Thyroid disease in pregnancy — Rapid

Synopsis

Interpret thyroid tests using pregnancy-specific physiology, treat maternal hormone deficiency or excess safely, and recognise antibody-mediated fetal risk and antithyroid-drug emergencies.

Interpret TSH and free T4 with trimester-specific and assay-specific reference ranges; normal early-pregnancy physiology may suppress TSH.
Established hypothyroidism needs prompt levothyroxine adjustment at a positive pregnancy test, often two extra usual daily doses per week under the pre-agreed plan.
RCOG recommends TSH and free T4 every four to six weeks until 20 weeks, then again at 28 weeks for levothyroxine-treated hypothyroidism.
For treated hypothyroidism aim for TSH below 2.5 mU/L while keeping free T4 within the appropriate pregnancy reference range.
If antithyroid treatment is necessary in early pregnancy, RCOG favours propylthiouracil; a carbimazole-to-PTU switch should occur promptly before 10 weeks when indicated.
Use the lowest effective antithyroid dose, targeting free T4 in the upper half of the pregnancy range rather than normalising TSH aggressively.
Gestational transient thyrotoxicosis generally needs supportive care, and euthyroid thyroid-peroxidase-antibody positivity alone does not justify levothyroxine.
Past Graves' disease still warrants TRAb assessment because antibodies can affect the fetus even after thyroidectomy or radioiodine.

Key red flags

Fever, confusion, severe tachycardia or pulmonary oedema in a thyrotoxic patient requires emergency treatment for possible thyroid storm.

Sore throat, mouth ulcers, fever or unexplained infection while taking carbimazole or propylthiouracil requires immediate blood-count assessment and withholding the medicine until reviewed.

Jaundice, dark urine, pruritus or right upper quadrant pain while taking propylthiouracil raises concern for severe hepatotoxicity.

Investigation priorities

TSH and free T4First step

Classify dysfunction and guide treatment using gestation-appropriate interpretation.

Management branches

Replacement adjustmentMeet the increased thyroxine requirement

Pregnancy is confirmed in someone with established levothyroxine-treated hypothyroidism.

Activate the agreed empirical dose increase and arrange prompt TSH and free T4 testing rather than waiting for routine antenatal booking.
Titrate to the RCOG pregnancy target and check every four to six weeks until 20 weeks, then at 28 weeks.
Review iron and calcium timing, vomiting and missed doses, and document the postnatal dose and testing plan.

Key medicines

Levothyroxine in established hypothyroidismAt a positive pregnancy test, take the usual oral daily dose on five days weekly and twice that dose on two days weekly when following the agreed RCOG plan; subsequently individualise against TSH and free T4.This schedule modifies an existing dose; newly diagnosed disease needs an individual starting prescription. Separate iron or calcium from levothyroxine and avoid biochemical overtreatment.

PropylthiouracilUse an endocrinologist-selected oral dose and titrate to maternal free T4 with the lowest effective exposure. The cited SmPC gives a general adult initiation range of 300–600 mg daily, once daily or divided; this is not a recommended pregnancy starting range, and pregnancy treatment may require substantially less.Do not automatically use the top of the general adult range. Watch for agranulocytosis, hepatotoxicity and fetal hypothyroidism, and consider carbimazole if ongoing treatment is needed beyond 20 weeks.

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