ObstetricsTier 2Topic

Thyroid function tests in pregnancy

Physiology in pregnancy

Thyroid physiology changes substantially in pregnancy - hCG has weak TSH-receptor agonist activity (raises fT4/suppresses TSH in the first trimester), and thyroxine-binding globulin roughly doubles (oestrogen-driven) -> total T4/T3 rise while free T4/T3 remain relatively stable
TSH physiologically falls in the first trimester (hCG cross-reactivity), especially with multiple pregnancy/hyperemesis - can mimic subclinical hyperthyroidism
2026 ATA update moves away from fixed trimester-specific TSH targets (2.5/3.0 mU/L) toward laboratory- and trimester-specific reference ranges; where unavailable, an upper TSH limit of ~4.0 mU/L is used

Overt hypothyroidism affects ~0.3-0.5% of pregnancies; subclinical hypothyroidism is more common; Graves' disease is the leading cause of hyperthyroidism in pregnancy

TSH is the primary screening test; free T4 to confirm/characterise
Universal screening is not recommended - test only with risk factors (personal/family history of thyroid disease, T1DM, other autoimmune disease, symptoms, goitre, infertility, recurrent miscarriage) or overt symptoms
TSH receptor antibodies (TRAb) checked in Graves' disease - predict fetal/neonatal thyrotoxicosis risk if elevated

Overt hypothyroidism -> levothyroxine, targeting TSH within the trimester-specific (or <4.0 mU/L default) range; doses often need increasing by ~30-50% from the pre-pregnancy baseline
Subclinical hypothyroidism - treatment considered particularly if TPO-antibody positive or TSH is significantly elevated; evidence for treating mild disease in antibody-negative women is weaker
Graves' disease - propylthiouracil in the first trimester (lower teratogenic risk than carbimazole), switch to carbimazole for the 2nd/3rd trimester (lower hepatotoxicity risk than long-term PTU)

Untreated overt hypothyroidism - miscarriage, pre-eclampsia, preterm birth, impaired neurodevelopment
TPO antibody positivity - increased miscarriage risk even if euthyroid

Postpartum thyroiditis follows in ~5-10% of women (especially if TPO-antibody positive) - a transient thyrotoxic phase, then a hypothyroid phase, then usually resolution

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