Management of Thyroid Disorders in Pregnancy
Physiology and Testing
Oestrogen-driven rises in thyroxine-binding globulin, and first-trimester hCG (which weakly stimulates the TSH receptor), mean TSH falls and fT4/fT3 rise transiently in early pregnancy before returning toward pre-pregnancy patterns by mid-gestation. Non-pregnant reference ranges are therefore invalid in pregnancy. Trimester- and manufacturer-specific reference ranges for TSH and fT4 are recommended for diagnosis [Grade B]. Where local ranges are unavailable, an upper TSH limit of 4.0 mU/L in the first trimester is a pragmatic default. Treatment targets (not diagnostic ranges) should guide dosing once a woman is on medication. Universal thyroid function testing at booking is not recommended; a risk-based approach is advised (Table 2: personal/family thyroid history, type 1 diabetes, SLE, previous late miscarriage/stillbirth, thyroid-disruptive drugs, goitre).
Iodine
Recommended total daily intake is 200–250 µg when planning pregnancy and throughout pregnancy/breastfeeding, via diet or a 150 µg potassium iodide prenatal supplement. Sustained intake above 500 µg/day should be avoided. Routine urinary iodine testing is not a valid individual marker.
Hypothyroidism
Categories by testing: overt hypothyroidism (OH) — TSH raised, fT4 low (prevalence 0.2–1%); subclinical hypothyroidism (SCH) — TSH raised, fT4 normal (2.2–10%), with severe SCH defined as TSH >10 mU/L; isolated hypothyroxinaemia — TSH normal, fT4 low (1.3–8%), for which routine levothyroxine is not recommended.
Levothyroxine is the only recommended preparation — desiccated thyroid extract and combined T4/T3 products should not be used, as insufficient T4 crosses to the fetal brain. For newly diagnosed OH or severe SCH, start at 1.6 µg/kg/day; for SCH (TSH between the upper trimester limit and 10 mU/L, especially if TPOAb-positive or diagnosed in the first trimester), consider 1.0–1.2 µg/kg/day. Women already on levothyroxine should self-initiate an empirical 25–30% dose increase (e.g., doubling the dose on two days each week) as soon as they have a positive pregnancy test [Grade A]. Check TSH/fT4 every 4–6 weeks until 20 weeks, then again at 28 weeks; target TSH <2.5 mU/L with fT4 in the normal trimester-specific range. Postpartum, revert to the pre-pregnancy dose and recheck TSH at 6–8 weeks.
Hyperthyroidism and Thyrotoxicosis
Graves' disease is the leading cause of overt hyperthyroidism in pregnancy (~0.1–1.3% prevalence); untreated disease raises risks of pre-eclampsia, stillbirth, preterm birth, low birth weight, and maternal heart failure. Propylthiouracil (PTU) is preferred in the first trimester (lower teratogenicity than carbimazole/methimazole, which are linked to aplasia cutis, choanal/oesophageal atresia, and abdominal wall defects); a woman conceiving on carbimazole should switch to PTU as soon as possible, ideally before 10 weeks' gestation (conversion ~20:1, e.g. 200 mg PTU = 10 mg carbimazole), with a switch back to carbimazole considered in the late second/third trimester given PTU's hepatotoxicity risk. Titrate to keep fT4 in the upper half of the reference range — not TSH, which may stay suppressed. Monitor every 2–4 weeks in the first half of pregnancy, 4–8 weekly after 20 weeks. Block-and-replace regimens are not recommended in pregnancy. Beta-blockers may be used short-term for symptom control.
New-onset suppressed TSH with raised fT4 requires distinguishing Graves' disease from gestational transient thyrotoxicosis (GTT), which is hCG-driven, peaks around 10 weeks, resolves by 18–20 weeks, and affects 1–5% of pregnancies (often with hyperemesis, no goitre, normal fT3/TRAb). GTT needs only symptomatic/supportive management — antiemetics, hydration, electrolyte correction — with no benefit from antithyroid drugs.
Postpartum Thyroiditis (PPT)
An autoimmune condition affecting 5–10% of previously euthyroid women within 12 months of birth (30–50% of TPOAb-positive women). Classic triphasic course: transient thyrotoxic phase (2–6 months postpartum, due to destructive hormone release, not treated with antithyroid drugs) → hypothyroid phase (3–12 months, permanent in up to 50%) → recovery. Routine screening is not recommended. Beta-blockers manage thyrotoxic symptoms; levothyroxine is used in the symptomatic hypothyroid phase or if trying to conceive, with annual TSH monitoring given the high risk of permanent hypothyroidism. Recurrence risk with subsequent pregnancies is up to 70%.
High-Yield Exam Points
- Diagnose using trimester- and manufacturer-specific TSH/fT4 ranges, not non-pregnant ranges; pragmatic first-trimester TSH upper limit = 4.0 mU/L if local range unavailable.
- Universal booking thyroid screening is not recommended — use risk-based testing.
- Levothyroxine only in pregnancy (never desiccated thyroid or T3-containing preparations); target TSH <2.5 mU/L.
- Known hypothyroid women: self-increase levothyroxine ~25–30% on a positive pregnancy test.
- PTU in first trimester (teratogenic risk), switch to carbimazole after 1st trimester if PTU hepatotoxicity a concern; conversion ratio ~20:1 (PTU:CMZ).
- Titrate antithyroid drugs to fT4 (upper half of range), not TSH.
- Distinguish Graves' disease from gestational transient thyrotoxicosis using TRAb, fT3, goitre, and eye signs — GTT is hCG-driven, self-limiting, supportive care only.
- PPT is triphasic (thyrotoxic → hypothyroid → euthyroid); the thyrotoxic phase is destructive, not overproduction, so antithyroid drugs are not indicated.
Source: RCOG Green-top Guideline No. 76 (First edition, published 17 April 2025 (Chan S-Y, Marsh MS, Gilbert J, Boelaert K, Evans C, Dhillon-Smith R, on behalf of the RCOG. *BJOG* 2025;132:e130–e161). A "next review" date is listed on the RCOG website but could not be independently verified from the primary source and is omitted here.)
Read the original on rcog.org.uk
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