Thyroid disease in pregnancy

Hypothyroidism is common in pregnancy with an estimated prevalence of 2-3% and 0.3-0.5% for subclinical and overt hypothyroidism respectively.[8] Endemic iodine deficiency accounts for most hypothyroidism in pregnant women worldwide while chronic autoimmune thyroiditis is the most common cause of hypothyroidism in iodine sufficient parts of the world.[9][10] The presentation of hypothyroidism in pregnancy is not always classical and may sometimes be difficult to distinguish from the symptoms of normal pregnancy. A high index of suspicion is therefore required especially in women at risk of thyroid disease e.g. women with a personal or family history of thyroid disease, goitre, or co-existing primary autoimmune disorder like type 1 diabetes.

Hypothyroidism is diagnosed by noting a high TSH associated with a subnormal T4 concentration. Subclinical hypothyroidism (SCH) is present when the TSH is high but the T4 level is in the normal range but usually low normal. SCH is the commonest form of hypothyroidism in pregnancy and is usually due to progressive thyroid destruction due to autoimmune thyroid disease.

Several studies, mostly retrospective, have shown an association between overt hypothyroidism and adverse fetal and obstetric outcomes (e.g. Glinoer 1991).[11] Maternal complications such as miscarriages, anaemia in pregnancy, pre-eclampsia, abruptio placenta and postpartum haemorrhage can occur in pregnant women with overt hypothyroidism.[12][13] Also, the offspring of these mothers can have complications such as premature birth, low birth weight and increased neonatal respiratory distress.[14] Similar complications have been reported in mothers with subclinical hypothyroidism. A three-fold risk of placental abruption and a two-fold risk of pre-term delivery were reported in mothers with subclinical hypothyroidism.[15] Another study showed a higher prevalence of subclinical hypothyroidism in women with pre-term delivery (before 32 weeks) compared to matched controls delivering at term.[16] An association with adverse obstetrics outcome has also been demonstrated in pregnant women with thyroid autoimmunity independent of thyroid function. Treatment of hypothyroidism reduces the risks of these adverse obstetric and fetal outcomes; a retrospective study of 150 pregnancies showed that treatment of hypothyroidism led to reduced rates of abortion and premature delivery. Also, a prospective intervention trial study showed that treatment of euthyroid antibody positive pregnant women led to fewer rates of miscarriage than non treated controls.[17]

It has long been known that cretinism (i.e. gross reduction in IQ) occurs in areas of severe iodine deficiency due to the fact that the mother is unable to make T4 for transport to the fetus, particularly in the first trimester. This neurointellectual impairment (on a more modest scale) has now been shown in an iodine sufficient area (USA) where a study showed that the IQ scores of 7- to 9-year-old children, born to mothers with undiagnosed and untreated hypothyroidism in pregnancy, were seven points lower than those of children of matched control women with normal thyroid function in pregnancy.[18] Another study showed that persistent hypothyroxinaemia at 12 weeks gestation was associated with an 8-10 point deficit in mental and motor function scores in infant offspring compared to children of mothers with normal thyroid function.[19] Even maternal thyroid peroxidase antibodies were shown to be associated with impaired intellectual development in the offspring of mothers with normal thyroid function.[20] It has been shown that it is only the maternal FT4 levels that are associated with child IQ and brain morphological outcomes, as opposed to maternal TSH levels.[5]

Medications to treat hypothyroidism have been found to be safe during pregnancy.[21] Levothyroxine is the treatment of choice for hypothyroidism in pregnancy. Thyroid function should be normalised prior to conception in women with pre-existing thyroid disease. Once pregnancy is confirmed the thyroxine dose should be increased by about 30-50% and subsequent titrations should be guided by thyroid function tests (FT4 and TSH) that should be monitored 4-6 weekly until euthyroidism is achieved. It is recommended that TSH levels are maintained below 2.5 mU/L in the first trimester of pregnancy and below 3 mU/L in later pregnancy.[22] The recommended maintenance dose of thyroxine in pregnancy is about 2.0-2.4 µg/kg daily. Thyroxine requirements may increase in late gestation and return to pre-pregnancy levels in the majority of women on delivery. Pregnant patients with subclinical hypothyroidism (normal FT4 and elevated TSH) should be treated as well, since supplementation with levothyroxine in such cases results in a significantly higher delivery rate, with a pooled relative chance of 2.76.[23]

Hyperthyroidism occurs in about 0.2-0.4% of all pregnancies. Most cases are due to Graves' disease although less common causes (e.g. toxic nodules and thyroiditis) may be seen.[24] Clinical assessment alone may occasionally be inadequate in differentiating hyperthyroidism from the hyperdynamic state of pregnancy. Distinctive clinical features of Graves' disease include the presence of ophthalmopathy, diffuse goitre and pretibial myxoedema. Also, hyperthyroidism must be distinguished from gestational transient thyrotoxicosis, a self-limiting hyperthyroid state due to the thyroid stimulatory effects of beta-hCG . This distinction is important since the latter condition is typically mild and will not usually require specific antithyroid treatment. Red cell zinc may also be useful in differentiating the two.[25] Hyperthyroidism due to Graves' disease may worsen in the first trimester of pregnancy, remit in later pregnancy, and subsequently relapse in the postpartum.

Uncontrolled hyperthyroidism in pregnancy is associated with an increased risk of severe pre-eclampsia and up to a four-fold increased risk of low birth weight deliveries. Some of these unfavourable outcomes are more marked in women who are diagnosed for the first time in pregnancy. A recent study has also shown that already high normal maternal FT4 levels are associated with a decrease in child IQ and gray matter and cortex volumes, similar to the effects of hypothyroidism.[5]

Uncontrolled and inadequately treated maternal hyperthyroidism may also result in fetal and neonatal hyperthyroidism[26] due to the transplacental transfer of stimulatory TSH receptor antibodies.[27] Clinical neonatal hyperthyroidism occurs in about 1% of infants born to mothers with Graves' disease. Rarely neonatal hypothyroidism may also be observed in the infants of mothers with Graves' hyperthyroidism. This may result from transplacental transfer of circulating maternal anti-thyroid drugs, pituitary-thyroid axis suppression from transfer of maternal thyroxine.

Ideally, a woman who is known to have hyperthyroidism should seek pre-pregnancy advice, although as yet there is no evidence for its benefit. Appropriate education should allay fears that are commonly present in these women. She should be referred for specialist care for frequent checking of her thyroid status, thyroid antibody evaluation and close monitoring of her medication needs. Medical therapy with anti-thyroid medications is the treatment of choice for hyperthyroidism in pregnancy.[28][29] Methimazole and propylthiouracil are effective in preventing pregnancy complications by hyperthyroidism.[30] Surgery is considered for patients who have severe adverse reactions to anti-thyroid drugs and this is best performed in the second trimester of pregnancy. Radioactive iodine is absolutely contraindicated in pregnancy and the puerperium. If a woman is already receiving carbimazole, a change to propylthiouracil is recommended but this should be changed back to carbimazole after the first trimester. On occasion, Carbimazole is associated with skin and also mid-line defects in the fetus but propylthiouracil long term also can cause liver side effects in the adult. Carbimazole and propylthiouracil are both secreted in breast milk but evidence suggests that antithyroid drugs are safe during lactation.[31] There are no adverse effects on IQ or psychomotor development in children whose mothers have received antithyroid drugs in pregnancy. Current guidelines suggest that a pregnant patient should be on propylthiouracil during the first trimester of pregnancy due to lower tetratogenic effect and then be switched to methimazole during the second and third trimester due to lower liver dysfunction side effects.