Overt hypothyroidism is seen in about 1% to 2% of pregnant women.8 Subclinical hypothyroidism is seen in another 2.5%.9
Most cases of hypothyroidism during pregnancy have the same cause as in hypothyroidism in general. Pregnancy increases the requirement of thyroid hormone because of the increased rate of metabolism in the mother and the transplacental transport of thyroid hormone, which is essential for the development and maturation of the different organs of the fetus. For women who are being treated for hypothyroidism, the dose of thyroxine should be increased approximately by 30% as soon as the pregnancy is confirmed.10
Thyroid function test results during pregnancy may be difficult to interpret. This is because pregnant patients may have a higher production of thyroid hormone from stimulation of the gland by human chorionic gonadotropin, which has a similar structure to that of thyroid-stimulating hormone. On top of that, increased estrogen during pregnancy results in higher levels of thyroid-binding globulin, which transports thyroid hormone in the blood. Therefore, a normal thyroid hormone level in a pregnant woman may not mean the patient is euthyroid, especially if the patient has symptoms of hypothyroidism. Thyroid hormone replacement may still be required in this case.
Hypothyroidism is diagnosed in pregnancy if patients have symptoms and, in general, have high levels of thyroid-stimulating hormone and low free thyroxine. Subclinical hypothyroidism in pregnancy can be identified if the test results show high levels of thyroid-stimulating hormone and normal free thyroxine. Subclinical hypothyroidism should be treated to ensure healthy pregnancy.
Synthetic thyroxine, which is identical to the thyroxine made by the thyroid gland, is used for pregnant women. It is safe for the fetus. Pregnant women with existing hypothyroidism require an increased dose of thyroxine during pregnancy, and the thyroid function is usually checked every 8 weeks.
Age, the presence of cardiac comorbidities, and a high dose of thyroxine are associated with a poor outcome in myxedema crisis.10 Standard doses of thyroxine, and especially of triiodothyronine, can precipitate cardiac arrhythmias. Start with no more than half the recommended dose of thyroxine or triiodothyronine for elderly patients.
PATIENTS WITH CARDIAC DISEASE
Thyroxine has fewer cardiac effects than triiodothyronine. Thyroxine is the preferred choice for thyroid hormone replacement in patients with heart disease.
THE ASYMPTOMATIC PATIENT WITH A PALPABLE NODULE IDENTIFIED IN THE ED
Solitary thyroid nodules are a common physical finding in the general population. Although most are benign colloid nodules that will disappear over time, a small percentage of solitary nodules are thyroid carcinomas. Biopsy results identify 70% of nodules to be benign, 5% to be malignant, and the remainder to be cytologically indeterminate.11 Therefore, referral for fine-needle aspiration biopsy is indicated for all patients with palpable nodules.
Synthetic levothyroxine is the most widely used agent for thyroid replacement. Deaths from overdose have not been reported. When taken in overdose, symptoms do not occur until 24 hours later as a result of metabolic conversion of thyroxine to triiodothyronine. Treatment is not standardized. For acute ingestion, activated charcoal can be given. Cholestyramine can decrease fecal elimination, and propranolol can control tachycardia and anxiety. Contact your local poison control center for specific treatment recommendations.
Acknowledgment: The author gratefully acknowledges the contributions of Horace K. Liang, the author of this chapter in the previous edition.