Causes and Symptoms of Congenital Hypothyroidism

The thyroid gland is an organ located in the front of the lower part of the neck, in front of the trachea, and consists of soft tissue shaped like a butterfly.

The thyroid gland forms during the first few weeks of fetal life and begins producing hormones by the end of the third month of pregnancy. The hormones produced by the thyroid gland are called thyroid hormone or thyroxine. Thyroid hormone circulates through the bloodstream to all tissues in the body and is an important hormone responsible for many functions; in particular, it is closely related to the body's metabolism. Especially during infancy, it plays a very important role in the normal growth and development of the brain. It is also involved in the growth of the body and skeletal system as a whole, maintains normal body temperature, and is responsible for functions such as normal distribution of muscle and fat.

The thyroid gland is regulated by an organ called the pituitary gland, which is located at the base of the brain. If thyroid hormone levels in the body are too low, the pituitary gland sends a signal called thyroid-stimulating hormone to stimulate the thyroid gland to produce enough thyroid hormone. Therefore, in patients with congenital hypothyroidism caused by thyroid abnormalities, thyroid hormone levels are low and thyroid-stimulating hormone levels are relatively elevated.

Among the various diseases present at birth, congenital hypothyroidism in particular can cause severe intellectual disability and delayed growth and development if not detected and treated early, so screening is performed between 3 and 7 days after birth. It is currently known to be found in about 1 out of every 4,000 newborns.

Causes

The most common cause of congenital hypothyroidism is improper thyroid formation, accounting for about 85% of cases. The thyroid begins to form at the base of the brain in early pregnancy and then moves downward into the neck as it develops. If a problem occurs during this developmental process, the thyroid may fail to form or may form incompletely. In some cases, some thyroid tissue remains in an abnormal location, such as the root of the tongue, under the tongue, or below the hyoid bone, during the migration process, and produces only a very small amount of thyroid hormone.

In about 10% of patients, the thyroid is present normally, but hypothyroidism occurs because there is a problem with synthesizing thyroid hormone within the thyroid gland. In rare cases, hypothyroidism can also occur due to a disorder of thyroid-stimulating hormone secreted by the pituitary gland.

In these cases, hypothyroidism usually can persist for life. In some cases (about 5%), if the mother has antibodies against the thyroid or has taken antithyroid medication, these substances pass to the fetus through the placenta and cause temporary hypothyroidism; in such cases, the condition may gradually improve.

Congenital hypothyroidism is mostly not a genetic disorder, but if other family members have thyroid disease, the chance of the child being born with the condition may be higher. In particular, when the cause is a deficiency of an enzyme involved in synthesizing thyroid hormone, it may show an autosomal recessive inheritance pattern in which both parents of the patient carry the genetic factor.

Symptoms

The symptoms of congenital hypothyroidism vary depending on the degree and duration of thyroid hormone deficiency. In most cases, birth weight and heart function are normal at birth, and cases showing clinical symptoms from birth are rare, at less than about 5%. Therefore, it is very difficult to diagnose based on clinical symptoms immediately after birth, so it is usually detected through newborn screening tests.

In infancy, jaundice persists for a long time, the baby does not cry much and seems to want to sleep all the time, and must be awakened often for feeding. The baby has little interest in food, does not suck well, and often chokes during feeding. The tongue becomes thick and enlarged, leaving the mouth open all the time, and the tongue may appear to protrude; this can lead to apnea or noisy breathing and may be accompanied by nasal congestion. There may be abdominal distension, umbilical hernia, and constipation that is often difficult to treat. The fontanelle and small fontanelle are widened, the distance between the eyes is wide, and the bridge of the nose appears low and broad. The eyelids, arms and legs, and the area around the genitals may also be swollen. Tooth development is delayed, and the voice may be rough, with a hoarse cry. The pulse is slow, blood pressure is low, circulation is poor, and the hands and feet are cold, with body temperature remaining low. The skin is cool and mottled on the extremities, and is dry and thick. A heart murmur may be heard, and the heart may appear enlarged on X-ray.

If congenital hypothyroidism is not detected and treated early, physical and mental developmental problems become more severe with age. After infancy, growth gradually slows, weight gain is poor, the arms and legs, fingers, and toes become shorter, and head circumference remains normal or becomes large. Muscle tone decreases, so when held, the baby may feel limp, and sitting or standing may be delayed. As neurological development does not proceed normally, muscle strength weakens, tremors and fine motor impairment appear, and gradually learning, behavioral, and language difficulties develop. Puberty may also occur too early or begin late.

So far, I have explained the causes and symptoms of congenital hypothyroidism.

In the next installment, we will look at the diagnosis and treatment of congenital hypothyroidism.

Source: Korea Disease Control and Prevention Agency National Health Information Portal