Hypothyroidism is more common among women than men, affecting 15 in every 1000 women compared with 1 in every 1000 men. In women, the condition can cause irregular or heavy periods as well as problems during pregnancy. The thyroid gland needs to produce 50% more than the usual amount of thyroid hormone during pregnancy, in order to ensure there is enough of the hormone developing fetus as well as the mother.

In pregnancy, a thyroid stimulating hormone (TSH) level of between 2.5 and 10 mlU/L and a normal thyroxine (T4) level is indicative of subclinical hypothyroidism, while a TSH level that exceeds mlU/L is considered a sign of hypothyroidism, even when the T4 level is normal.

If this condition is left untreated in pregnant women, several complications may occur, some of which are described below.

• Miscarriage or stillbirth
• Premature birth and low birth weight
• Pre-eclampsia may develop, a condition where high blood pressure and fluid retention in the mother put both the mother and baby at risk of several complications
• The mother may become anemic
• The baby may develop an underactive thyroid
• The risk of birth defect is increased
• Bleeding after birth

Babies born with hypothyroidism may have delayed growth and impaired intellectual ability, symptoms that are referred to as cretinism in severe cases. In the UK, all newborns are screened for the condition using a blood test taken form the heel when the infant is aged around five days old.

About 5% of women who give birth develop thyroiditis afterwards (postpartum thyroiditis), a condition that can develop any time up to nine months after the birth. Here, hyperthyroidism develops initially and is followed by the onset of hypothyroidism. Up to one half of these women have hypothyroidism for the rest of their lives.