How to manage hyperthyroid disease in pregnancy

The authors report no financial relationships relevant to this article.

CASE Life on the line

A 32-year-old woman in the 24th week of her fourth pregnancy arrives at the emergency department complaining of cough and congestion, shortness of breath, and swelling in her face, hands, and feet. The swelling has become worse over the past 2 weeks, and she had several episodes of bloody vomiting the day before her visit. The patient says she has not experienced any leakage of fluid, vaginal bleeding, or contractions. She reports good fetal movement.

The patient’s medical history is unremarkable, but a review of systems reveals a 15-lb weight loss over the past 2 weeks, racing heart, worsening edema and shortness of breath, and diarrhea.

Physical findings include exophthalmia and an enlarged thyroid with a nodule on the right side, as well as bilateral rales, tachycardia, tremor, and increased deep tendon reflexes. There is no evidence of fetal cardiac failure or goiter.

A computed tomography (CT) scan of the mother shows bilateral pleural effusions indicative of high-output cardiac failure. Thyroid ultrasonography (US) reveals a diffusely enlarged thyroid gland with a right-sided mass.

The thyroid-stimulating hormone (TSH) level is undetectable. Fetal heart rate is in the 160s, with normal variability and occasional variable deceleration. Fetal US is consistent with the estimated gestational age and shows adequate amniotic fluid and no gross fetal anomalies.

What is the likely diagnosis?

This is a classic example of undiagnosed hyperthyroidism in pregnancy manifesting as thyroid storm.

As the case illustrates, uncontrolled hyperthyroidism in pregnancy poses a significant challenge for the obstetrician. The condition can cause miscarriage, preterm delivery, intrauterine growth restriction, preeclampsia, and—at its most dangerous—thyroid storm.¹ Thyroid storm is a life-threatening emergency, and treatment must be initiated even before hyperthyroidism is confirmed by thyroid function testing.² The good news is that these complications can be successfully avoided with adequate control of thyroid function.

Overt hyperthyroidism, seen in 0.2% of pregnancies, requires active intervention to avert adverse pregnancy outcome and neurologic damage to the fetus. Subclinical disease, seen in 1.7% of pregnancies, can also create serious obstetrical problems.¹

The effects of hyperthyroidism in pregnancy vary in severity, ranging from the fairly innocuous, transient, and self-limited state called gestational transient thyrotoxicosis to the life-threatening emergency of thyroid storm. This review will update you on how to manage this disorder for optimal pregnancy outcome.

To screen or not to screen

Routine screening for thyroid dysfunction has been recommended for women who have infertility, menstrual disorders, or type 1 diabetes mellitus, and for pregnant women who have signs and symptoms of the disorder. Some authors recommend screening all pregnant women, but routine screening is not endorsed by the American College of Obstetricians and Gynecologists.^2,3

Thyroid testing in pregnancy is recommended in women who:

• have a family history of autoimmune thyroid disease
• are on thyroid therapy
• have a goiter or
• have insulin-dependent diabetes mellitus.

Pregnant women who have a history of high-dose neck radiation, thyroid therapy, postpartum thyroiditis, or an infant born with thyroid disease should also be tested at the first prenatal visit.⁴

Telltale signs and laboratory tests

The signs and symptoms of hyperthyroidism can include nervousness, heat intolerance, tachycardia, palpitations, goiter, weight loss, thyromegaly, exophthalmia, increased appetite, nausea and vomiting, sweating, and tremor.¹ The difficulty here? Many of these symptoms are also seen in pregnant women who have normal thyroid function, so that symptoms alone are not a reliable guide.

Instead, the diagnosis of overt hyperthyroidism is made on the basis of laboratory tests indicating suppressed TSH and elevated levels of free thyroxine (FT₄) and free triiodothyronine (FT₃). Subclinical hyperthyroidism is defined as a suppressed TSH level with normal FT₄ and FT₃ levels.²

The effects of hyperthyroidism on laboratory values are shown in TABLE 1. A form of hyperthyroidism called the T₃– toxicosis syndrome is diagnosed by suppressed TSH, normal FT₄, and elevated FT₃ levels.⁴

TABLE 1

Is your pregnant patient hyperthyroid? Five-test lab panel offers a guide

What are the causes?

The most common cause of hyperthyroidism in pregnancy—accounting for some 95% of cases—is Graves’ disease.² This autoimmune disorder is characterized by autoantibodies that activate the TSH receptor. These autoantibodies cross the placenta and can cause fetal and neonatal thyroid dysfunction even when the mother herself is in a euthyroid condition.⁴