Effects of hypothyroidism in pregnancy and pregnancy outcome

Associations of untreated hypothyroidism include:

• infertility
• increased risk of early and late obstetrics complications e.g. - recurrent miscarriages, anaemia, gestational hypertension, placental abruption, premature delivery, postpartum haemorrhage (1)

These complications are more frequent in overt disease than with subclinical hypothyroidism (2).

Frequency of these complications can be reduced by restoring the normal thyroid function with adequate thyroxin treatment of the pregnant woman (1)

• according to a prospective randomised intervention trial in women with autoimmune thyroiditis who received thyroxine (started at 5 to 10 weeks) throughout the pregnancy, the miscarriage rate was decreased by 75% and preterm delivery by 69% than in the control group

The adverse effects of maternal hypothyroidism on the foetus are considered to depend on the severity and early onset of the maternal disease (1). Recent studies have revealed that

• undiagnosed (resulting in untreated) maternal hypothyroidism in the first half of pregnancy is linked with poorer neurodevelopmental in the foetus
• in untreated (or suboptimally treated) women with hypothyroidism due to chronic autoimmune thyroiditis, the foetus is at risk of clinically relevant cognitive deficits
• the presence of isolated hypothyroxinaemia (free thyroxine near the lower limit of normality, normal TSH, and no detectable thyroid antibodies) in early pregnancy is associated with a lower psychodevelopmental index in the foetus

The free throxine values return to normal levels spontaneously in later gestation and the fetus develops normally in most of these women. There is no concrete evidence available on benefits of treating women with isolated hypothyroxinaemia (1).

Iodine deficiency affects both maternal and fetal thyroid glands early

• severe iodine deficiency causes irreversible fetal brain damage resulting in endemic cretinism
• even less severe iodine deficiency has shown to be associated with an overall reduction in cognitive functions (1).

Reference:

• 1. Glinoer D, Abalovich M. Unresolved questions in managing hypothyroidism during pregnancy. BMJ. 2007;335(7614):300-2.
• 2. Abalovich M et al. Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2007;92(8 Suppl):S1-47.