Thyroid and pregnancy

JH Lazarus

Cardiff University School of Medicine, Llandough Hospital

Summer School 11-14 July 2006
The Møller Centre, Storeys Way, Cambridge, UK

Pregnancy has an effect on thyroid economy with significant changes in iodine metabolism, serum thyroid binding proteins and the development of maternal goitre especially in iodine deficient areas. Pregnancy is also accompanied by immunological changes, mainly characterised by a shift from a Th1 to a Th2 state.

Thyroid peroxidase antibodies (TPOAb) are present in 10% of women at 14 weeks gestation and are associated with (a) increased pregnancy failure (i.e. abortion), (b) increased incidence of gestational thyroid dysfunction and (c) predisposition to postpartum thyroiditis (PPT). Thyroid function should be measured in women with severe hyperemesis gravidarum. Graves’ hyperthyroidism during pregnancy is best managed with propylthiouracil given throughout gestation. TSH receptor antibody measurements at 36 weeks gestation are predictive of transient neonatal hyperthyroidism and should be checked even in previously treated patients receiving thyroxine.

Postpartum exacerbation of hyperthyroidism is common and should be evaluated in women with Graves’ disease not on treatment. Radioiodine therapy in pregnancy is absolutely contraindicated. Hypothyroidism (including subclinical hypothyroidism) occurs in about 2.5% of pregnancies and may lead to obstetric and neonatal complications as well as being a cause of infertility.

During the last few decades evidence has been presented to underpin the critical importance of adequate foetal thyroid hormone concentrations in order to ensure normal central and peripheral nervous system maturation. In iodine deficient and iodine sufficient areas low maternal circulating thyroxine concentration has been associated with significant decrement in child IQ and development. These data suggest a requirement for an antenatal thyroid screening programme with thyroxine treatment in women with hypothyroidism in early pregnancy. Hypothyroidism in pregnancy is treated with a larger dose of thyroxine than in the non-pregnant state.

Postpartum thyroid dysfunction (PPTD) occurs in 50% of women found to have TPOAb in early pregnancy. The hypothyroid phase of PPTD is symptomatic and requires thyroxine therapy. A high incidence of permanent hypothyroidism has been noted in these women. Women having transient PPTD with hypothyroidism should be checked frequently as there is a 50% chance of these patients developing hypothyroidism during the next seven years.

The opinions expressed in this paper are those of the speaker and do not necessarily reflect the views of the Society

Revised: 23-Aug-2006