How to manage hypothyroid disease in pregnancy

TABLE 2

Laboratory diagnosis of hypothyroidism

What causes hypothyroidism?

The most common cause of hypothyroidism in most of the world is iodine deficiency. In developed countries, however, where lack of iodine in the diet is not a problem, Hashimoto’s thyroiditis, also known as chronic autoimmune thyroiditis, is the most common cause. Hashimoto’s thyroiditis is characterized by the presence of antithyroid antibodies, including both thyroid antimicrosomial and antithyroglobulin antibodies. Both iodine deficiency and Hashimoto’s thyroiditis are associated with goiter.⁵ Other causes of hypothyroidism include radioactive iodine therapy for Graves’ disease, a condition we will discuss in Part 2 of this series in February; thyroidectomy; viral thyroiditis; pituitary tumors; Sheehan’s syndrome; and a number of medications.

Causes of hypothyroidism are summarized in TABLE 3.³

TABLE 3

Causes of hypothyroidism

Effects vary by medication

Medications alter thyroid function in different ways. Iodine and lithium inhibit thyroid function and, along with dopamine antagonists, increase TSH levels. Conversely, thioamides, glucocorticoids, dopamine agonists, and somatostatins decrease TSH levels. Finally, ferrous sulfate, sucrafate, cholestyramine, and aluminum hydroxide antacids all inhibit gastrointestinal absorption of thyroid hormone and therefore should not be taken within 4 hours of thyroid medication.⁶

Maternal hypothyroidism: Effects on fetus, newborn

The impact of maternal hypothyroidism on the fetus depends on the severity of the condition.

• Uncontrolled hypothyroidism. The consequences of this condition can be dire. The possibilities include intrauterine fetal demise and stillbirth, preterm delivery, low birth weight, preeclampsia, and developmental anomalies including reduced intelligence quotient (IQ).^1,2,4,6 Blazer and colleagues correlated intrauterine growth with maternal TSH and fetal FT₄ and concluded that impaired intrauterine growth is related to abnormal thyroid function and might reflect an insufficient level of hormone production by hypothyroid mothers during pregnancy.⁹ Maternal and congenital hypothyroidism resulting from severe iodine deficiency are associated with profound neurologic impairment and mental retardation.^1,3,10 If the condition is left untreated, cretinism can occur. Congenital cretinism is associated with growth failure, mental retardation, and other neuropsychologic deficits including deaf-mutism.^3,4 However, if cretinism is identified and treated in the first 3 months of life, near-normal growth and intelligence can be expected.⁶ For this reason, all 50 states and the District of Columbia require newborn screening for congenital hypothyroidism.⁶
• Asymptomatic overt hypothyroidism. Several studies have evaluated neonatal outcomes in pregnancy complicated by asymptomatic overt hypothyroidism—that is, women who had previously been diagnosed with hypothyroidism, who have abnormal TSH and FT₄ levels, but who do not have symptoms. Pop and colleagues have shown impaired psychomotor development at 10 months in infants born to mothers who had low T₄ during the first 12 weeks of gestation.⁷ Haddow and colleagues correlated elevated maternal TSH levels at less than 17 weeks’ gestation with low IQ scores in the offspring at 7 to 9 years of age.⁸ Klein and colleagues demonstrated an inverse correlation between a woman’s TSH level during pregnancy and the IQ of her offspring.¹¹ Kooistra and colleagues confirmed that maternal hypothyroxinemia is a risk for neurodevelopmental abnormalities that can be identified as early as 3 weeks of age.¹² Studies of this relationship are summarized in TABLE 4.
• Subclinical hypothyroidism. During the past decade, researchers have focused attention on neonatal neurologic function in infants born to mothers who had subclinical disease. Mitchell and Klein evaluated the prevalence of subclinical hypothyroidism at less than 17 weeks’ gestation and subsequently compared the IQs in these children with those of controls.⁴ They found the mean and standard-deviation IQs of the children in the control and treated groups to be significantly higher than those of the children whose mothers were not treated. Casey and colleagues evaluated pregnancy outcomes in women who had undiagnosed subclinical hypothyroidism.¹⁰ They found that such pregnancies were more likely to be complicated by placental abruption and preterm birth, and speculated that the reduced IQ demonstrated in the Mitchell and Klein study might have been related to the effects of prematurity.

TABLE 4

Fetal and neonatal effects of asymptomatic overt hypothyroidism