https://www.health.gov.au/resources/pregnancy-care-guidelines/part-g-targeted-maternal-health-tests/thyroid-dysfunction extracted 6 February 2020
There is currently insufficient evidence to support routine testing for thyroid dysfunction. As there is an association between thyroid dysfunction and adverse pregnancy and fetal outcomes, the focus is on identifying and treating women at high risk of the condition.
Thyroid dysfunction in pregnancy often results from a pre-existing condition but may arise during pregnancy. Thyroid dysfunction involves either over or under activity of the thyroid gland.
- Hyperthyroidism, in which thyroid hormone levels are raised, is most commonly caused by Graves’ disease, an autoimmune disorder (Marx et al 2008) but may also be induced by excessive exposure to iodine (de Benoist et al 2008). Symptoms include weight loss, heat intolerance and hypertension. It is generally diagnosed and treated before conception (Mestman 2004; Marx et al 2008).
- Hypothyroidism is a thyroid hormone deficiency, which may be overt (with symptoms including cold sensitivity, fatigue and dry skin) (De Groot et al 2012), or subclinical with few or no symptoms but abnormal levels of thyroid hormones (Reid et al 2013). It is most commonly caused by endemic iodine deficiency (Lazarus 2011). Autoimmune thyroid disease (eg Hashimoto’s disease) is the most common cause when iodine intake is adequate (Reid et al 2013). Detection of thyroid autoantibodies (to thyroid peroxidase or thyroglobulin) confirms the autoimmune origin of hypothyroidism, or in euthyroid women may indicate increased risk of thyroid dysfunction (Reid et al 2013).
- Thyroid dysfunction is the second most common endocrine condition (after diabetes mellitus) affecting women of reproductive age (Reid et al 2013).
- The incidence of hyperthyroidism in pregnancy is in the range of 0.1–0.4% (De Groot et al 2012).
- Studies in relatively iodine-sufficient populations estimate an incidence of 0.3–0.5% for overt hypothyroidism and 3–5% for subclinical hypothyroidism (De Groot et al 2012). It is likely that incidence would be higher in areas of iodine insufficiency.
- The Australian National Health Survey (ABS 2014) found that, in 2011–2012, iodine levels were relatively low among women of childbearing age. Although women aged 16–44 years had sufficient iodine levels overall, around 18% had iodine levels considered moderately deficient (compared to the national average of 13%) and nearly two thirds (62%) had an iodine level below that recommended by WHO for pregnant and breastfeeding women.
- The WHO Global Database on Iodine Deficiency identifies moderate iodine deficiency in some African countries (Algeria, Chad, Senegal), Afghanistan, Belarus and Vietnam (de Benoist et al 2008). Urinary iodine levels associated with a high risk of iodine-induced hyperthyroidism or autoimmune thyroid disease were identified in Brazil, Chile, Ecuador, Liberia and Uganda.
- Thyroid autoantibodies are present in 5–15% of women of childbearing age (De Groot et al 2012).
46.1.2 Risks associated with thyroid dysfunction in pregnancy
- Overt hypothyroidism and hyperthyroidism are associated with a range of adverse obstetric outcomes (miscarriage, pre-eclampsia, placental abruption, preterm birth and post-partum haemorrhage) and risks to the baby (low birth weight, increased neonatal respiratory distress and decreased cognitive function) (Lazarus 2011; Lazarus et al 2012).
- Studies are now focusing on the potential effect of subclinical thyroid dysfunction and autoimmune disease. A systematic review found that subclinical hypothyroidism in pregnancy is associated with pre-eclampsia (OR 1.7; 95%CI 1.1 to 2.6) and perinatal mortality (OR 2.7; 95%CI 1.6 to 4.7) and the presence of maternal thyroid autoantibodies is associated with miscarriage (OR 3.73; 95%CI 1.8 to 7.6) and preterm birth (OR 1.9; 95%CI 1.1 to 3.5) (van den Boogaard et al 2011). A meta-analysis of cohort studies had similar findings for miscarriage (OR 3.90; 95%CI 2.48 to 6.12) (Thangaratinam et al 2011) and another for preterm birth (RR 1.41; 95%CI 1.08 to 1.84) (He et al 2012).
46.2 Testing for thyroid dysfunction
Routine testing for thyroid dysfunction is not recommended by the RANZCOG (RANZCOG 2015) or in the United States (ACOG 2015) and is not addressed in the United Kingdom antenatal guidelines (NICE updated 2016).
46.2.1 Benefits and harms of testing for thyroid dysfunction
More evidence is needed to assess the benefits or harms of different approaches to testing for thyroid dysfunction in pregnancy on maternal, infant and child health outcomes. A recent Cochrane review (Spencer et al 2015) found that:
- compared to case finding, universal testing increased diagnosis and subsequent treatment of thyroid dysfunction (moderate to high quality) but there were no clear differences in outcomes reported (pre-eclampsia, preterm birth, miscarriage, fetal or neonatal death) (moderate to high quality)
- compared to no testing, universal testing similarly increased diagnosis and subsequent treatment but there was no clear difference in neurosensory disability for the infant as a child (IQ<85 at 3 years) and other outcomes were not reported.
A subsequent RCT reported that the risk of miscarriage (3.1 vs 8.5%, RR 0.36, 95%CI 0.23 to 0.58, p< 0.001) was lower and the risk of caesarean section higher (41.0 vs 33.5%, RR 1.22, 95%CI 1.08 to 1.39, p<0.001) in the testing group than in the control group (low quality evidence) (Ma et al 2016). The difference in risk of preterm birth did not reach significance (p=0.772) (very low quality).
Take a holistic approach
While iodine fortification of bread in Australia means that women will likely enter pregnancy with adequate iodine intake, supplementation (150 micrograms a day) is still recommended during pregnancy and breastfeeding. Women who have recently arrived in Australia may have previous exposure to inadequate or excessive iodine, depending on their country of origin.
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