Interpreting thyroid function tests in pregnancy

At a glance

• Read pregnant TFTs against laboratory and trimester-specific ranges. If the lab has none, use 4.0 mU/L as the TSH upper limit (ATA 2017, kept in ATA 2026).
• hCG lowers TSH in the first trimester, most at 10 to 12 weeks and more with twins or hyperemesis. A low TSH at this stage is often normal.
• TBG doubles, so total T4 and T3 rise by about 50%. Free T4 immunoassays drift low in later pregnancy.
• TSH is the main test. Free T4 matters when TSH is abnormal, in central disease and when monitoring Graves treatment.
• Testing is targeted (ATA 2026, RCOG 2025). Diagnosis and treatment of each disorder is in thyroid disorders in pregnancy; general TFT patterns and assay traps are in interpreting thyroid function tests.

What changes and why

Change	Timing	What it does to the result
hCG stimulates the TSH receptor	Peaks at 10 to 12 weeks	TSH falls; free T4 rises slightly. Lower limit of TSH may be 0.1 mU/L or less
Oestrogen doubles TBG	From 6 to 8 weeks, plateau by 20	Total T4 and T3 about 1.5 times non-pregnant
Albumin falls, TBG rises	Second and third trimesters	Free T4 immunoassays under-read
Placental type 3 deiodinase, larger volume	Throughout	Treated hypothyroid women need 20 to 30% more levothyroxine
Higher renal iodide clearance	Throughout	150 µg iodine supplement daily (NHMRC)

Where no local range exists, ATA 2017 suggested lowering the non-pregnant TSH range by about 0.4 mU/L at the bottom and 0.5 mU/L at the top in the first trimester, which lands close to 4.0. TSH drifts back towards the non-pregnant range in the second and third trimesters.

Reading the result, step by step

1. Gestation and number of fetuses. A TSH of 0.08 mU/L means something different at 9 weeks with twins than at 32 weeks.
2. Context. Vomiting, known thyroid disease, previous Graves treated with radioiodine or surgery, levothyroxine dose and timing, amiodarone, lithium, checkpoint inhibitors and biotin (hair and nail supplements of 5 to 10 mg, not the small amount in prenatal vitamins).
3. TSH against the right range. Use the laboratory's pregnancy range, or 4.0 mU/L as the upper limit.
4. Free T4 against a method-specific pregnancy range. Use the same laboratory through the pregnancy so trends are comparable. If the free T4 looks implausible in the third trimester, total T4 times 1.5 is an alternative.
5. Free T3 only when TSH is suppressed: a high T3 relative to T4 points to Graves or a toxic nodule.
6. Antibodies for the question being asked. TPO for risk of progression and postpartum thyroiditis. TRAb to separate Graves from hCG-driven thyrotoxicosis and to predict fetal thyrotoxicosis.

TSH	Free T4	Most likely in pregnancy	What to do
Low, first trimester	Normal or slightly high	Physiological or gestational transient thyrotoxicosis	TRAb if doubt; repeat in 3 to 4 weeks
Low	High, with high T3	Graves (TRAb positive)	Antithyroid drug, fetal surveillance plan
Above range, 10 or below, first trimester	Normal	Subclinical hypothyroidism	Repeat within 3 weeks; TPO; consider levothyroxine if confirmed
Above range, 10 or below, after first trimester	Normal	Subclinical hypothyroidism	Monitor TSH every 4 to 6 weeks (ATA 2026, RCOG 2025)
Above 10, or above range with low free T4	Low	Overt hypothyroidism	Levothyroxine now, recheck in 4 weeks
Normal	Low	Assay drift or isolated hypothyroxinaemia	Check method range and iodine; don't treat routinely
Normal or high	High	Interference (biotin, heterophile antibodies)	Repeat on another platform

Worked examples

Example 1. 9 weeks, vomiting 10 times a day, weight loss 6%: TSH under 0.01 mU/L, free T4 27 pmol/L (non-pregnant 10 to 20), free T3 6.4 pmol/L (3.0 to 6.0), no goitre, TRAb negative. Gestational transient thyrotoxicosis. The T3 is only just raised and TRAb is negative. Treat the hyperemesis, give propranolol for troublesome palpitations, and repeat TFTs; expect resolution by 14 to 18 weeks.

Example 2. 7 weeks, two previous miscarriages: TSH 5.8 mU/L (lab pregnancy range 0.1 to 3.9), free T4 13 pmol/L, TPO antibodies 320 IU/mL. Subclinical hypothyroidism with thyroid autoimmunity. Repeat TSH within about 3 weeks; if it stays above range, levothyroxine (about 50 µg daily) is reasonable. After birth, a trial off levothyroxine is advised if it was started for subclinical hypothyroidism; check TSH about 6 weeks later and watch for postpartum thyroiditis.

Example 3. 31 weeks, routine bloods for tiredness: TSH 1.9 mU/L, free T4 9.2 pmol/L (non-pregnant 10 to 20). TSH is normal. This is the expected fall in measured free T4 in late pregnancy. No treatment.

Example 4. 6 weeks, hypothyroid on levothyroxine 100 µg, hasn't changed her dose: TSH 4.4 mU/L. She is under-replaced for pregnancy. Increase to 9 tablets a week (two extra doses) and recheck TSH and free T4 in 4 weeks, aiming for 0.5 to 2.5 mU/L.

Example 5. 12 weeks, Graves treated with radioiodine 5 years ago, euthyroid on levothyroxine: TSH 1.2 mU/L, free T4 15 pmol/L, TRAb 4.5 times the upper limit. The mother's TFTs are normal but the antibody crosses the placenta. Repeat TRAb at 18 to 22 weeks and arrange fetal heart rate, growth and goitre surveillance.

Accuracy and pitfalls of the tests

• TSH is the most reliable marker of primary thyroid disease in pregnancy, as outside it, apart from the first-trimester hCG effect.
• Free T4 immunoassays depend on binding proteins. Different platforms give results that differ by 20% or more in the third trimester, so a non-pregnant range applied to a late-pregnancy result over-calls hypothyroxinaemia.
• TRAb (TBII) has sensitivity and specificity above 95% for Graves. A negative result in a vomiting woman at 9 weeks is reassuring.
• TPO antibodies are present in about 10% of pregnant women. A positive result predicts rising TSH and postpartum thyroiditis but doesn't by itself need levothyroxine (TABLET).
• Thyroid radionuclide scans (radioiodine or technetium) aren't done in pregnancy; the fetal thyroid takes up tracer from about 10 to 12 weeks. Separate Graves from gestational thyrotoxicosis clinically and with TRAb.

When it changes management

• Start or increase levothyroxine: overt hypothyroidism, TSH above 10 mU/L, under-replacement in known hypothyroidism.
• Antithyroid drug: Graves disease with raised free T4, using PTU in the first trimester.
• Fetal surveillance: TRAb more than about 3 times the upper limit.
• No action: physiological first-trimester TSH suppression, gestational transient thyrotoxicosis, low free T4 with normal TSH in late pregnancy.

How it's tested

• A table of first-trimester TFTs in a vomiting woman: choose observation over carbimazole or PTU.
• A TSH of 3.2 mU/L at 8 weeks: recognise that the old 2.5 cut-off no longer defines disease.
• A euthyroid woman after radioiodine: choose TRAb as the next test.

Pitfalls

• Reading a first-trimester TSH against the non-pregnant range.
• Treating a low measured free T4 at 30 weeks when TSH is normal.
• Diagnosing hypothyroidism on a TSH of 2.8 mU/L using the ATA 2011 limit.
• Assuming normal maternal TFTs mean the fetus is safe in a woman with past Graves.
• Ordering a thyroid uptake scan (radioiodine or technetium) to separate Graves from hCG-driven thyrotoxicosis.