Thyroid Disease in Pregnancy

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Published: 07 October 2021

Thyroid disorders are common in women of childbearing age and careful assessment of thyroid function is an important aspect of antenatal care.

It’s normal for thyroid physiology to change significantly during pregnancy, and uncorrected thyroid dysfunction can have adverse effects on both fetal and maternal well-being. It can even extend beyond pregnancy and affect neuro-intellectual development in the early life of the infant (Lazarus 2010).

Thyroid function is defined by levels of circulating thyroid hormones, which are:

  • Free thyroxine (fT4)
  • Free triiodothyronine (fT3)
  • Thyroid stimulating hormone (TSH).

If these levels are abnormal, then thyroid function is classified as either thyroid deficiency (overt hypothyroidism), or as thyroid hormone excess (overt hyperthyroidism).

In cases where only thyroid stimulating hormone levels are abnormal, but levels of fT4 and fT3 are normal, then the disorder is referred to as either subclinical hypothyroidism (TSH high) or subclinical hyperthyroidism (TSH low) (Hyer 2018).

How Does Pregnancy Affect Thyroid Function?

Thyroid dysfunction typically affects 2 to 3% of pregnant women (Forehan 2012). Detecting, and if necessary, correcting any abnormal levels early in the antenatal period is essential for the normal development of the fetal brain and nervous system (NIDDK 2021).

During the first trimester, the fetus is completely dependent on the maternal supply of thyroid hormone via the placenta (Pregnancy, Birth and Baby 2021). It’s not until the 12th week of pregnancy that the fetus begins to manufacture its own supply of thyroid hormone, but it still doesn’t make enough thyroid hormone until 18 to 20 weeks.

Making a Diagnosis

Enquiring about the mother’s thyroid history is an essential first step in determining the need for further investigations. Thyroid problems can be hard to diagnose in pregnancy as many of the symptoms can be caused by the pregnancy itself - for example, high blood pressure and aversion to heat may also occur in pregnant women with normal thyroid function (NIDDK 2021).

Typically, women with an underactive thyroid report having very low energy levels, constipation or hair loss, as well as feeling unusually cold. Symptoms of an overactive thyroid, on the other hand, might include weight loss, heat intolerance and high blood pressure. This means that regular screening is recommended for women with a higher than normal risk of thyroid dysfunction. For example, those who have:

  • Symptoms of thyroid dysfunction
  • Personal or family history of thyroid disease
  • Previous thyroid surgery
  • Goitre
  • History of miscarriage or premature delivery
  • History of infertility
  • Type 1 diabetes.

(Pregnancy, Birth and Baby 2021)

Women who are severely overweight or who are over 30 years of age are also at greater risk. Smith et al. (2017) additionally recommend thyroid function screening for women who come from an area with moderate to severe iodine insufficiency, or who have previously had radiotherapy to the head and neck.

woman with goitre
A goitre (swelling in the neck from an enlarged thyroid) is a risk factor for thyroid dysfunction.

Hyperthyroidism In Pregnancy

Hyperthyroidism in pregnancy is usually caused by Graves’ disease, an autoimmune disorder in which the woman’s immune system makes antibodies that cause the thyroid to produce an excess of thyroid hormone (NIDDK 2021).

Left untreated, hyperthyroidism during pregnancy can lead to:

  • Premature birth
  • Miscarriage
  • Low birth weight
  • Pre-eclampsia
  • Congestive heart failure
  • A sudden and severe worsening of symptoms (thyroid storm).

As radioiodine cannot be given during pregnancy, treatment involves the administration of anti-thyroid drugs (Hyer 2018) using the minimum dose required to achieve stabilisation of hormonal levels within a normal range.

As well as difficulty coping with heat and tiredness, which can occur in a normal pregnancy, other warning signs for hyperthyroidism might include:

  • Shaky hands
  • Fast and irregular heartbeat
  • Unexplained weight loss
  • Failure to gain normal amounts of weight during pregnancy.

Hypothyroidism in Pregnancy

Hypothyroidism is common in women of reproductive age and occurs in 2 to 3% of all pregnancies (Tran et al. 2019). Adequate thyroid function is especially important during the first trimester, as the fetus cannot synthesise thyroid hormones until the second trimester.

For some women, a lack of iodine is the cause of low thyroid hormone levels. For example, a study found that, on average, Australian women consume 100 micrograms of iodine a day. However, the World Health Organization recommends a daily intake of at least 250 micrograms of iodine during pregnancy and breastfeeding (Pregnancy, Birth and Baby 2021).

Another cause of hypothyroidism is Hashimoto’s disease, an autoimmune disorder in which the immune system makes antibodies that attack the thyroid, causing inflammation and damage that impairs the production of thyroid hormones.

Whatever the cause, thyroid hormones must be supplemented by medicine to ensure fetal wellbeing. Most cases of hypothyroidism in pregnancy are mild, may not need treatment and may even be asymptomatic, often mimicking the normal discomforts or minor ailments of pregnancy. For example:

  • Extreme tiredness
  • Sensitivity to cold
  • Muscle cramps
  • Severe constipation
  • Problems with memory or concentration.

(NIDDK 2021)

If severe cases are left untreated, however, then outcomes such as pre-eclampsia, miscarriage or stillbirth become more likely.

Most women with hypothyroidism will have their thyroid hormone levels checked every 4 to 6 weeks for the first 20 weeks of pregnancy, and at least once after 30 weeks gestation to ensure optimal dosage of medication (University Hospitals Sussex NHS Foundation 2021).

Caring for Women with Thyroid Disorders in Pregnancy

endocrinologist checking patient's thyroid

Maintaining thyroid hormone levels within the normal range is essential for a healthy pregnancy, and as thyroid disorders are relatively common in women of childbearing age, midwives need to be aware of the implications that both an under and overactive thyroid can have on the pregnancy. Ideally, this involves interprofessional teamwork, in which the midwife plays a central role, liaising with the patient’s general practitioner, endocrinologist and obstetrician (Hyer 2018).

Most pregnant women recover their regular normal thyroid function soon after delivery, however, as many as 1 in 20 continue to experience persistent irregular thyroid activity, known as postpartum thyroiditis (Pregnancy, Birth and Baby 2021).

Conclusion

Pregnancy can be a challenging time for women with thyroid disease. Without adequate monitoring and intervention when needed, the development of the fetal nervous system can be impaired and the pregnancy put in jeopardy.

This is why, if opportunity allows, thyroid hormone levels should always be checked before conception, and women should be advised about the importance of maintaining normal thyroid hormone levels before getting pregnant.

For many women, these simple screening and treatment measures can go a long way in minimising pregnancy loss and avoiding many of the minor ailments of pregnancy.


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