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13 The patient with thyroid disease
Shiao‐yng Chan
Department of Obstetrics and Gynaecology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
Case History 1: A 35‐year‐old woman with unexplained infertility is about to undergo IVF treatment. She has been diagnosed with hypothyroidism 2 years ago. She has maintained normal thyroid function test results with levothyroxine 100 μg/day for the past 18 months and has been clinically euthyroid. Her most recent test showed a thyroid stimulating hormone (TSH) level of 3.8 mU/L (normal range: 0.4–4.2 mU/L) and a free thyroxine (T4) level of 12.2 pmol/L (9.5–20 pmol/L).
Case History 2: A 26‐year‐old woman was diagnosed with Graves’ disease 6 months following the delivery of her only child 4 years ago. After 2 years of carbimazole treatment her disease became quiescent but recurred 6 months ago. She is currently on carbimazole 30 mg/day and is clinically euthyroid with a small soft diffuse palpable goiter. She has been with a new partner for the last 2 years and would like to proceed with ICSI for male factor infertility. Thyroid function tests performed within the last month showed a TSH level of 0.03 mU/L (0.4–4.2 mU/L), free T4 of 18 pmol/L (9–20 pmol/L) and free T3 of 6 pmol/L (3.5–6.8 pmol/L).
Background
The management of women with thyroid disease preconceptually and during pregnancy is a clinical area that is advancing rapidly. Recommendations are being refined as new evidence continues to emerge. Several controversies, including those surrounding the universal screening of pregnant women for thyroid disease, thresholds at which treatment should be commenced in newly diagnosed subclinical disease and whether to treat euthyroid thyroid peroxidase (TPO) antibody positive women, remain unresolved even after results have emerged from recently completed clinical trials. Updated clinical guidelines by different professional bodies have been published but the consensus opinions expressed are not identical [1,2]. Thus, there are differences in the specific management strategies adopted by various centers, but the general basic principles are broadly similar. It should also be noted that reference ranges for thyroid function tests vary according to the laboratory assay, ethnicity of the population and stage of pregnancy.
Hypothyroidism
In iodine replete countries, the incidence of hypothyroidism diagnosed before pregnancy is about 1%, the commonest cause being primary thyroidal failure due to autoimmune thyroiditis. In addition, approximately 2.5% of pregnant women have subclinical hypothyroidism, defined by an elevated serum TSH concentration but normal free T4 and free T3 concentrations [3]. This can be indicative of the early stages of thyroid insufficiency or inadequate thyroxine replacement in previously diagnosed overt hypothyroidism. Worldwide iodine deficiency remains the leading cause of hypothyroidism. In affected countries the WHO has recommended routine iodine supplementation to ensure a daily iodine intake of 250 μg during pregnancy and lactation [4].
The risks of untreated overt hypothyroidism in pregnancy are well documented (Table 13.1). Several studies have now also associated subclinical hypothyroidism during the first half of pregnancy with an increased risk of miscarriage [5], preterm delivery [5,6], and neuropsychological deficiencies in the offspring [7]. Maternal thyroid hormones are believed to be crucial for the normal development of the placenta and the fetus, particularly the central nervous system, especially in the first trimester of pregnancy prior to the onset of fetal thyroid hormone production in mid‐trimester [8]. There is good evidence that adequate treatment of overt hypothyroidism and the rapid normalization of thyroid function during pregnancy is associated with good obstetric outcomes [3,5]. However, two large clinical trials that assessed if the screening for and treatment of subclinical hypothyroidism from the first and early second trimesters have shown no treatment benefit in relation to offspring neurodevelopmental outcomes [9,10]. The secondary outcomes of pregnancy complications also showed no significant differences between treatment arms. Some have argued that this is because treatment was commenced too late in pregnancy. The issue of whether to treat subclinical hypothyroidism, if found preconception, remains controversial. In cases of ART, a meta‐analysis of studies suggested that levothyroxine treatment to achieve a TSH less than 2.5 mU/L prior to conception could improve live birth rates [11]. On the other hand, in cases of euthyroidism, simply having TPO antibodies does not warrant levothyroxine treatment preconception or during pregnancy as this does not change pregnancy outcome [12].
Table 13.1 Obstetric complications associated with untreated overt maternal hypothyroidism [21].
| Miscarriage (first and second trimesters) Pregnancy‐induced hypertension Preeclampsia Anemia Postpartum hemorrhage | Preterm birth Low birthweight Stillbirth Perinatal death |
Hyperthyroidism
Over 90% of hyperthyroidism in pregnancy is secondary to Graves’ disease. In this condition, TSH receptor antibodies stimulate the thyroid gland resulting in elevated free T4 and free T3 concentrations in the circulation, which suppress TSH production by the pituitary gland. Uncontrolled maternal thyrotoxicosis is associated with many complications in pregnancy (Table 13.2). The transplacental passage of TSH receptor antibodies may cause fetal or neonatal hyperthyroidism in less than 1% of cases while anti‐thyroid drugs can induce fetal hypothyroidism.
There has been an unproven causal link between carbimazole (CBZ) or methimazole (MMI) and the rare benign scalp condition of aplasia cutis, esophageal atresia, choanal atresia and dysmorphic facial features in the fetus [13]. Associations of CBZ/MMI with other congenital anomalies involving the musculoskeletal, urinary, cardiovascular and respiratory systems have also been described [14]. Similarly, propylthiouracil (PTU) has also been associated with mainly head/neck and urinary anomalies, with overall a slightly lower teratogenic risk than CBZ/MMI. Compared with the unexposed, offspring exposed to either drug had around 1.5 times the risk of having a congenital anomaly [14]. Both drugs show no significant differences in their potential to induce fetal hypothyroidism [15]. The evaluation of children exposed to either drug also showed no difference in neurodevelopmental assessments compared to unexposed siblings [16]. Hence PTU is currently favored over carbimazole/MMI in women planning a pregnancy. Because PTU is associated with the rare side effect of severe liver impairment (1:10,000), some have advocated converting treatment back to carbimazole in the second and third trimesters.
Table 13.2 Complications associated with uncontrolled maternal thyrotoxicosis [22]
| Thyroid storm (first and second trimesters) Maternal congestive cardiac failure Preeclmpsia Placental abruption Preterm delivery | Miscarriage Fetal growth restriction Fetal thyrotoxicosis Fetal hypothyroidism Stillbirth Perinatal death |
Management options
Adequate treatment and control of both hypothyroid and hyperthyroid disease in pregnancy is associated with good obstetric outcome. Because the optimization of thyroid
