Thyroid in Pregnancy
What Changes, What the Risks Are, and How to Monitor Properly

The targets differ from general care. In the first trimester, the recommended upper limit is 2.5 mIU/L (compared to 4.0 to 4.5 in standard care). In the second and third trimesters, the upper limit is 3.0 mIU/L. These tighter targets exist because the fetal thyroid doesn't become functional until around week 12, meaning the fetus depends entirely on maternal thyroid hormone during the critical first trimester window of brain and nervous system development. A TSH of 3.5 that would be 'fine' outside pregnancy may require dose adjustment during the first trimester. This is not always communicated clearly in routine obstetric care.

Yes, and the effects are serious enough to warrant proactive monitoring. Thyroid hormone is critical for fetal brain and nervous system development in the first trimester. Maternal hypothyroidism during this window, even subclinical, has been associated with reduced IQ and developmental delays in children. Beyond neurodevelopment: untreated hypothyroidism in pregnancy is associated with significantly higher rates of miscarriage, preterm birth, placental abruption, gestational hypertension, and low birth weight. These are outcomes that change with treatment. The risk-benefit calculation for treating subclinical hypothyroidism in pregnancy is very different from the same question outside pregnancy.

Almost certainly yes, if you're on thyroxine. Thyroid hormone demand increases by 30 to 50 percent in pregnancy, and most women with hypothyroidism need a dose increase starting in the first trimester. Standard advice is to increase the dose by 25 to 30 percent as soon as pregnancy is confirmed, then verify with a TSH test at 4 to 6 weeks. The increase is often needed before a standard antenatal appointment is scheduled, which is why awareness of this helps. Women with Hashimoto's who are trying to conceive should have their thyroid function optimised before conception, with TSH below 2.5 as a starting point.

Graves' disease, the most common cause of hyperthyroidism, typically improves during pregnancy because immune tolerance shifts toward the fetal semi-allograft (the immune system is partially suppressed to tolerate the fetus). Many women with Graves' disease need reduced antithyroid medication or none at all in the second half of pregnancy. But the first trimester can be more challenging, and untreated hyperthyroidism carries risks: fetal heart rate abnormalities, fetal growth restriction, and in severe cases, premature delivery. TRAb antibodies (from Graves' disease) can cross the placenta and cause neonatal hyperthyroidism in the baby, which requires neonatal monitoring. A rebound of Graves' disease is common postpartum as immune suppression lifts.

Postpartum thyroiditis is an autoimmune thyroid inflammation that occurs in 5 to 10 percent of women in the 12 months after delivery. It's most common in women who had elevated anti-TPO antibodies during pregnancy. The typical pattern is a hyperthyroid phase (months 1 to 4 postpartum) as immune suppression lifts and the thyroid is attacked, followed by a hypothyroid phase (months 4 to 8) as thyroid reserve is depleted. The hyperthyroid phase often produces palpitations and anxiety mistaken for stress. The hypothyroid phase produces fatigue and low mood often attributed to postnatal depression without thyroid function being checked. Around 25 percent of women with postpartum thyroiditis go on to develop permanent hypothyroidism.

Yes, particularly if you have a family history of thyroid disease, any autoimmune condition, previous pregnancy loss, or any symptoms suggesting thyroid dysfunction. Pre-conception thyroid testing should include TSH, free T4, and anti-TPO antibodies. If TSH is above 2.5 before conception, treatment is generally recommended before proceeding. Women with Hashimoto's who have a TSH in the upper normal range may need dose adjustment to bring TSH below 2.5 before pregnancy to ensure adequate reserves for the first trimester demand increase. Identifying and addressing thyroid issues before conception rather than during it gives the best outcomes for both mother and baby.