What happens when a woman with hypothyroidism becomes pregnant?

Endogenous thyroid hormone production during pregnancy cannot be increased in a woman with hypothyroidism. In this case, the dose of exogenously administered levothyroxine (LT4) will need to be adjusted. If this does not happen, the woman will end up with hypothyroidism while receiving the exact same dose that kept her euthyroid before pregnancy, resulting in insufficient thyroid hormones for the fetal growth.


What are the risks of hypothyroidism to the mother and the fetus?

Maternal hypothyroidism leads to insufficient thyroid hormones for the development of the fetal organs, especially the fetal brain. Even subclinical hypothyroidism is associated with:

  • Fetal neurodevelopmental abnormalities and
  • lower IQs of the child.

Studies show that increased thyroxine or levothyroxine needs increase gradually from 4-6 to 16-20 weeks of gestation, where they remain stable until delivery.

Hypothyroidism is associated with significant complications, such as:

  • premature birth
  • miscarriage
  • perinatal mortality
  • pre-eclampsia
  • post-partum hemorrhage

Some risks also appear to be higher in women with thyroid antibodies which also increase the risk of a usually transient thyroiditis up to one year after delivery.


What should a woman with known or not known hypothyroidism who wants to get pregnant do?

Therefore, ideally, a woman with known hypothyroidism who is planning pregnancy should consult her endocrinologist to check her TSH levels and adjust her levothyroxine dose to bring TSH to the desired levels for pregnancy. During pregnancy, and mainly during the first half of pregnancy, as thyroxine needs gradually increase, regular monitoring of thyroid function and dose adjustments are necessary.

But in addition to the known pre-existing hypothyroidism, hypothyroidism can also develop in pregnancy. In countries such as Greece with adequate dietary iodine intake, the most common cause of hypothyroidism in pregnancy is chronic autoimmune thyroiditis (Hashimoto’s). Women with a family history of thyroid disease or with symptoms suggestive of hypothyroidism should have their thyroid function measured in blood.

Therefore, early diagnosis and treatment of thyroid dysfunction in pregnancy is of dual importance, since it protects both the foetus and the mother.