The aetiology, diagnosis and management of hypothyroidism in pregnant women.

Introduction

Primary hypothyroidism is highly prevalent worldwide. Prevalence increases with age, with women having a higher incidence than men. The most common cause is iodine deficiency. Other causes include autoimmunity, radioactive iodine treatment and surgery.

Clinical and diagnostic features

Primary hypothyroidism can be classified into overt (OH) and sub-clinical (SH) (Table 1). In OH, thyroid-stimulating hormone (TSH) is elevated and thyroxine (fT4) and tri-iodothyronine (fT3) are low. These patients are usually symptomatic. in SH, TSH is elevated, but fT4 and fT3 remain normal and patients are usually asymptomatic.

Clinical features of hypothyroidism can be very non-specific, including weight gain, constipation and tiredness. These symptoms can also be experienced during pregnancy. many women can also remain relatively asymptomatic despite quite marked biochemical abnormalities. Biochemical testing is the only means of confirming the diagnosis.

Effects of hypothyroidism during pregnancy

It is estimated that during pregnancy the prevalence of OH is 0.3-0.5% and 2-3% for SH.^1-3 Studies have shown that there are consequences for both the mother and fetus if maternal hypothyroidism occurs. These adverse events and outcomes are more pronounced in those with OH.

Maternal

There is an association between hypothyroidism and decreased fertility. During pregnancy, women with OH have increased prevalence of miscarriage, gestational hypertension, placental abruption, anaemia and postpartum haemorrhage. Treatment with thyroxine to achieve euthyroid levels reduces the obstetric risks.

Offspring

OH that has been untreated during pregnancy has been associated with preterm labour, low birth weight and neonatal respiratory distress. in one study, isolated low fT4 with a normal TSH was associated with preterm labour and macrosomia.3 SH has been associated with preterm delivery.⁴

Iodine is important in fetal brain development, and there is evidence to also support an important role for thyroid hormone. a number of studies have looked at neurological development of neonates born to women with thyroid dysfunction during pregnancy. One study showed that infants born to women with low T4 at 12 weeks of gestation had lower scores on the Neonatal Behavioural assessment Scale orientation index.⁵

There have been few studies which have looked at the longer term and have shown reductions in iQ scores in children born to women with OH. Children born to women with SH and isolated T4 reductions may also have subtle cognitive deficits.

Management

Hypothyroidism is easily treated with thyroxine administration. Women who have known OH should ensure they are euthyroid prior to conception, aiming for a TSH of6Women should be advised to increase their dose of thyroxine around 4-6 weeks' gestation by 30-50 per cent. Requirements increase in pregnancy because of increased volume of distribution and increased thyroid binding globulin (TBG).

Women who have SH should also be commenced on thyroxine preconception to normalise TSH, again aiming for levels of

 Screening

Many individuals with thyroid disease can remain undiagnosed. at present in Australia, there is no universal screening recommendation for thyroid disease in pregnant women. The Endocrine Society recommends targeted case-finding in women who are at high risk of thyroid dysfunction (Table 2).

Conclusion

Thyroid dysfunction is a relatively common occurrence in pregnancy. Evidence supports the treatment of OH and SH to improve obstetric and neonatal complications. Preconception management is essential to ensure women are euthyroid. Currently, the evidence does not support universal screening for thyroid disease in pregnancy, but rather targeted case-finding in high-risk women.

 Endometriosis and Infertility 235.84 Kb

References

1. Canaris GJ, Manowitz NR. Mayor G, Ridgway EC.  The Colorado thyroid disease prevalence study.  Arch Intern Med 2000;160:526-34.

2. Klein RZ, Haddow JE, Faix JD et al.  Prevalence of thyroid deficiency in pregnant women.  Clin Endo (Oxf) 1991:35:41-6.

3. Cleary-Goldman J, Malone FD, Lambert-Messerlian G et al.  Maternal thyroid hypofuncion and pregnancy outcome.  Obs & Gyne 2008;112;1;85-92.

4. Casey B, Dashe JS, Wells CE, McIntire DD, Byrd W, Leveno KJ, Cunningham FG. 2005 Subclinical hypothyroidism and pregnancy outcomes.  Obstet Gynecol 105:239-45.

5. Kooistra L, Crawford S, van Baar AL, Brouwers EP, Popp VJ Neonatal effects of maternal hypothyroxinemia during early pregnancy.  Pediatrics 2006;117:161-67.

6. Abalovich M et al. Management of thyroid dysfunction during pregnancy and postpartum.  JCEM 2007;92:8:S1-47.

See also Medical Observer http://www.medicalobserver.com.au/

Content Updated March 10, 2010