TSH (thyroid-stimulating hormone) in pregnancy has no single normal value: each trimester has its own range, and the best range is the one your laboratory sets for pregnant women. When a lab has none, the American Thyroid Association (ATA) uses an upper limit of about 4.0 mIU/L, roughly 0.5 below the usual non-pregnant limit.
This page sets out the limits five guidelines publish, side by side and attributed, with a checker for your own result. It explains why TSH falls in the first trimester, what high and low results lead to, how levothyroxine and iodine fit in, and which symptoms need a call the same day.
The short answer: read TSH against the pregnancy range printed on your report. Without one, ATA 2017 puts the upper limit at about 4.0 mIU/L. The Polish guideline uses 3.18, 3.44 and 3.53 for the three trimesters. The old fixed limits of 2.5 and 3.0 come from ATA 2011, which ATA replaced in 2017.
TSH normal range in pregnancy by trimester
Each body below publishes its own numbers for a different situation. ATA 2017 and the European Thyroid Association (ETA) give a fallback for labs without a pregnancy range; the Polish Society of Endocrinology (PTE) gives a range measured in healthy Polish women; ATA 2011 is the older set many reports and articles still quote. The table keeps them apart rather than averaging them into one range nobody published.
| Guideline | First trimester | Second trimester | Third trimester |
|---|---|---|---|
| ATA 2017, when the lab has no pregnancy range | ≤ 4.0* | ≤ 4.0* | ≤ 4.0* |
| Polish Society of Endocrinology 2021 (Polish women) | 0.009 – 3.18 | 0.05 – 3.44 | 0.11 – 3.53 |
| European Thyroid Association 2014, when the lab has no trimester range | ≤ 2.5 | ≤ 3.0 | ≤ 3.5 |
| ATA 2011 (replaced in 2017) | 0.1 – 2.5 | 0.2 – 3.0 | 0.3 – 3.0 |
* ATA 2017 gives one figure. Its text applies it from weeks 7 to 12 and expects TSH to drift back towards the non-pregnant range in the second and third trimesters. "≤" means the guideline publishes an upper limit only. mIU/L, mU/L and µIU/mL are the same number.
The Endocrine Society's 2012 guideline used the same upper limits as ATA 2011: 2.5 mIU/L in the first trimester and 3.0 after it. Those limits came from 6 studies of about 5,500 women. ATA dropped them in 2017 after cohorts of more than 60,000 women showed higher upper limits in many populations, and the shift only appears from around week 7.
In Russia, the Ministry of Health guideline on normal pregnancy (2023) asks for TSH and TPO antibodies once at the first visit. It refers a woman to an endocrinologist when TSH is above 2.5 mIU/L with TPO antibodies, or above 4.0 whatever the antibody result. That is a referral rule rather than a reference range, so it appears in the checker and not in the table.
Why your lab's range comes first
TSH tests from different manufacturers give slightly different numbers, and populations differ in iodine intake, body weight and how common thyroid antibodies are. ATA 2017 therefore ranks the options: a trimester range from your own laboratory first, then a range from a similar population measured with a similar assay, and only then the 4.0 mIU/L fallback. The range printed on your report is the one your doctor will use.
Population studies show how far apart healthy women can be. All of these excluded women with thyroid antibodies:
| Population | Weeks | Median | 2.5th–97.5th percentile |
|---|---|---|---|
| Netherlands, Generation R study (5,393 women) | 9–18 | — | 0.03 – 4.04 |
| China, Li et al. (4,800 women screened) | 4–6 | 2.15 | 0.56 – 5.31 |
| China, Li et al. | 7–12 | 1.47 | 0.1 – 4.34 |
| China, Li et al., women who were not pregnant | — | 2.07 | 0.69 – 5.64 |
Sources: Medici et al. 2012 (Generation R); Li et al. 2014. Both are among the studies ATA 2017 lists when it explains why it moved away from fixed limits.
The Chinese study shows what a fixed limit does. With a cut-off of 2.5 mIU/L, 27.8% of the 4,800 women would have been labelled as having subclinical hypothyroidism; with the laboratory's own pregnancy range, 4.0%. In Russia, a 2022 study of 2,008 women in Moscow, Ivanovo and Smolensk found that TSH in healthy pregnant women did not exceed 3.8 mIU/L in any trimester.
TSH checker
Enter your TSH, the trimester you were in when the blood was taken and, if you know it, your TPO antibody result. The checker places the number against each guideline above and names the ATA 2017 band it falls in. It does not diagnose, and it cannot see your free T4 or your lab's own range.
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If you already take levothyroxine, ATA 2017 aims for the lower half of your lab's trimester range, or below 2.5 mIU/L when there is none. Vomiting you cannot keep down, a racing heart or swelling in the neck needs a call the same day.
Why TSH falls in the first trimester
hCG, the pregnancy hormone a home test detects, has a structure close to TSH's and stimulates the thyroid's TSH receptor directly. The thyroid makes more hormone, and the pituitary answers by releasing less TSH. hCG peaks between about weeks 7 and 11, which is when TSH sits lowest; our hCG levels by week chart shows the same curve from the other side.
ATA 2017 reports a TSH below 0.4 mIU/L in up to 15% of healthy women in the first trimester and below 0.1 in about 5% by week 11. The share with a suppressed TSH falls to about 10% in the second trimester and 5% in the third. Twins, with more hCG, push TSH lower still. Before week 7 the fall has often not begun: in the Chinese study, TSH at weeks 4 to 6 matched non-pregnant women.
Free T4 in pregnancy
Free T4 is the thyroid hormone itself, the fraction not bound to proteins. Pregnancy raises the binding protein and lowers albumin, and most labs measure free T4 with immunoassays that these changes throw off, so results drift down in later pregnancy partly because of the method. ATA 2017 calls for ranges specific to both the method and the trimester. The Polish reference study published both hormones together:
| Trimester | TSH, mIU/L | Free T4, pmol/L |
|---|---|---|
| First trimester | 0.009 – 3.18 | 11.99 – 21.89 |
| Second trimester | 0.05 – 3.44 | 10.46 – 16.67 |
| Third trimester | 0.11 – 3.53 | 8.96 – 17.23 |
Source: Kostecka-Matyja et al. 2017, adopted as Table 1 of the PTE 2021 guideline. Free T4 ranges from another assay can differ.
A low free T4 with a normal TSH is called isolated hypothyroxinaemia. ATA 2017 advises against treating it routinely, because the two trials that did found no benefit for the child.
High TSH: subclinical and overt hypothyroidism
A TSH above the pregnancy range with a normal free T4 is subclinical hypothyroidism. A high TSH with a low free T4 is overt hypothyroidism; ETA 2014 also counts any TSH above 10.0 mIU/L as overt, whatever the free T4. Overt hypothyroidism affects about 0.3–0.5% of pregnancies and is linked to miscarriage, preterm birth, low birth weight and lower IQ in the child when it goes untreated, so every guideline treats it.
For subclinical hypothyroidism, ATA 2017 lets the TPO antibody result decide:
- Levothyroxine recommended: TPO antibodies with a TSH above the pregnancy range; or no antibodies with a TSH above 10.0 mIU/L.
- Levothyroxine may be considered: TPO antibodies with a TSH above 2.5 but still inside the range; or no antibodies with a TSH above the range but below 10.0.
- Levothyroxine not recommended: no antibodies and a TSH inside the range, or below 4.0 when the lab has no range.
The Polish guideline is stricter. With its own range it treats a TSH above 3.18, 3.44 or 3.53 mIU/L, by trimester, as subclinical hypothyroidism needing levothyroxine, and asks for close monitoring between 2.5 and 3.18 in the first trimester, with treatment considered if you have TPO antibodies or another autoimmune disease.
TPO antibodies
Antibodies against thyroid peroxidase (TPO) mark autoimmune thyroid disease, usually Hashimoto's. ATA 2017 finds them in 2–17% of pregnant women and advises the test for anyone with a TSH above 2.5 mIU/L. A thyroid with antibodies may not keep up with pregnancy's extra demand, so ATA advises a TSH when pregnancy is confirmed and about every 4 weeks until mid-pregnancy, even when the first result is normal.
Low TSH: hCG, hyperemesis and an overactive thyroid
Most low first-trimester results reflect hCG. When free T4 is also raised in the first half of pregnancy, and there is no sign of Graves' disease, the usual cause is gestational transient thyrotoxicosis, found in about 1–3% of pregnancies. It often comes with hyperemesis gravidarum, the severe vomiting that affects 3–10 in 1,000 pregnancies. ATA 2017 treats it with fluids and anti-sickness care rather than antithyroid drugs, because free T4 settles by weeks 14 to 18 on its own. Our morning sickness guide covers when vomiting stops being ordinary.
Graves' disease, the autoimmune overactive thyroid, is less common in pregnancy and is managed by a specialist, often with a change of medicine. It shows up as a low TSH with high hormones beyond the first trimester, or with eye changes, a goitre or TSH-receptor antibodies.
Levothyroxine: what is standard and what to avoid
Levothyroxine is the treatment for an underactive thyroid in pregnancy, and the NHS states it can be taken during pregnancy. Our medicines in pregnancy table covers everyday medicines; thyroid treatment follows its own rules:
What is standard
- Levothyroxine (T4) on its own. ATA 2017 and the Polish guideline both say so.
- Tell your doctor as soon as a pregnancy test is positive if you already take it. The need rises from weeks 4 to 6, and ATA 2017 expects the dose to go up by about 20–30%. Agree the plan before you conceive, so you know what to do on the day.
- A TSH about every 4 weeks until mid-pregnancy and once near week 30, aiming for the lower half of the trimester range.
- At least 4 hours apart from iron and calcium, including a prenatal vitamin that contains them: both bind levothyroxine in the gut.
- Back to the pre-pregnancy dose after birth, with a TSH about 6 weeks later.
What to avoid
- Stopping levothyroxine, or changing the dose beyond the plan your doctor gave you.
- T3 (liothyronine) or desiccated thyroid. ATA 2017 and the Polish guideline both say not to use them in pregnancy.
- Kelp, seaweed tablets or high-dose iodine. Their iodine content varies and can be thousands of times the daily limit.
- Selenium to treat TPO antibodies. ATA 2017 does not recommend it.
Iodine
The thyroid needs iodine to make hormone, and the baby depends on yours. ATA 2017 recommends about 250 µg of iodine a day in pregnancy, a daily supplement of 150 µg as potassium iodide in most regions, ideally from three months before conception, and no more than 500 µg a day for any length of time. The Polish guideline also advises 150 µg, Russia's 200 µg, and Germany's maternity guideline an extra 100–200 µg. The guidelines split on levothyroxine: ATA 2017 sees no need for extra iodine if you take it, while the Polish guideline still advises iodine. Our prenatal supplements guide covers the rest of the list.
Who gets tested, and when
Guidelines disagree on testing everyone. ATA 2017 makes no recommendation for or against universal screening and tests by risk instead: a thyroid history or symptoms, a goitre or known antibodies, neck radiation or thyroid surgery, age over 30, type 1 diabetes or another autoimmune disease, miscarriage, preterm birth or infertility, two or more previous pregnancies, a family history of thyroid disease, a BMI of 40 or more, amiodarone or lithium, recent iodine contrast, or living where iodine is short.
Poland's guideline tests every woman at weeks 4 to 8, and Russia's tests TSH with TPO antibodies at the first visit. Germany's maternity guideline does not include TSH among its routine tests, and neither does Ukraine's 2022 normal-pregnancy guideline, so there you ask for it. Our guide to blood tests in pregnancy covers the rest of the booking bloods, and the tests and scans hub shows where each one falls.
After the birth: postpartum thyroiditis
About 5% of women develop postpartum thyroiditis, rising to 33–50% of those with TPO antibodies in early pregnancy. A brief overactive phase at two to six months can give palpitations or irritability, and an underactive phase between three and twelve months can bring tiredness, feeling cold and poor concentration. ATA 2017 advises a thyroid test for anyone with postnatal depression.
When to get help the same day
Call your doctor or maternity unit the same day if: you cannot keep food or fluids down; vomiting comes with a racing heart, shaking or weight loss; you take carbimazole, thiamazole or propylthiouracil and develop a high temperature, sore throat or mouth ulcers (a sign of a low white-cell count), or yellow skin or eyes or dark urine; or you take levothyroxine, have a new positive pregnancy test and no plan yet.
Call 999 or 911, or your local emergency number, if: a swelling in your neck makes it hard to breathe, or a very fast heartbeat comes with a high temperature, agitation or confusion.
Tiredness, feeling cold, constipation or dry skin are reasons to ask about a TSH test at your next appointment. They are common in pregnancy anyway and do not need the same-day line.
Frequently Asked Questions
What should TSH be in pregnancy?
Inside the pregnancy range printed on your lab report. If the lab gives none, ATA 2017 uses an upper limit of about 4.0 mIU/L; ETA 2014 uses 2.5, 3.0 and 3.5 for the three trimesters; the Polish guideline uses 3.18, 3.44 and 3.53. TSH is usually lowest from about week 7 to week 12.
Does TSH go up or down in pregnancy?
Down first. From around week 7, hCG stimulates the thyroid directly and TSH falls. In a Chinese study of 4,800 women the upper limit dropped from 5.31 mIU/L at weeks 4 to 6 to 4.34 at weeks 7 to 12. TSH then rises again through the second and third trimesters but stays below the usual non-pregnant range.
Is a TSH of 3 high in early pregnancy?
It depends on whose limit you use. It sits below the ATA 2017 fallback of 4.0 and inside the Polish first-trimester range, but above 2.5, the point where ATA advises a TPO antibody test and where the Polish guideline asks for closer monitoring. Under the old ATA 2011 and ETA 2014 limit of 2.5 it counts as high. Your lab's own range settles it.
What does a low TSH in the first trimester mean?
Usually that hCG is stimulating your thyroid, which is normal. ATA 2017 reports a TSH below 0.4 in up to 15% of healthy women in the first trimester, and below 0.1 in about 5% by week 11. A free T4 test separates this from an overactive thyroid, which needs a specialist.
What TSH should I have before getting pregnant?
If you already take levothyroxine, ATA 2017 advises a TSH between the lower limit of the normal range and 2.5 mIU/L before you conceive. For everyone else ATA makes no recommendation on testing before pregnancy, except before assisted reproduction or with known TPO antibodies; the Polish guideline suggests a TSH test for every woman planning pregnancy.
How often should TSH be checked in pregnancy?
On levothyroxine, or with TPO antibodies or past thyroid surgery, ATA 2017 and the Polish guideline advise a TSH about every 4 weeks until mid-pregnancy and once near week 30. Without a thyroid history, a single test at the first visit is the rule where screening is routine, as in Poland and Russia.
What if my free T4 is low but my TSH is normal?
That is called isolated hypothyroxinaemia. ATA 2017 advises against treating it routinely, because the two trials that gave levothyroxine for it found no benefit to the child. Free T4 tests are also less reliable in pregnancy, since the proteins that carry the hormone change, so the result has to be read against a range for that method and trimester.
How can I lower my TSH in pregnancy?
No food or supplement lowers TSH safely. If treatment is needed, it is levothyroxine prescribed and adjusted by your doctor. Do not take kelp or high-dose iodine to fix it: ATA 2017 advises against more than 500 µg of iodine a day, and it does not recommend selenium for women with TPO antibodies.
TSH in pregnancy: at a glance
TSH in pregnancy is read against a trimester range, ideally your laboratory's own. Without one, ATA 2017 uses an upper limit of about 4.0 mIU/L; ETA 2014 uses 2.5, 3.0 and 3.5; Poland uses 3.18, 3.44 and 3.53. hCG lowers TSH from about week 7, so a low first-trimester value is often normal. Above 2.5, ATA advises a TPO antibody test, and the result decides whether levothyroxine is recommended. Women already on levothyroxine contact their doctor as soon as a test is positive.
Inside Baby Novum: the lab results journal has a thyroid (TSH) entry for each result with its date and notes, and the supplement tracker's library includes iodine, with room to add a medicine your doctor prescribed, such as levothyroxine. Reminders come with Premium.
Read next
Sources
- Alexander et al., Thyroid 2017 — 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum
- Stagnaro-Green et al., Thyroid 2011 — 2011 Guidelines of the American Thyroid Association for thyroid disease during pregnancy and postpartum
- Lazarus et al., European Thyroid Journal 2014 — European Thyroid Association guidelines on subclinical hypothyroidism in pregnancy and in children
- De Groot et al., Journal of Clinical Endocrinology & Metabolism 2012 — Endocrine Society guideline on thyroid dysfunction during pregnancy and postpartum
- Hubalewska-Dydejczyk et al., Endokrynologia Polska 2021 — Thyroid diseases in pregnancy: guidelines of the Polish Society of Endocrinology
- Kostecka-Matyja et al., European Thyroid Journal 2017 — Reference values for TSH and free thyroid hormones in healthy pregnant women in Poland
- Medici et al., Journal of Clinical Endocrinology & Metabolism 2012 — Maternal early pregnancy and newborn thyroid hormone parameters: the Generation R study
- Li et al., Journal of Clinical Endocrinology & Metabolism 2014 — The rational upper limit of serum TSH in the first trimester in Chinese pregnant women
- Rybakova et al., Problems of Endocrinology 2022 — Reference intervals of TSH in pregnant women in the central regions of the Russian Federation
- Ministry of Health of the Russian Federation — Clinical guideline "Normal pregnancy" (2023, in Russian)
- Ministry of Health of Ukraine — Evidence-based clinical guideline "Normal pregnancy" (2022, in Ukrainian)
- Federal Joint Committee (G-BA), Germany — Maternity guideline (Mutterschafts-Richtlinie, in German)
- NHS — Levothyroxine
- MedlinePlus — Levothyroxine
- Levoxyl (levothyroxine) US prescribing information — Drug interactions
- NHS — Severe vomiting in pregnancy (hyperemesis gravidarum)
- NHS — Overactive thyroid: complications
- NHS — Side effects of carbimazole
- NHS — Goitre
Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.