Thyroid Dysfunction in Pregnancy

The full reasoning pathway — pregnancy changes thyroid physiology and lowers treatment thresholds. Refer urgently, increase levothyroxine early, and use PTU for hyperthyroidism; monitor across trimesters and postpartum, and safety-net.StartDecisionInvestigateActionReferStop / Admit

PresentationThyroid disease in pregnancy

↑oestrogen & TBG → ↑FT4/FT3; a fall in TSH is normal in T1. Apply trimester-specific reference ranges. Established disease, new diagnosis, or postpartum — all need specialist input.

Step 1 · Safety — Emergency / 2WW?Thyroid storm · hyperemesis · suspected malignancy?

Severe hyperthyroidism (thyroid storm) or intractable vomiting (hyperemesis gravidarum) → emergency admission. Thyroid nodule/goitre with suspected malignancy → 2WW.

YES

Stop · Admit / 2WWEmergency or cancer pathway

Thyroid storm / hyperemesis → emergency admission. Suspected thyroid malignancy (nodule/goitre) → 2WW thyroid (NICE NG12).

ClassifyHypo · Hyper · Postpartum

Check TSH + FT4 (+ FT3 if hyper) immediately, apply trimester ranges, and refer urgently to antenatal + endocrinology.

Step 3 · manage by pattern

Hypothyroidism (overt or subclinical)

↑ Levothyroxine early

Urgent antenatal + endocrine referral. Increase levothyroxine by 30–50% from as early as 4–6 weeks on specialist advice to keep TSH normal. Postpartum: drop back to pre-pregnancy dose, check TFTs at 6w.

Hyperthyroidism (overt or subclinical)

Specialist · PTU

Refer all (incl. euthyroid). Check TSH/FT3/FT4 immediately. Propylthiouracil preferred (carbimazole teratogenicity — MHRA), lowest dose for euthyroidism. Specialist monitoring.

Postpartum thyroiditis (PPT)

Thyrotoxic → hypothyroid

Usually resolves <1yr. Thyrotoxic phase → refer to differentiate from Graves; recheck 4–8w for hypothyroid phase. Hypothyroid phase → specialist advice on levothyroxine.

Step 6 · Refer · monitoringUrgent endocrine + antenatal; TFTs at booking, each trimester & postpartum

Refer urgently. Measure TFTs at pregnancy confirmation, antenatal booking, ≥once in 2nd & 3rd trimester, and 2–4 weeks postpartum. T1DM = 3× PPT risk → check TSH/FT4/TPOAb pre-pregnancy, at diagnosis of pregnancy, and 3 & 6 months postpartum.

Step 8 · preconception & modifiable factors

Step 8 · Preconception & modifiable factorsOptimise before and during pregnancy

Preconception counselling for known thyroid disease — optimise control and adjust treatment before conceiving (hypothyroid women often need an immediate dose increase once pregnant). Adequate iodine intake; take levothyroxine on an empty stomach, separated from iron/calcium/prenatal vitamins. Adherence is critical. Discuss the carbimazole/PTU teratogenicity balance with the specialist; support smoking cessation.

Step 9 · review & safety-net

Step 9 · Review & safety-netMonitor closely; urgent return advice

Check TFTs at booking, each trimester and postpartum; titrate with the specialist to trimester-specific ranges. Same-day / admit for thyroid storm (fever, tachyarrhythmia, agitation), intractable vomiting/dehydration (hyperemesis), or fetal concerns. Drop levothyroxine to the pre-pregnancy dose after delivery and recheck at ~6 weeks; review postpartum thyroiditis and screen high-risk (T1DM) women.

⚠️ Inadequately-controlled disease harms mother and fetus: undetected/under-treated hypothyroidism impairs fetal neurodevelopment; poorly-controlled hyperthyroidism raises miscarriage, pre-eclampsia, IUGR, preterm delivery and fetal death. Treat early and refer urgently.

Safety

Emergencies, the 2WW Flag & Why Control Matters

Done

Pregnancy lowers the threshold for harm from thyroid disease. Screen first for the situations that need emergency or cancer-pathway action.

Thyroid storm Severe signs & symptoms of hyperthyroidism (fever, tachyarrhythmia, agitation, vomiting) → emergency admission.

Hyperemesis gravidarum Intractable vomiting (can overlap with gestational thyrotoxicosis) → emergency admission for rehydration & assessment.

Suspected thyroid malignancy Thyroid nodule or goitre with suspicious features → 2WW thyroid (NICE NG12), independent of pregnancy.

Inadequately-controlled hyperthyroidism Raises miscarriage, pre-eclampsia, IUGR, preterm delivery, low birth weight and fetal death → urgent control.

Undetected/under-treated hypothyroidism May adversely affect fetal neuropsychological development & survival; associated with ovulatory dysfunction & infertility.

Adrenergic symptoms Tremor / tachycardia needing symptomatic treatment while awaiting specialist assessment → seek endocrinology advice (e.g. beta-blocker).

Maternal thyroid hormone is critical for fetal brain development, especially in the first trimester before the fetal thyroid is functional — which is why both under-treated hypothyroidism and poorly-controlled hyperthyroidism carry serious fetal risks and demand prompt action. Thyroid storm and hyperemesis are the genuine emergencies, and a suspicious nodule still follows the 2WW cancer pathway in pregnancy.

Diagnose

Pregnancy Thyroid Physiology & Trimester-Specific Ranges

Done

TBG & thyroid hormones

↑ oestrogen production → ↑ thyroxine-binding globulin (TBG) → rise in total (and free) FT4 and FT3.

TSH falls in 1st trimester

A fall in TSH is normal in the first trimester (hCG cross-stimulates the TSH receptor). Don't misread a low T1 TSH as pathological hyperthyroidism.

Use trimester ranges

Apply trimester-specific reference ranges for TSH and for total/free thyroid hormones — non-pregnant ranges mislead.

Immune changes

Altered immune function can change the course of existing autoimmune thyroid disease (may improve in pregnancy, relapse postpartum).

After delivery

Thyroid hormone & TSH levels normally return to the pre-pregnant state postpartum (watch for PPT, Step 5).

What to request

TSH + FT4 for hypothyroid pictures; add FT3 for hyperthyroid pictures. Check immediately and send results to the specialist.

Pregnancy physiology rewrites the reference ranges: hCG suppresses TSH in the first trimester and rising TBG lifts total thyroid hormones, so trimester-specific ranges are essential to avoid both over-diagnosing hyperthyroidism and missing genuine hypothyroidism. This is the single most common interpretive error in pregnancy thyroid testing.

Treat

Hypothyroidism — Increase Levothyroxine Early

Done

Overt or subclinical hypothyroidism needs prompt action — the treatment threshold is lower than outside pregnancy.

Refer urgently

Urgent referral to antenatal + endocrinology. Check TSH & FT4 immediately and discuss with the specialist while awaiting assessment.

Increase the dose early

On specialist advice, an increase in levothyroxine of 30–50% may be needed from as early as 4–6 weeks' gestation to maintain a normal serum TSH.

Already on levothyroxine?

Discuss dose increase & monitoring as soon as pregnancy is confirmed — don't wait for booking. A common practical approach is to increase the dose immediately on a positive test, then titrate.

Newly diagnosed?

Whilst awaiting review, discuss initiation of levothyroxine with the specialist; the aim is rapid normalisation of TSH.

Target

Keep TSH within the trimester-specific range (specialist-led). Recheck TFTs frequently through pregnancy (Step 6).

How to take

Levothyroxine ≥30 min before food/caffeine; separate from iron & calcium (incl. pregnancy supplements) by ~4 hours to preserve absorption.

ℹ️ The rising demand is driven by increased TBG and transfer to the fetus, so the dose often needs to go up in the first trimester — the opposite of the instinct to minimise medication in pregnancy.

Thyroxine requirements rise by roughly a third early in pregnancy because of increased binding-globulin and transplacental transfer to a fetus that can't yet make its own hormone. Increasing the dose by 30–50% from 4–6 weeks (often by simply adding extra doses per week on a positive test) keeps the TSH in range and protects fetal neurodevelopment — under-treatment here is the harm to avoid.

Treat

Hyperthyroidism — PTU & Specialist Management

Done

Refer all women with hyperthyroidism in pregnancy — including those currently euthyroid — to antenatal + endocrinology. Antithyroid drugs are specialist-initiated.

Refer everyone

Urgent referral of all patients to antenatal & endocrinology, including women who are euthyroid (e.g. treated Graves). Check TSH, FT3 & FT4 immediately and send to the specialist.

Drug of choice

Propylthiouracil (PTU) is usually used in pregnant women, due to the increased risk of congenital abnormalities with carbimazole (see MHRA alert). Use the lowest possible dose to maintain euthyroidism.

Risk–benefit

The risks vs benefits of individual antithyroid treatments should be considered before prescribing — both PTU and carbimazole cross the placenta; specialist balances agent & timing.

Already on treatment

If already on antithyroid therapy, ongoing blood monitoring is arranged and managed in secondary care.

Symptom control

For adrenergic symptoms (tremor, tachycardia) awaiting specialist review, seek endocrinology advice — a beta-blocker may be used short-term.

Aetiology

Distinguish Graves' disease from gestational transient thyrotoxicosis (hCG-driven, often with hyperemesis) — specialist-led, as management differs.

Carbimazole (and its active metabolite methimazole) is associated with a recognised embryopathy, so the MHRA advises effective contraception and specialist-guided use; PTU is generally preferred in the first trimester despite its own (hepatic) risks. Because the agent choice, dose and timing are a fine specialist balance — and Graves antibodies can affect the fetus — every case is referred, even when the mother is currently euthyroid.

Diagnose

Postpartum Thyroiditis — Thyrotoxic Then Hypothyroid

Done

Postpartum thyroiditis (PPT) is common and classically biphasic. It usually resolves spontaneously within 1 year postpartum.

Thyrotoxic pattern

If initial TFTs show a thyrotoxic pattern → refer to a specialist for further tests to differentiate from Graves' disease. After the thyrotoxic phase resolves, recheck TFTs at 4–8 weeks (or if new symptoms) to screen for the hypothyroid phase.

Hypothyroid pattern

If TFTs show a hypothyroid pattern → seek specialist advice on whether to start levothyroxine.

Who to suspect

Check TFTs after delivery — particularly with a goitre, non-specific symptoms suggestive of thyroiditis, history of PPT or autoimmune thyroid disease, or TPOAb positive.

When to check

Check TSH & FT4 at 6–8 weeks postpartum in at-risk women.

History of (resolved) PPT

Offer annual TFT monitoring; screen before future pregnancy and at 6–8 weeks after any pregnancy — high risk of permanent hypothyroidism & recurrence.

Significant relapse risk

There is a significant risk of relapse — don't discharge from follow-up too early.

PPT typically runs a thyrotoxic phase (destructive release of stored hormone) followed by a hypothyroid phase before recovery — and distinguishing the thyrotoxic phase from new Graves' disease matters because Graves needs antithyroid drugs whereas PPT thyrotoxicosis does not. The high rate of eventual permanent hypothyroidism and recurrence is why resolved PPT earns annual monitoring and pre-pregnancy screening.

Lifestyle

Monitoring Schedule & Higher-Risk Groups

Done

At pregnancy confirmation Measure TFTs as soon as pregnancy is confirmed — the earliest window for dose adjustment.

At antenatal booking Recheck TFTs at booking and act on trimester-specific ranges.

Each of 2nd & 3rd trimester At least once in the 2nd and once in the 3rd trimester (more often if titrating).

2–4 weeks postpartum Recheck TFTs 2–4 weeks after delivery; watch for PPT and adjust doses.

Type 1 diabetes = 3× PPT risk Check TSH, FT4 & TPOAb prior to pregnancy, at diagnosis of pregnancy, and 3 & 6 months postpartum.

TPOAb-positive women Higher risk of PPT and progression — lower threshold for postpartum testing & follow-up.

Pre-conception optimisation In known thyroid disease, optimise TFTs and adjust levothyroxine before conception where possible.

Coordinate care Keep antenatal, endocrinology and primary care aligned on targets, doses and the monitoring plan.

Frequent, scheduled TFTs (confirmation, booking, each trimester, postpartum) are what keep a moving target in range as requirements change through pregnancy and fall again after delivery. Women with type 1 diabetes carry a threefold PPT risk, so their antibody and TFT screening is more intensive and extends to 3 and 6 months postpartum.

Treat

Postpartum — Step Levothyroxine Back Down

Done

Was on levothyroxine before pregnancy

The dose will need to be decreased back to the pre-pregnancy dose as advised by a specialist. Check TFTs at 6 weeks postpartum.

Started levothyroxine in pregnancy

Discuss with the specialist whether any dose changes are required postpartum and the frequency of TFT monitoring.

Hyperthyroidism postpartum

Significant risk of relapse of Graves' after delivery — check TFTs after delivery; continue specialist-led monitoring.

Breastfeeding

Levothyroxine is safe; PTU/carbimazole are used at the lowest effective dose under specialist guidance — don't stop thyroid treatment to breastfeed without advice.

PPT hypothyroid phase

If levothyroxine started for PPT-related hypothyroidism, the specialist will advise on duration and a later trial of withdrawal (many recover).

Document the plan

Record the postpartum dose change, the 6-week TFT, and the ongoing monitoring interval.

The pregnancy-driven rise in thyroxine requirement reverses after delivery, so a woman left on her pregnancy dose will become over-replaced — hence the prompt step back to the pre-pregnancy dose and a 6-week TFT check. Graves' disease characteristically relapses postpartum, which is why hyperthyroid women stay under specialist monitoring after delivery.

Refer

Referral & Emergency Criteria

Done

Urgent antenatal + endocrinology

Refer all patients with overt or subclinical hypo- or hyperthyroidism — including euthyroid women with a history of thyroid disease. Check TSH/FT4 (+FT3 if hyper) immediately and discuss while awaiting assessment.

New diagnosis

New hypo- or hyperthyroidism in pregnancy → contact endocrinology for advice & guidance and start the urgent referral.

Emergency admission

Severe hyperthyroidism / thyroid storm, or intractable vomiting suggesting hyperemesis gravidarum.

2WW thyroid

Thyroid nodule or goitre with suspected malignancy (NICE NG12).

Refer if TFTs abnormal (PPT)

In postpartum thyroiditis, refer to endocrinology if TFTs are abnormal — urgency by clinical judgement.

Pre-conception

Known thyroid disease planning pregnancy → optimise & refer for pre-pregnancy counselling and dose planning.

Thyroid disease in pregnancy is shared antenatal–endocrine–primary-care territory, and the threshold to refer is deliberately low (everyone, even the euthyroid) because the stakes for mother and fetus are high and the management — trimester targets, antithyroid drug choice, Graves antibody surveillance — is specialist. Thyroid storm and hyperemesis are the emergencies; a suspicious nodule still goes down the 2WW cancer pathway.

Safety

Risks of Inadequate Control & Safety-Netting

Done

Keep the maternal and fetal stakes front of mind — they justify early treatment and urgent referral.

Under-treated hypothyroidism

May adversely affect fetal neuropsychological development & survival; associated with ovulatory dysfunction & infertility pre-conception.

Poorly-controlled hyperthyroidism

Increases miscarriage, pre-eclampsia, intrauterine growth restriction, preterm delivery, low birth weight and fetal death.

Drug safety

Carbimazole embryopathy risk (MHRA) → PTU preferred; lowest effective antithyroid dose. Levothyroxine is safe and often needs increasing.

Re-test / escalate if

Symptoms worsen, vomiting becomes intractable, or TFTs drift out of the trimester range — bring testing forward and re-discuss with the specialist.

Postpartum vigilance

Check TFTs after delivery; high relapse/PPT risk. T1DM and TPOAb-positive women need closer surveillance.

Document & coordinate

Record the diagnosis, dose plan, monitoring schedule and referrals; keep antenatal, endocrine & primary care aligned.

Spelling out the fetal and maternal consequences of under-control — neurodevelopmental harm from hypothyroidism; miscarriage, pre-eclampsia and growth restriction from hyperthyroidism — is what justifies the assertive, refer-early, treat-early posture of this whole pathway. Good documentation and joined-up care between antenatal, endocrinology and primary care close the loop.