Congenital hypothyroid syndrome

Definition

Congenital hypothyroid syndrome is a condition in which a child is born with a deficiency in thyroid gland activity or thyroid hormone levels.

Description

The thyroid gland is a small gland in the front of the neck that secretes thyroid hormones called thyroxine (T4) and triiodothyronine (T3) into the bloodstream. Some of the T4 is converted into T3 by the liver and kidney. These thyroid hormones help regulate a great number of processes. A deficiency in the level of these hormones can affect the brain, heart, muscles, skeleton, digestive tract, kidneys, reproductive function, blood cells, other hormone systems, heat production, and energy metabolism.

In most cases of congenital hypothyroidism, the thyroid gland is either completely absent or severely under-developed. Sometimes thyroid tissue is located in ectopic, or abnormal, locations along the neck.

Other abnormalities can lead to congenital hypothyroidism including:

• abnormal synthesis of thyroid hormones;
• abnormal synthesis of thyroid-stimulating hormone (TSH) or thyrotropin-releasing hormone (TRH), which are regulatory hormones that affect the production of thyroid hormones;
• abnormal response to thyroid hormones, TSH or TRH;
• inadvertent administration of harmful drugs or substances to the pregnant mother, possibly resulting in temporary congenital hypothyroidism in the newborn;
• dietary deficiency of iodine, a raw component vital to the manufacture thyroid hormones.

Genetic profile

Most causes of congenital hypothyroidism are not inherited. Some abnormalities in thyroid hormone synthesis (TSH synthesis), or the response to TSH, are inherited in autosomal recessive fashion. This means that both parents have one copy of the changed (mutated) gene but do not have the condition. Abnormal response to thyroid hormone may be an autosomal dominant condition, meaning that only one parent has to pass on the gene mutation in order for the child to be affected with the syndrome.

Demographics

Congenital hypothyroidism occurs in one in every 4,000 newborns in the United States. It is twice as common in girls as in boys. The condition is less common in African Americans and more common in Hispanics and Native Americans.

Signs and symptoms

The signs and symptoms of congenital hypothyroidism are difficult to observe because the mother passes along some of her thyroid hormones to the fetus during pregnancy. Even if the newborn is completely lacking a thyroid gland, it may not be obvious in the early stages of life. Ectopic thyroid tissue may also provide enough thyroid hormones for a short period of time.

Rarely, the affected newborn will exhibit jaundice (yellow skin), noisy breathing, and enlarged tongue. If hypothyroidism continues undetected and untreated, the infant may gradually demonstrate feeding problems, constipation, sluggishness, sleepiness, cool hands and feet, and failure to thrive. Other signs include protruding abdomen, slow pulse, enlarged heart, dry skin, delayed teething, and coarse hair. Affected children may also have myxedema, which is swelling of the face, hands, feet, and genitals. Hypothyroidism eventually leads to marked retardation in physical growth, mental development, and sexual maturation.

Diagnosis

Prompt diagnosis and treatment are critical to avoid the profound consequences of hypothyroidism. The signs and symptoms of hypothyroidism are often subtle in newborns, only to manifest themselves later in life when permanent damage has been done. Before the implementation of screening for hypothyroidism in the 1970s, most children with the disease suffered growth and mental retardation, as well as neurological and psychological deficits.

Most cases of congenital hypothyroid syndrome are now detected by a screening test performed during a newborn's first few days of life. Every state offers testing, and most states require it. The test for hypothyroidism is part of a battery of standard screening tests designed to diagnose important conditions. A sample of the child's blood is analyzed for levels of thyroxine (T4), thyroid-stimulating hormone (TSH), or both, depending on the individual state or country. Some states also require a second round of screening performed one to four weeks later.

Once the diagnosis of congenital hypothyroidism is made, other tests can pinpoint the nature of the abnormality. X rays of the hip, shoulder, or skull often reveal characteristically abnormal patterns of bone development. Scintigraphy is a method by which images of the thyroid gland and any ectopic thyroid tissue are obtained to determine if the thyroid is absent or ectopic. But treatment should not be delayed for these other tests. Early treatment offers a good probability of normal development.

Treatment and management

Treatment of congenital hypothyroidism requires replacement of deficient thyroid hormones with levothyroxine, an oral tablet form of T4. There is no need to directly replace T3, since T4 is converted to T3 by the liver and kidney. Hypothyroid children usually require more levothyroxine per pound of body weight than hypothyroid adults do. The importance of prompt and adequate treatment cannot be overemphasized. Delays in treatment result in permanent stunting of physical, mental, and sexual development.

Blood levels of T4 should be checked regularly to ensure appropriate replacement. The blood levels of TSH should also be monitored since TSH is an indicator of the effectiveness of T4 replacement. As the child develops, the physical growth rate also provides a good measure of treatment.

Prognosis

If congenital hypothyroidism is detected and treated early in life, the prognosis is quite good. Most children will develop normally. However, the most severely affected infants may have mild mental retardation, speech difficulty, hearing deficit, short attention span, or coordination problems.

Resources

BOOKS

"Hypothyroidism." In Nelson Textbook of Pediatrics, edited by Richard E. Behrman, et al. 16th ed. Philadelphia: W.B. Saunders Company, 2000.

"The Thyroid." In Cecil Textbook of Medicine, edited by Lee Goldman, et al. 21st ed. Philadelphia: W.B. Saunders Company, 2000.

"Thyroid Hormone Deficiency." In Williams Textbook of Endocrinology, edited by Jean D. Wilson, et al. 19th ed. Philadelphia: W.B. Saunders Company, 1998.

ORGANIZATIONS

U.S. Preventive Services Task Force, Guidelines from Guide to Clinical Preventive Services. Williams and Wilkins, 1996.

Kevin O. Hwang, MD