Thyroid Disease in Pregnancy: Tests and Care

Thyroid Disease in Pregnancy: Tests and Care

Pregnancy places extraordinary demands on the thyroid gland. Thyroid hormones are essential for fetal brain development, particularly in the first trimester when the fetus depends entirely on maternal thyroid hormones. Thyroid dysfunction during pregnancy — whether hypothyroidism, hyperthyroidism, or autoimmune thyroiditis — can have serious consequences for both mother and baby. Understanding the tests, risks, and management is critical for every expectant mother.

How Pregnancy Changes Thyroid Function

Several physiological changes occur during pregnancy:

• hCG (human chorionic gonadotropin): This pregnancy hormone has TSH-like activity and stimulates the thyroid, causing TSH to drop in the first trimester. hCG peaks at 10-12 weeks.
• Estrogen: Increases thyroid-binding globulin (TBG), raising total T3 and T4 levels by 50% by mid-pregnancy.
• Iodine requirements: Increase by 50% during pregnancy due to increased renal clearance and fetal needs.
• Placental deiodinases: Convert T4 to T3 and reverse T3, modulating thyroid hormone availability to the fetus.

Because of these changes, TSH reference ranges are trimester-specific. Using non-pregnancy ranges leads to misdiagnosis in 15-20% of pregnant women.

Hypothyroidism in Pregnancy

Prevalence: Overt hypothyroidism affects 0.3-0.5% of pregnancies; subclinical hypothyroidism affects 2-3%.

Risks of untreated hypothyroidism:

• Miscarriage (2-3x increased risk)
• Preterm delivery
• Preeclampsia
• Low birth weight
• Impaired fetal neurodevelopment (reduced IQ, attention deficits)
• Postpartum hemorrhage

A landmark study in The New England Journal of Medicine (1999) found that children of mothers with untreated hypothyroidism during pregnancy had IQ scores 7 points lower than children of euthyroid mothers at age 7-9.

Treatment: Levothyroxine is safe and essential during pregnancy. Dose typically increases by 25-30% in the first trimester. TSH should be checked every 4 weeks in the first half of pregnancy, then at least once in the second half.

Hyperthyroidism in Pregnancy

Prevalence: 0.1-0.4% of pregnancies (Graves' disease is most common).

Risks:

• Maternal: thyroid storm, heart failure, preeclampsia
• Fetal: tachycardia, growth restriction, prematurity, stillbirth
• Neonatal: transient hyperthyroidism (from maternal antibodies crossing placenta)

Treatment: PTU (propylthiouracil) is preferred in the first trimester due to methimazole's teratogenic risk. Methimazole is preferred in the second and third trimesters due to PTU's hepatotoxicity risk. Radioactive iodine is contraindicated.

Recommended Tests During Pregnancy

• TSH: First trimester screening (target 0.1-2.5 mIU/L)
• Free T4: If TSH is abnormal
• TPO antibodies: Screen for Hashimoto's (positive in 5-15% of pregnant women)
• TRAb: If Graves' disease suspected or history of Graves'

The ATA recommends universal TSH screening in the first trimester for women with risk factors (personal/family history of thyroid disease, autoimmune conditions, goitre, symptoms). Universal screening for all pregnant women remains debated.

Postpartum Thyroiditis

Occurs in 5-10% of women within 12 months of delivery. It typically presents with a transient hyperthyroid phase (2-4 months postpartum) followed by a hypothyroid phase (4-8 months). Most women recover, but 20-30% develop permanent hypothyroidism.

A 2019 study in Thyroid found that women with positive TPO antibodies during pregnancy had a 50% risk of developing postpartum thyroiditis.

Conclusion

Thyroid disease during pregnancy is common and treatable, but requires careful monitoring with trimester-specific targets. All pregnant women should be screened for thyroid dysfunction, especially those with risk factors. Early treatment with levothyroxine or antithyroid medications protects both mother and baby.

References:

• Alexander, E.K., et al. (2017). Guidelines of the ATA for diagnosis and management of thyroid disease during pregnancy. Thyroid, 27(3), 315-389.
• Stagnaro-Green, A., et al. (2011). Guidelines of the ATA for the diagnosis and management of thyroid disease during pregnancy. Thyroid, 21(10), 1081-1125.
• Haddow, J.E., et al. (1999). Maternal thyroid deficiency during pregnancy and subsequent neuropsychological development. The New England Journal of Medicine, 341(8), 549-555.
• Casey, B.M., et al. (2005). Subclinical hypothyroidism and pregnancy outcomes. Obstetrics & Gynecology, 105(2), 239-245.
• Negro, R., et al. (2006). Levothyroxine treatment in euthyroid pregnant women with autoimmune thyroid disease. The Journal of Clinical Endocrinology & Metabolism, 91(7), 2587-2591.