesis gravidarum or high β-hCG states

  • Usually self-limiting by ~12–16 weeks

  • Symptomatic control (e.g. propranolol) if needed

• Neonatal Hyperthyroidism Risk

  • In mothers with Graves’ disease (active or past), TRAb (TSH receptor antibodies) can cross placenta

  • Monitor neonate for signs of hyperthyroidism (irritability, poor feeding, tachycardia)

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General Considerations

• Iodine Supplementation: Current guidelines recommend 150 µg/day of iodine during pregnancy and breastfeeding to support adequate thyroid hormone production

• Monitoring

  • Repeat thyroid function tests at least every trimester or more frequently if adjusting therapy

  • Watch for postpartum thyroiditis, which can occur in women with TPO antibodies or autoimmune history

• Complications of Untreated Thyroid Dysfunction

  • Hypothyroidism: Miscarriage, pre-eclampsia, preterm birth, low birth weight, neurodevelopmental impairment

  • Hyperthyroidism: Pre-eclampsia, placental abruption, heart failure, foetal growth restriction

• Postpartum

  • Reassess thyroid function 6–8 weeks postpartum, especially if changes made during pregnancy

  • Refer to endocrinology if persistent abnormalities or complications

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Notes:

• Consider targeted screening in early pregnancy for women with risk factors (thyroid disease history, T1DM, TPO antibody positive)

• Use trimester-specific reference ranges for TSH to guide diagnosis and management

• Adjust levothyroxine doses promptly in pregnancy; aim for TSH <2.5 mU/L in 1st trimester

• Antithyroid drug choice depends on gestational age (PTU in 1st trimester, carbimazole thereafter)

• Monitor closely for foetal and maternal complications (especially pre-eclampsia, growth restriction, postpartum thyroiditis)

• Provide iodine supplementation and ensure adequate follow-up postpartum for thyroid status re-evaluation