Thyroid Function in Pregnancy Interpreter (TSH by Trimester)
Thyroid Function in Pregnancy Interpreter (TSH by Trimester)
Read a TSH in pregnancy against a trimester-specific interval — your laboratory’s, or the fallback the 2026 American Thyroid Association guideline gives when there is none — with the FT4, the trimester and whether it is a first result. It applies the 2026 treatment thresholds, which drop the antibody-based rules of the 2017 guideline, and it handles the levothyroxine target and the suppressed TSH of early pregnancy.
Thyroid function in pregnancy
TSH + trimester + FT4 → actionFirst trimester, TSH 5.2 mIU/L, no trimester-specific interval (fallback 0.1–4.0), FT4 normal, first result, not on levothyroxine
What the 2026 guideline changed
| Question | ATA 2017 (superseded) | ATA 2026 |
|---|---|---|
| Interval when no local trimester-specific one exists | Upper limit of about 4.0 mU/L | 0.1–4.0 mU/L in the first and second trimesters; the non-pregnant interval in the third |
| Does TPO antibody status decide treatment of subclinical hypothyroidism? | Yes — treatment thresholds differed by antibody status (2.5, the reference limit, and 10 mU/L) | No — "TPOAb status is no longer used to guide LT4 treatment decision-making" |
| Subclinical hypothyroidism found in the first trimester | By antibody status and TSH | Treat if persistent (strong recommendation) |
| Subclinical hypothyroidism found later | By antibody status and TSH | May not require treatment (conditional) |
| New overt hypothyroidism | Treat | Treat; with TSH below 6 mU/L it may be confirmed within 3 weeks first |
| TSH target on levothyroxine | Lower half of the trimester-specific range, or below 2.5 | 0.5–2.5 mU/L (good practice statement) |
Why the interval changes in pregnancy, and what changed in 2026
Pregnancy moves thyroid function tests for physiological reasons. hCG, which peaks late in the first trimester, stimulates the TSH receptor, so the thyroid makes a little more hormone and TSH falls; thyroxine-binding globulin rises under oestrogen, which disturbs FT4 immunoassays; and the fetus depends entirely on maternal thyroxine until its own thyroid starts working. A TSH read against a non-pregnant interval is therefore wrong in both directions: a normal early-pregnancy TSH of 0.2 looks suppressed, and a raised one can look normal.
The American Thyroid Association replaced its 2017 pregnancy guideline in May 2026. It still prefers an interval specific to the laboratory and the trimester. When there is none, it gives a fallback of 0.1 to 4.0 mIU/L for the first and second trimesters — the non-pregnant interval shifted down by about 0.4 at the bottom and 0.5 at the top — and the non-pregnant interval for the third. This page asks for the interval rather than building one in, so that a laboratory’s own trimester-specific limits can be used and the fallback is only a default.
The bigger change is in treatment. The 2017 guideline sorted subclinical hypothyroidism by thyroid peroxidase antibody status and by TSH thresholds of 2.5, the reference limit and 10. The 2026 guideline states that antibody status is no longer used to decide, and reorganises the decision around timing: persistent subclinical hypothyroidism found in the first trimester should be treated, while subclinical hypothyroidism first found later may not need treatment, because levothyroxine started after about the first trimester has not been shown to improve outcomes. It also favours confirming a new, mildly raised TSH before treating. The target on levothyroxine is 0.5 to 2.5 mIU/L.
This page was written from the 2026 guideline’s recommendation tables as extracted, cross-checked against a published summary; the full text could not be read end to end, and where a point could not be confirmed — subclinical hyperthyroidism, for instance — the page says so. For non-pregnant adults use the thyroid function test interpreter; for units, the TSH unit converter and free T4 unit converter.
Frequently asked questions
What is a normal TSH in pregnancy?
Ideally, whatever your laboratory’s trimester-specific interval says. When there is none, the 2026 American Thyroid Association guideline suggests 0.1 to 4.0 mIU/L in the first and second trimesters, and the ordinary non-pregnant interval in the third. TSH is lowest late in the first trimester because hCG stimulates the thyroid.
Is a TSH of 2.5 still the cut-off in pregnancy?
Not for diagnosis. The 2.5 mIU/L figure survives as the upper end of the target for women already taking levothyroxine (0.5 to 2.5). In the 2017 guideline it was also a treatment threshold for antibody-positive women; the 2026 guideline no longer uses antibody status to decide treatment.
Should subclinical hypothyroidism be treated in pregnancy?
Under the 2026 ATA guideline, yes if it is found in the first trimester and persists on repeat testing. If it is first found in the second or third trimester, it may not need treatment, because levothyroxine started after about the first trimester has not been shown to improve pregnancy or child outcomes.
Why is my TSH low in early pregnancy?
hCG, the pregnancy hormone, stimulates the thyroid, so TSH falls — most at the end of the first trimester. A low TSH with a normal FT4 at that stage is usually normal. A low TSH with a raised FT4 may be gestational transient thyrotoxicosis, which usually settles by itself, or Graves’ disease, which TSH receptor antibodies help to identify.
Do thyroid peroxidase antibodies matter in pregnancy any more?
They no longer decide whether subclinical hypothyroidism is treated under the 2026 guideline. They still indicate autoimmune thyroid disease and a higher chance of the TSH rising later, and the guideline says antibody testing may be done in women with infertility or recurrent miscarriage.
Related calculators
References
- Korevaar TIM, Leung AM, Alexander EK, et al. American Thyroid Association 2026 guidelines for thyroid disease in preconception, pregnancy, and postpartum. Thyroid. 2026;36(5):481–544. doi:10.1177/10507256261445624. — lab- and trimester-specific TSH and FT4 intervals preferred; if unavailable, TSH 0.1–4.0 mU/L in the first and second trimesters and the non-pregnant interval in the third; new overt hypothyroidism with TSH below 6 mU/L may be confirmed within 3 weeks, and with TSH 6 or more, or persisting, treated; persistent first-trimester subclinical hypothyroidism should be treated, and subclinical hypothyroidism diagnosed in the second or third trimester may not require treatment; TPOAb status no longer guides treatment of subclinical hypothyroidism; levothyroxine target TSH 0.5–2.5 mU/L preconception and in pregnancy (good practice statement).
- MedCentral. 2026 ATA guideline update: thyroid disease in preconception, pregnancy, and postpartum. 2026. — secondary summary used to cross-check the 2026 recommendations: repeat testing before starting levothyroxine for a new mild TSH rise, particularly below 6 mU/L; treatment started after about the first trimester not shown to improve outcomes; levothyroxine from the second trimester does not improve outcomes in isolated hypothyroxinaemia.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389. Superseded by the 2026 guideline; cited for comparison only.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/