The most common cause of hypothyroidism in pregnancy in the United States is Hashimoto's Thyroiditis. The body produces antibodies against the thyroid gland rendering it unable to manufacture the hormone. However, the most common cause of hypothyroidism world wide is iodine deficiency. Iodine is essential for the manufacture of the hormone.
Other causes include subacute thyroiditis (viral illness of the thyroid gland), certain drugs (ferrous sulfate, phenytoin, rifampin), pituitary or hypothalamic disease, or prior treatment with radioactive iodine to treat Graves Disease.
A blood test is used to detect hypothyroidism. Diagnosis in pregnancy is made by an elevated TSH (thyroid stimulating hormone which is made by the pituitary) and a corresponding low thyroid hormone level (T4).
One can also measure antibody levels in the thyroid hormones (antithyroglobulin, antithyroid peroxidase). Measurement of antibody levels is important because women who have antibodies are at increased risk of pregnancy complications and also increased risk of postpartum thyroid dysfunction. Women can have a goiter or large swelling in the neck area.
Having one autoimmune disease increases the chance of developing another. Women with type I diabetes have a 5 to 8% chance of developing hypothyroidism during pregnancy and a 25% chance of developing postpartum thyroid disease.
The treatment of this disease is to replace the thyroid hormone with levothyroxine till the TSH levels are normal. Generally levels are followed each trimester of the pregnancy since the demands of pregnancy may necessitate an increase in dosage. If the cause of the hypothyroidism is due to iodine deficiency, iodine supplementation is essential not only during pregnancy but also after birth.
This is a subgroup of thyroid impairment found in 2–5% of pregnant women. Generally the T4 is normal but the TSH is elevated. Subclinical hypothyroidism has been linked to faulty placental development. There is a three fold increased risk of abruption, higher miscarriage rate, and a two fold increase in the incidence of preterm birth leading to impaired neurodevelopment in the child.
There is currently a great debate on whether women who present with this condition should be treated since studies have not shown a benefit with replacement. These patients should be followed after delivery because of the increased incidence of developing overt thyroid disease postpartum.
Pregnancy has a beneficial effect on women with preexisting thyroid disease. Due to the suppression of the immune system, the antibodies found in Hashimoto's disease decline but, immediately postpartum, there can be a resurgence with marked worsening of the condition. There can be a noticeable reduction in goiter size during the pregnancy.
Congenital hypothyroidism occurs in one in 4,000 births. There can be multiple etiologies from genetic, immunologic, environmental, and drug induced causes. It is critically important not to miss this diagnosis in the infant since developmental retardation can occur if the condition goes untreated.