The most common cause of hyperthyroidism in women of childbearing age is Graves’ disease, which has an incidence of 30 to 80 per 100,000 person-years. Toxic nodular disease is rarer with less than two cases per 100,000 person-years.16 The incidence of Graves’ disease decreases during pregnancy due to the immunosuppressive effects of pregnancy.
Transient gestational hyperthyroidism is a common cause of mild hyperthyroidism secondary to thyroid stimulation by beta human chorionic gonadotrophin. Transient gestational hyperthyroidism is generally limited to the first half of pregnancy and is seen more often in women with hyperemesis and those with high beta human chorionic gonadotrophin levels due to molar pregnancy or multiple gestation.
If TSH levels are low during pregnancy, the TSH receptor antibody (TRAb) level should be assessed. If the TSH level is persistently undetectable and/or free T3/T4 levels are elevated and/or TRAb levels are positive then it is recommended that the patient be referred to an endocrinologist. Nuclear medicine thyroid scans are contraindicated in pregnancy.
Antithyroid medication, such as carbimazole or propylthiouracil, may be required in cases of overt hyperthyroidism. Both medications are associated with a small increase in rates of fetal malformations (2 to 4% above background rates). The birth defects associated with propylthiouracil are generally milder and more easily corrected so it is used preferentially before a planned pregnancy and during the first trimester. If antithyroid medication is required in the second or third trimesters, when the risk of malformations is lower, carbimazole is preferred due to its lower risk of hepatotoxicity. The aim of treatment with antithyroid medications is to maintain a free T4 level at the upper end (or within 10%) of the nonpregnant reference range.
Women with a history of Graves’ disease treated with surgery or radioactive iodine ablative therapy should have TRAb levels measured in early pregnancy. If positive, TRAb measurement should be repeated at 18 to 22 weeks’ gestation. As TRAb can cross the placenta and cause fetal hyperthyroidism and neonatal Graves’ disease, women with active Graves’ disease or positive TRAb at 18 to 22 weeks’ gestation should have monitoring for fetal hyperthyroidism by a maternal-fetal medicine specialist. If the TRAb level is elevated at 18 to 22 weeks’ gestation or in women with active Graves’ disease on treatment, measurement of TRAb levels at 30 to 34 weeks’ gestation can guide decisions about neonatal and postnatal monitoring.
Thyroid nodules and thyroid cancer in pregnancy
Thyroid nodules found during examination can be further assessed by ultrasound. Referral to an endocrinologist should be considered for women with nodules detected during pregnancy.
Fine-needle aspiration biopsy can be safely performed at any time during pregnancy. If differentiated thyroid cancer (papillary or follicular thyroid cancer) is detected during pregnancy, surgery can be delayed until the postpartum period as such a delay is unlikely to affect the long-term prognosis of differentiated thyroid cancer. Surgery in the second trimester may be considered for advanced differentiated thyroid cancer, medullary thyroid cancer or poorly differentiated thyroid cancer.2