Thyroid and pregnancy: tests, targets, and monitoring
Pregnancy significantly increases the workload of the thyroid. Every clinician and patient should know the target values, treatment doses, and monitoring intervals.
Physiology of pregnancy
During pregnancy, the need for thyroid hormones increases by 25–50%. T4 and T3 levels rise because the amount of thyroxine-binding globulin increases, but the free hormone fractions remain roughly the same. In early pregnancy, human chorionic gonadotropin (hCG) stimulates TSH receptors, which causes TSH to drop slightly in the first trimester.
TSH reference values in pregnancy
TSH should be in the range of 0.1–2.5 mIU/L in the first trimester, 0.2–3.0 mIU/L in the second trimester, and 0.3–3.0 mIU/L in the third trimester. If trimester-specific reference ranges exist in your region, those should be used. If TSH is below the lower limit of the relevant trimester but fT4 is normal, this is physiological and does not require treatment.
Hypothyroidism during pregnancy
Both overt and subclinical hypothyroidism are treated, because leaving them untreated increases the risk of miscarriage, preterm birth, low birth weight, placental abruption, and impaired fetal brain development. Women who are already taking levothyroxine should increase their daily dose by 25–50% as soon as they learn that they are pregnant. Anti-TPO-positive women with a normal TSH should be carefully monitored. If their TSH rises above 2.5 mIU/L, starting treatment is considered. Check TSH and fT4 every 4–8 weeks until mid-pregnancy and then at least once per trimester.
Hyperthyroidism during pregnancy
The main causes are Graves' disease and gestational transient hyperthyroidism, which is due to elevated hCG levels. It is important to distinguish between them: gestational transient hyperthyroidism is usually mild, resolves on its own, is associated with nausea, and TSI is normal. Graves' disease, on the other hand, requires targeted treatment. In the first trimester, propylthiouracil is preferred because it carries a lower risk of malformations. From the second trimester onward, methimazole (Thyrozol) may be used. Radioactive iodine therapy is absolutely contraindicated. TSI is measured again at the end of the second trimester or the beginning of the third trimester, because maternal antibodies can cross the placental barrier and cause hyperthyroidism in the fetus or newborn.
Blood tests and ultrasound
Not all pregnant women need testing. Screening is performed in women with a known thyroid disease, a family history, anti-TPO positivity, type 1 diabetes, or any other autoimmune disease; those who have previously had radiation therapy to the head or neck, infertility, recurrent miscarriages, age over 30, severe obesity, or living in an iodine-deficient area. In these women, thyroid function should be assessed before the 10th week of pregnancy. Thyroid ultrasound is safe during pregnancy and is used to evaluate nodules and goiter.
After delivery
About 5–10% of women develop postpartum thyroiditis, which usually occurs 3–12 months after delivery (see separate article). After delivery, women on levothyroxine should return to the dose they were taking before pregnancy and have their TSH rechecked 3–6 months after delivery. Women with Graves' disease being treated with thionamides can usually continue methimazole at a low dose (≤20 mg per day) while breastfeeding, but the infant's thyroid function is monitored by the pediatrician.
Follow-up
If you have known hypothyroidism, you should be checked every 4–8 weeks until the values are stable, and then at least once per trimester. If you have Graves' disease, fT4/fT3 should be measured every 4–8 weeks, with the goal of keeping the level in the upper third of the reference range. In addition, TSI should be measured at the initial assessment and again at the end of the second trimester or the beginning of the third trimester. For thyroid disease newly diagnosed during pregnancy, closer monitoring is needed (every 2–4 weeks) until a stable state is reached.
Medical disclaimer. This article is for general educational purposes and does not replace consultation, diagnosis or treatment by a qualified healthcare professional. Always discuss your individual situation with your own doctor.