Hypothyroidism, a condition in which the thyroid gland fails to produce enough thyroid hormones, is a common issue among women. In fact, one in eight women will experience a thyroid disorder at some point in their lives.
Pregnancy often triggers the diagnosis of hypothyroidism in women. If not properly managed, an underactive thyroid in a pregnant woman can adversely affect the developing baby. This raises some important questions: Can a mother’s hypothyroidism be “passed down” to her newborn? What are the risk factors for hypothyroidism at birth, also known as congenital hypothyroidism? We’ll address these questions here.
Maternal Hypothyroidism
The health of a pregnant woman's thyroid is crucial for the fetus’s development. During the first 18 to 20 weeks of pregnancy, the fetus relies on the mother’s thyroxine (T4) — a storage thyroid hormone — for brain development. After this period, the fetus begins to produce its own thyroid hormones but still depends on the mother’s iodine intake. Without adequate dietary iodine, the fetus cannot produce enough thyroid hormones.
Maternal hypothyroidism can occur during pregnancy or in women with pre-existing hypothyroidism. In the U.S., it is often associated with Hashimoto’s thyroiditis, an autoimmune disorder affecting the thyroid.
Complications of Untreated Hypothyroidism During Pregnancy
Failure to diagnose or properly treat hypothyroidism during pregnancy can lead to several complications, including:
Preeclampsia
Pre-term birth
Low birth weight
Newborn respiratory distress
Muscle pain
Placental abnormalities
Increased risk of miscarriage
Excessive postpartum bleeding
The severity, onset, and duration of maternal hypothyroidism can impact fetal brain development. Even mild hypothyroidism can affect fetal brain growth and may result from:
Mild iodine deficiency
Hashimoto’s thyroiditis
Insufficient thyroid medication
Treatment for Maternal Hypothyroidism
The primary treatment for maternal hypothyroidism involves thyroid hormone replacement medication, which helps normalize thyroid hormone levels. Pregnant women with hypothyroidism typically require more frequent thyroid function tests, as their medication dosages may need adjustment due to increased thyroid hormone demands during pregnancy.
Prenatal vitamins are essential during pregnancy, but if you are taking thyroid medication like levothyroxine, it’s important to take your prenatal supplements and thyroid medication separately. Minerals in prenatal vitamins, such as iron and calcium, can interfere with thyroid medication absorption, leading to lower thyroid hormone levels.
Iodine Requirements
Iodine is a crucial trace element needed for thyroid hormone production. Thyroid-stimulating hormone (TSH) from the pituitary gland increases iodine uptake by the thyroid, which is essential for producing thyroid hormones T4 and T3. During pregnancy and lactation, iodine requirements increase to 220 mcg and 290 mcg, respectively. Since the body cannot produce iodine, it must be obtained from dietary sources or supplements. Starting iodine supplements before or early in pregnancy may increase the amount of maternal T4 available for the fetus.
Can Hypothyroidism Be Passed to the Baby?
Not necessarily.
If a pregnant woman has low thyroid hormone levels, the baby might be born with symptoms related to maternal hypothyroidism, such as developmental issues. However, this does not mean the baby has hypothyroidism. There could be a genetic link to hypothyroidism, but this is not fully understood.
Hypothyroidism often has a genetic component. Many people with thyroid conditions report a family history of similar disorders. While a family history of thyroid issues may increase the risk, it does not guarantee that the baby will have hypothyroidism.
Congenital Hypothyroidism
Congenital hypothyroidism, which is rare, occurs in about 1 in 2,500 to 3,000 newborns. This condition arises when the thyroid gland does not develop properly during pregnancy, resulting in no thyroid gland, an underdeveloped gland, an incorrectly located gland, or a fully developed gland that cannot produce normal amounts of thyroid hormone. It is usually due to a genetic problem, although it is not typically inherited through families. If untreated, congenital hypothyroidism can cause severe issues like mental disabilities, growth delays, or hearing loss. Early diagnosis and treatment are critical.
Risk Factors for Neonatal Hypothyroidism
Certain factors can increase the risk of hypothyroidism in newborns, including:
Maternal iodine deficiency during pregnancy
Medications taken during pregnancy, which may cause temporary neonatal hypothyroidism
Causes of Congenital Hypothyroidism
The two most common causes of congenital hypothyroidism are:
·Thyroid Dysgenesis (TD): This accounts for about 65% of congenital hypothyroidism cases. TD involves either an absent or severely underdeveloped thyroid gland. Approximately 5 to 10% of TD cases are due to genetic mutations affecting thyroid development, following a dominant inheritance pattern.
· Thyroid Dyshormonogenesis (DH): This makes up about 35% of cases. In DH, the thyroid gland, which may be of normal or enlarged size, cannot produce and secrete thyroid hormones properly due to defects in iodine utilization. DH is caused by autosomal recessive genetic mutations, requiring two copies of an abnormal gene (one from each parent).
Are Premature Babies at Higher Risk?
Premature infants may have an immature hypothalamus-pituitary-thyroid (HPT) axis, which can lead to low thyroid hormone levels shortly after birth. These low levels might be temporary, but if high TSH levels persist, treatment with thyroid medication may be necessary. The exact need for thyroid hormone medication in premature infants is still uncertain.
Signs and Symptoms of Congenital Hypothyroidism
Congenital hypothyroidism may not be immediately apparent, so many countries, including the U.S., screen newborns for this condition. Symptoms can include:
Jaundice
Poor muscle tone
Feeding difficulties
Slow or poor growth
Puffy face
Swelling around the eyes
Enlarged fontanelles (“soft spots”)
Excessive sleeping
Large, swollen tongue
Constipation
Cool, pale skin
Protruding belly button
Diagnosis and Treatment
Since the 1970s, neonatal screening programs have significantly improved outcomes for babies with congenital hypothyroidism. Diagnosis involves a heel prick blood test to measure T4 and TSH levels. Low T4 and high TSH levels indicate congenital hypothyroidism. A confirmatory blood test from a vein may be ordered before treatment begins.
Treatment for congenital hypothyroidism is similar to that for adults, involving daily thyroid hormone replacement medication to normalize hormone levels and support appropriate growth and development. Regular checkups with a pediatric endocrinologist are necessary as thyroid hormone requirements change with age.
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