Thyroid disorders are relatively frequent in women so that one woman in eight will develop thyroid disorder during her lifetime. Hypothyroidism (underactive thyroid) is not rare during pregnancy and represents a serious risk for both mother and a child.
Pregnancy represents a stress test for the thyroid gland which normally increases in size and increases the production of thyroid hormones by 50%. It is therefore not surprizing that thyroid problems can start or get worse during pregnancy. In a recent study 15.5% of all pregnant women were found to have subclinical (early or mild) hypothyroidism (defined as a TSH>2.75).
It is recommended that thyroid function is checked in women who are either planning pregnancy or newly pregnant and who are at high risk of hypothyroidism. High risk women are those with a goitre, > 30 years old, with positive thyroid antibodies, with a history of miscarriage and/or preterm delivery or family history of thyroid disease, with type 1 diabetes or other autoimmune disorder, with infertility, who are currently on thyroxine replacement, who had previous thyroid surgery or previous radiation to the head or neck or who are living in areas of iodine deficiency.
Hypothyroid women are predisposed to infertility, miscarriage, preterm delivery, postpartum haemorrhage and other complications. It is therefore important that thyroid hormone levels are normal both before and during pregnancy. If a pregnant woman has low level of thyroid hormone her baby may have inadequate level of thyroxine during a critical time of brain development (usually first 12 weeks of pregnancy) which may have a negative impact of mental development of child. For those women who are already on thyroxine replacement thyroid hormone levels should be checked before they try to conceive. If TSH is found to be higher than accepted the dose of thyroxine should be adjusted and pregnancy has to be deferred until thyroid function tests are normalized.
Women who have known positive thyroid peroxidase (TPO) antibodies but otherwise normal thyroid function should have extra thyroid function screening before pregnancy as well as during pregnancy as thyroid dysfunction in pregnancy is more frequent in those women. Women with positive TPO antibodies are at an increased risk of miscarriage, preterm delivery, progression to hypothyroidism and postpartum thyroiditis. In a pregnant woman with positive TPO antibodies thyroxine replacement is recommended and the dose has to be re-evaluated every four to six weeks of pregnancy (up to 20th week) because an increase in dose may be necessary. Most of these women do not need thyroxine after pregnancy but should be monitored as they are at high risk of developing postpartum thyroiditis.