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

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 wellbeing. It can even extend beyond pregnancy and affect neuro-intellectual development in the infant's early life (Singh & Sandhu 2023).

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

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

(Hyer 2018)

If these levels are abnormal, then thyroid function is classified as either thyroid deficiency (overt hypothyroidism) or thyroid hormone excess (overt hyperthyroidism) (Hyer 2018).

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 occurs in about 2 to 3% of pregnancies (Pregnancy, Birth and Baby 2023). 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 2017).

During the first trimester, the fetus completely depends on the maternal supply of thyroid hormone via the placenta. It’s not until the 10th to 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 the third trimester (Pregnancy, Birth and Baby 2023).

Making a Diagnosis

Enquiring about the birthing parent’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 people with normal thyroid function (NIDDK 2017).

Typically, people 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 people at 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 2023)

Those who are overweight or obese or who are over 30 years of age are also at greater risk (Pregnancy, Birth and Baby 2023).

Smith et al. (2017) additionally recommend thyroid function screening for those who come from an area with moderate to severe iodine insufficiency, or who have previously had radiotherapy to the head and neck.

Hyperthyroidism In Pregnancy

Hyperthyroidism in pregnancy is usually caused by Graves’ disease, an autoimmune disorder in which the person’s immune system makes antibodies that cause the thyroid to produce an excess of thyroid hormone (NIDDK 2017).

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).

(Singh & Sandhu 2023; NIDDK 2017)

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.

(NIDDK 2017)

Hypothyroidism in Pregnancy

For some people, a lack of iodine causes 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 Organisation recommends a daily intake of at least 250 micrograms of iodine during pregnancy and breastfeeding (Pregnancy, Birth and Baby 202e).

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 (NIDDK 2017).

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 2017)

If severe cases are left untreated, however, then outcomes such as pre-eclampsia, miscarriage or stillbirth become more likely (Singh & Sandhu 2023).

Most patients 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 around 30 weeks gestation to ensure optimal dosage of medication (Singh & Sandhu 2023).

Caring for People with Thyroid Disorders in Pregnancy

Maintaining thyroid hormone levels within the normal range is essential for a healthy pregnancy, and as thyroid disorders are relatively common in females of childbearing age, midwives need to be aware of the implications that both an underactive 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 people recover their regular 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 2023).

Conclusion

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

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

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