What you need to know about thyroid disorders in pregnancy

Overt hypothyroidism is primarily diagnosed with laboratory tests—specifically, low free thyroxine (FT₄) or free triiodothyronine (FT₃), or both, resulting in elevated TSH levels.

If untreated, overt hypothyroidism is associated with significant morbidity in both the nonpregnant and pregnant states (TABLE 3). Levothyroxine is easily administered and well tolerated, with no to few adverse effects with appropriate follow-up.¹⁰

In women with subclinical hypothyroidism, only 1 of the thyroid function tests is elevated—either elevated TSH with normal free thyroid hormone levels (mild thyroid failure) or normal TSH with low FT₄ levels (hypothyroxemia). Most cases of mild thyroid failure are thought to be related to thyroid dysfunction, whereas hypothyroxemia is usually associated with a deficiency of iodine.

Subclinical hypothyroidism can occur in women with a history of thyroid disease, after surgery or radioactive iodine therapy for toxic goiter, or as the result of an inadequate dosage of thyroid medication. It can also occur in women with no history of thyroid dysfunction, detected in routine testing in women with no symptoms or with nonspecific complaints that could be related to thyroid disease. Experts agree that women with secondary subclinical disease should be treated to achieve a euthyroid state because approximately 5% per year will develop overt disease. Considerable controversy clouds management of women with primary subclinical hypothyroidism.

Subclinical hypothyroidism is more common among white women (~67%) than among black women.

TABLE 2

Know these signs and symptoms of thyroid dysfunction

TABLE 3

Consequences of untreated thyroid dysfunction are significant

Subclinical hyperthyroidism is more elusive

Overt hyperthyroidism can be detected through symptom-based screening (TABLE 2).

Subclinical hyperthyroidism is defined as low TSH with normal thyroid hormone levels. The pituitary appears to be more sensitive to the presence of thyroid hormones than to their absence. Subclinical hyperthyroidism is most common in black women and smokers. Approximately 50% of women with subclinical disease will have normal TSH levels several weeks to 1 year later.

Because subclinical hyperthyroidism can occur in up to 20% of women on thyroid replacement therapy, the dosage should be adjusted to achieve a euthyroid state.

CASE 2 Diabetes, with a family history of hypothyroidism

M.H., 30, is 12 weeks’ pregnant with her second child and reports a 12-year history of diabetes. Before she became pregnant, she was taking insulin, with HbA_1C=8.5%. Her history includes a mother with hypothyroidism.

How should she be managed?

Besides the obvious need for good diabetes control, this case merits screening for thyroid dysfunction, as the patient has 2 risk factors (TABLE 4).

One prominent controversy of the 21st century is whether all pregnant women should undergo routine screening for hypothyroidism. The controversy extends to screening all women of childbearing age.

TABLE 4

Risk factors for hypothyroidism include other autoimmune disorders

$64,000 question: Should all women be screened?

Screening would involve testing thyroid function in women with no history and few or no signs and symptoms of thyroid dysfunction. Such screening could be population-based (using special methods to recruit, contact, and follow patients) or case-finding (performed on patients who present for unrelated reasons). The decision to screen a woman who is pregnant or planning to conceive should be based on many factors, most notably whether treatment prevents impaired neonatal neurodevelopment and preterm delivery.

A mother’s elevated TSH level can have lasting effects in the child

Haddow and colleagues¹ measured the IQ of 47 children, ages 7 to 9 years, whose mothers had had an elevated serum TSH concentration in the second trimester, 15 children whose mothers had high serum TSH values in combination with low thyroxine levels in the second trimester, and 124 children whose mothers had normal TSH values. None of these children had hypothyroidism at birth. The children of the women with an elevated TSH concentration had lower IQs. Interestingly, the group with hypothyroxemia was not evaluated at the time, and the mean FT₄ level was low in the entire group, suggesting overt hypothyroidism rather than subclinical disease.