Thyroid Disorders in Pregnancy
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Direct answer
Pregnancy alters thyroid physiology — oestrogen raises thyroid-binding globulin, human chorionic gonadotropin weakly stimulates the thyroid, and levothyroxine requirements rise by about 30-50 per cent in hypothyroid women. Overt hypothyroidism (raised TSH with low free T4) risks miscarriage, pre-eclampsia, abruption, stillbirth and impaired neurodevelopment, treated with levothyroxine to keep TSH below about 2.5 mIU/L in the first trimester (below 3 later). Hyperthyroidism is usually Graves disease; propylthiouracil is preferred in the first trimester with a switch to carbimazole or methimazole afterwards, radioactive iodine being absolutely contraindicated.
What you must remember
- Physiology: thyroid-binding globulin rises with oestrogen (total hormone rises while free T4 stays normal); hCG transiently suppresses TSH in early pregnancy; iodine requirement rises to about 250 micrograms daily, met by iodised salt.
- Hypothyroidism: overt disease (high TSH with low free T4) carries miscarriage, anaemia, pre-eclampsia, abruption, postpartum haemorrhage, low birth weight, stillbirth and neurodevelopmental harm; subclinical disease (high TSH, normal free T4) is treated per TSH level and antibody status.
- Treatment targets: levothyroxine on an empty stomach; classically titrate TSH below 2.5 mIU/L in the first trimester and below 3 mIU/L later (current guidance prefers trimester-specific ranges where available); the dose typically rises by a third or more once pregnancy is confirmed.
- Screening: case-finding of high-risk women (goitre, known disease, type 1 diabetes, prior head or neck radiation, family history, infertility) versus universal screening remains debated; testing is by TSH with free T4.
- Hyperthyroidism: Graves disease predominates; treat with the lowest effective dose — propylthiouracil in the first trimester (rare hepatotoxicity, monitor liver function), switching to carbimazole or methimazole afterwards (fetal aplasia cutis and choanal atresia cluster in the first trimester).
- Fetal and neonatal considerations: TSH-receptor antibodies cross the placenta, causing fetal and neonatal hyperthyroidism — monitor antibodies, fetal heart rate and growth; radioiodine is contraindicated in pregnancy and lactation; thyroid storm is treated with propylthiouracil, beta blockade, iodine and supportive care.
- Related syndromes: gestational transient thyrotoxicosis — self-limiting hCG-driven overdrive of hyperemesis needing supportive care, not antithyroid drugs; postpartum thyroiditis — painless lymphocytic thyroiditis in the first postpartum year (roughly 5-8 per cent, more with positive TPO antibodies), passing through thyrotoxic then hypothyroid phases before recovery.
Common confusion
Antithyroid drug selection by trimester is the perennial slip: propylthiouracil first (safer for the first-trimester fetus), carbimazole later (protecting the mother's liver) — both reach the fetus, so doses stay minimal. The mirror-image error is treating gestational transient thyrotoxicosis of hyperemesis with antithyroid drugs; it is hCG-driven and self-limiting. In hypothyroidism, not adjusting levothyroxine upward early costs fetal neurodevelopment time.
Exam-focused takeaway
Stems test the TSH targets by trimester, the levothyroxine increase on conception, the propylthiouracil-then-carbimazole switch with its reasons, radioiodine contraindication and antibody transfer causing neonatal Graves. Expect one-liners on postpartum thyroiditis phases and iodine needs.
Frequently asked questions
Why does levothyroxine dose rise in pregnancy?
Increased thyroxine-binding globulin, placental deiodinase activity and fetal needs raise requirements by about 30-50 per cent, so the dose is increased as soon as pregnancy is confirmed.
What TSH targets are used?
Classically below 2.5 mIU/L in the first trimester and below 3 mIU/L in the second and third, with current guidance preferring trimester-specific reference ranges.
Which antithyroid drug for which trimester?
Propylthiouracil in the first trimester, then a switch to carbimazole or methimazole — balancing fetal embryopathy risk against maternal liver injury.
Is radioiodine ever acceptable in pregnancy?
Never in pregnancy or lactation — the fetal thyroid concentrates iodine from about 10-12 weeks and would be destroyed.
What is postpartum thyroiditis?
Painless autoimmune thyroiditis after delivery, passing through thyrotoxic then hypothyroid phases before most women recover; commoner with positive TPO antibodies.