The immune system, which plays a role in thyroid disease, is suppressed during pregnancy in order to protect the developing fetus. As a result of the loss of this protective effect at the end of pregnancy, there is a tendency for thyroid disease to occur after delivery in those women who have had previous thyroid disease or who are at risk for developing thyroid disease. Silent autoimmune thyroiditis is particularly common after pregnancy. This “postpartum thyroiditis” tends to get better after a few weeks although recurrence in subsequent pregnancies and progression to permanent hypothyroidism is possible.

It is important to recognize thyroid disorders during pregnancy as untreated hypothyroidism may impair full and normal development of the fetus, to some degree, and may increase maternal complications.

Iodine intake should be increased during pregnancy and breastfeeding of 150 micrograms to 250 micrograms per day, but should not exceed 500 micrograms per day.

GRAVES’ DISEASE AND PREGNANCY

Treatment of Graves’ hyperthyroidism during pregnancy is different from that in non pregnant women, since radioactive iodine cannot be given and surgery should be avoided (particularly in the first and third trimesters of the pregnancy for fear of inducing a miscarriage). Because of the immunosuppressive effect of pregnancy, antithyroid drugs can be given in doses lower than with non pregnant patients. Over treatment of the hyperthyroidism with antithyroid drugs can affect the baby’s thyroid since the drugs cross the placenta into the baby’s bloodstream.

While antithyroid drugs (methimazole, propylthiouracil) can rarely cause birth defects, the risk of uncontrolled hyperthyroidism is worse than the risk of birth defects from antithyroid drugs. Pregnant women with hyperthyroidism require careful monitoring to minimize the risks of hyperthyroidism and antithyroid drugs.

THYROXINE TREATMENT IN PREGNANCY

There is no contra-indication to taking thyroxine throughout pregnancy. If hypothyroidism has been diagnosed before pregnancy The Endocrine Society recommends to adjust the dose to reach a TSH not higher than 2.5 mIU/L before pregnancy. It has been observed that thyroxine requirements increase during the pregnancy so most women with thyroid diseases need dose adjustment and monitoring. The baby’s thyroid becomes functional at approximately 12 weeks of gestation. Thyroid hormones play an important role in fetal brain development, so the thyroid hormones provided by the mother during the first trimester of pregnancy are especially important. Thyroxine treatment is adjusted to obtain TSH levels specific to each trimester (less than 2.5 mIU/L in the first trimester or 3 mIU/L during the second and third trimester.

SCREENING DURING PREGNANCY FOR THYROID DISORDERS

Screening guidelines for thyroid disease differ among the various associations and expert groups. The Endocrine Society recommends screening for thyroid disorders in women at high risk:

Women with prior thyroid disease or surgery, goitre, family history, positive thyroid antibodies,other autoimmune diseases, symptoms or signs suggestive of thyroid dysfunction, previous head and neck irradiation for other disease. Screening is performed by measuring the TSH level.

BREAST FEEDING AND THYROID DISEASE

Radioactive isotopes are secreted in milk therefore no isotope tests or isotope scans should be performed on someone who is breastfeeding. Antithyroid drugs can be used when breast feeding, as only negligible amounts actually get into the milk. Thyroxine is also secreted in the milk, but providing the dosage in the mother is in the physiologic range, it appears to be quite safe for the mother on thyroxine to breast feed.

INFERTILITY

Patients with either hyper- or hypothyroidism can have fertility problems although it is certainly possible to have these diseases and still get pregnant. Sub-clinical hypothyroidism can sometimes cause infertility and miscarriages and is, thus, usually treated in women of childbearing age that desire to become pregnant.

In addition, both men and women with untreated thyroid disease often have decreased sexual desire (libido). Hyper- or hypothyroidism is also a cause for male infertility since sperm development requires normal thyroid hormone levels.

Graves’ disease can be treated with radioactive iodine or by surgery before pregnancy to avoid the use of antithyroid medication during pregnancy. It is generally recommended to wait six months after radioactive iodine treatment before becoming pregnant.

One other cause of infertility in patients with thyroid disease is the uncommon condition of primary ovary failure. This is an autoimmune disorder, like Graves’ disease and Hashimoto’s thyroiditis, caused by proteins and white cells in the blood that attack proteins in the patient’s ovaries. This leads to the decreased size of the ovaries, the failure to ovulate, premature menopause, and infertility.

MENSTRUATION

Menstruation tends to be increased in hypothyroidism and decreased in hyperthyroidism. The effects of thyroid hormones on menstrual periods, ovarian function and the endocrine system in general are complicated but important. With too much or too little thyroid hormone a variety of effects on the reproductive system can occur. Girls who become hyper- or hypothyroid during puberty may have delayed menstrual function.

Updated in November 2019 by Deric Morrison, MD, FRCPC, Div. of Endocrinology, Dept. of Medicine, University of Western Ontario. Original text written by Irving B. Rosen, MD, FRCS(C), FACS, and Paul G. Walfish CM, MD, FRCP(C), FACP, FRSM.