How does thyroid function affect fertility and pregnancy?

Thyroid function matters in pregnancy for two reasons. The fetal thyroid does not start working until roughly the middle of the first half of pregnancy, so early fetal brain development depends on maternal thyroid hormone. And thyroid hormone requirement rises during pregnancy, so a dose that was correct before conception often is not. The 2026 American Thyroid Association guideline for preconception, pregnancy and postpartum is the current reference and it changed several long-standing habits.

Before pregnancy

For a woman already taking levothyroxine and planning pregnancy, the 2026 guideline describes a TSH between 0.5 and 2.5 mU/L as a reasonable treatment target, and suggests rechecking TSH and free T4 every 3 to 6 months while pregnancy is being planned. It also moved away from universal screening: clinicians are advised to evaluate every newly pregnant patient for thyroid disease risk factors and offer TSH testing to those at increased risk.

The change on thyroid antibodies

This is the most significant reversal. Euthyroid women with positive TPO or thyroglobulin antibodies were often given levothyroxine to reduce miscarriage risk. High-quality randomised trials did not show benefit. The 2026 guideline gives a strong recommendation, supported by high-quality evidence, that "for euthyroid TPOAb and/or TgAb positive women with infertility, levothyroxine treatment should not be offered." The same applies to fertility treatment and recurrent pregnancy loss. Monitoring replaces medication.

During pregnancy

Where local trimester-specific reference intervals are unavailable, the 2026 guideline states that a TSH interval of 0.1 to 4.0 mU/L can be used in the first and second trimesters. Levothyroxine is recommended when TSH exceeds 10 mU/L regardless of trimester. For milder elevations the guideline advises confirmatory retesting within 4 to 6 weeks before starting treatment, noting that at least half of mild abnormalities normalise within several weeks. Thyroid function testing is performed at pregnancy confirmation and about every 4 weeks through the first half of pregnancy. T3-containing therapy should be avoided.

If you take levothyroxine and are trying to conceive, arrange thyroid testing as soon as a pregnancy test is positive rather than waiting for the first prenatal visit. Iodine intake of 250 mcg daily is recommended in pregnancy and lactation, usually as a prenatal vitamin containing 150 mcg.