78	M. D. Katz

Discussion

Interpretation of Thyroid Function Tests (TFTs)

Women like Alicia are commonly encountered by medical and reproductive endocrinologists, and obstetricians and gynecologists. Their TFTs are within the acceptable normal range for the general nonpregnant population. However, since the fetal thyroid does not manufacture thyroid hormones, thyroxine (T4), and triiodothyronine (T3), until 18–20 weeks gestation, women contemplating pregnancy who have possible thyroid dysfunction should be viewed in a different context. The maternal contribution to bioavailable thyroid hormone is paramount to ensure that fetal neurodevelopment proceeds properly. Although the fetal thyroid gland makes increasingly larger amounts of thyroid hormone from the second trimester on, studies show that the mother’s contribution is essential until delivery.

Due to physiologic changes that accompany pregnancy, it is imperative to use population and trimester-speci c normal ranges when evaluating TFTs in a pregnant woman. The rst trimester reference range is appropriate for women planning to conceive and those diagnosed with infertility. Many laboratories publish their own trimester-speci c normal values. In the absence of these, the 2017 American Thyroid Association (ATA) guidelines for pregnancy suggested a normal range of TSH at 0.1–0.4 mIU/L for weeks 7–12 [1]. In general, measuring TSH, free T4, and/or total T4 usually suf ces. Total T4 values can increase by up to 50% in pregnancy. This is due to elevated estrogen levels which induce increased hepatic production of thyroid-binding globulin (TBG). Thus, more thyroid hormone is bound to TBG and inactive, necessitating increased production by the thyroid gland in order to meet the demands of pregnancy. Furthermore, human chorionic gonadotropin (hCG) stimulates thyrocytes, as there is signi - cant homology between the beta subunits of hCG and TSH. This is responsible for the transient decrease in TSH which may be observed in early pregnancy corresponding to the highest hCG levels (Fig. 12.1). In multiple gestations, where the hCG is most elevated, the TSH can be suppressed [2].

The majority of pregnant women with previously diagnosed hypothyroidism therefore require increased doses of thyroid hormone as early as 4 weeks gestation. Ideally, hypothyroid women should have a TSH <2.5 mIU/L prior to conception. Although T3 has a role in the treatment of hypothyroidism, it is not appropriate for pregnancy, and all pregnant women should receive adequate doses of levothyroxine (LT4). Women who conceive via assisted reproductive technology (ART) often require earlier and greater augmentation of their dose due to the increased demands ART places on the hypothalamic–pituitary–thyroid axis.

12  Thyroid and Reproduction		79
		TSH and b-hCG
1.5		50

1.0

(mIU/L)TSH 0.5

0	10	20				30										40

WEEKS OF GESTATION

Fig. 12.1  TSH and β-hCG (Glinoer et al. JCEM 71:276, 1990)

	40	(I.U/Lx1000)
	30
	20	hCG
	10
	0

Iodine Supplementation for Pregnancy and Lactation

Dietary iodine requirements likewise increase in pregnancy, as iodine is the substrate for thyroid hormone. The increase in glomerular ltration rate (GFR) associated with pregnancy leads to increased iodine excretion. The consequences of iodine de ciency in pregnancy include fetal hypothyroidism, goiter, and impaired neurological development. Therefore, the World Health Organization (WHO) recommends 250 μg daily iodine supplementation for pregnancy and lactation. Most American women have iodine incorporated into their diet and thus the ATA recommends a daily supplement of 150 μg iodine for pregnancy and lactation, commonly found in prenatal vitamins in the United States.

A recent meta-analysis evaluated maternal iodine status and child intelligence quotient (IQ) at 1.5–8 years in Dutch, Spanish, and British mother–child pairs. A positive association between maternal urinary iodine/creatinine ratio and mean verbal IQ was seen only up to 14 weeks gestation. This underlines the crucial role of iodine for neurological development in the rst trimester.

Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/

80	M. D. Katz

Table 12.1  Indications for preconception or early pregnancy screening for hypothyroidism

Signs and symptoms of hypothyroidism

Residence in an iodine insuf cient region

Personal or family history of hypothyroidism or thyroid autoimmunity

Infertility

Recurrent miscarriage

History of preterm birth

Type 1 diabetes mellitus or other autoimmune disorder

Goiter

History of head or neck radiation treatment

History of thyroid surgery

History of lithium or amiodarone usage

Screening for Maternal Hypothyroidism

It has been well established that overt maternal hypothyroidism can cause oligoor amenorrhea. Overt hypothyroidism (TSH above normal range and free T4 below normal range) complicates up to 5% of pregnancies and can also be associated with adverse outcomes including pregnancy loss, preterm birth, low birth weight, and neurodevelopmental consequences such as low IQ in the offspring. However, there is a lack of consensus regarding screening asymptomatic women for hypothyroidism as intervention trials have not unanimously shown bene t. Therefore, universal screening of asymptomatic women either prior to or during pregnancy remains controversial and is not recommended by the American College of Obstetricians and Gynecologists (ACOG) or the Endocrine Society. Risk factors listed below warrant screening for hypothyroidism. Pre-conception or early pregnancy screening decisions should be made on an individual basis (Table 12.1).

Maternal Subclinical Hypothyroidism

Several randomized studies have evaluated whether screening and treatment of asymptomatic pregnant women with subclinical hypothyroidism (elevated TSH, normal free T4), as in Alicia’s case would be bene cial. Subclinical hypothyroidism occurs in up to 10% of pregnancies and similar to hypothyroxinemia (normal TSH, low free T4), can be associated with miscarriage, preterm delivery and diminished cognitive ability in the offspring [3]. Among the larger and more recent studies, a predominantly Welsh cohort of pregnant women with a mean gestational age of 12 weeks 3 days was screened for subclinical maternal hypothyroidism and hypothyroxinemia [4]. Cognitive development, as measured by IQ at 3 and 9 years did not vary between the 390 children whose mothers were treated with LT4 compared

12  Thyroid and Reproduction

81

with the 404 children whose mothers did not receive treatment, at a median gestational age of 13 weeks, 3 days. Likewise, an American study of 339 pregnant women with subclinical hypothyroidism and 263 pregnant women with hypothyroxinemia, in which LT4 was initiated at a median gestational age of 16 weeks 4 days and 18 weeks, respectively, failed to show any difference in pregnancy outcome or IQ at 5 years [2]. A confounding factor in both these studies may be that the intervention occurred too late.

Thyroid Autoimmunity

In the United States and other iodine replete regions, the main etiology of hypothyroidism is thyroid autoimmunity (Hashimoto’s thyroiditis), in contrast to iodine de ciency, which is the prevailing cause in developing areas. The thyroid antibodies involved in Hashimoto’s thyroiditis are thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TGAb). Most reproductive studies have focused on the former. Women of reproductive age with overt hypothyroidism should be treated with thyroid hormone. Euthyroid pregnant women with TPOAb are at greater risk for adverse pregnancy outcome, speci cally miscarriage and pre-term birth, than TPOAb negative women; however, treatment is controversial. TPOAb-positive women comprise up to 18% of all pregnant women and may lack adequate thyroid hormone reserve, rendering them incapable of appropriately increasing production of thyroid hormone once they conceive. Studies have demonstrated that the risk of miscarriage is doubled in TPOAb-positive women [5]. Therefore, the ATA recommends that all pregnant women with a TSH >2.5 mIU/L, be evaluated for TPOAb. Although insuf cient evidence exists for the ATA to either recommend or not recommend LT4 for euthyroid TPOAb-positive pregnant women, their guidelines do note that the risk of adverse pregnancy outcomes, especially pregnancy loss and preterm birth, are increased with a TSH >2.5 mIU/L and TPOAb positivity. Monitoring for the development of hypothyroidism as often as monthly was recommended.

A recently published study from the United Kingdom investigated whether administering LT4 to euthyroid TPOAb-positive women with a history of recurrent pregnancy loss or infertility, would result in a higher live birth rate [6]. A total of 952 pregnant women were randomized to the treatment and placebo groups. LT4 was not shown to improve live birth rate; however, the ATA guidelines state that use of LT4 in euthyroid TPOAb-positive women with a prior history of pregnancy loss is reasonable. A metaanalysis, which looked at subclinical hypothyroidism, hypothyroxinemia, and TPOAb positivity, showed that all were signi cantly associated with an increased risk of preterm birth [7] (Fig. 12.2). The ATA recommends treatment with LT4 titrated to a TSH <2.5 for women with subclinical hypothyroidism who are undergoing intracytoplasmic sperm injection (ICSI) or in vitro fertilization (IVF).

Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/