Babies who are born with underactive thyroid function have a disorder known as congenital hypothyroidism. The usual cause of this condition is the failure of the thyroid gland to develop during pregnancy. At birth the infants look normal and then slowly over a period of weeks the clinical features of hypothyroidism appear. Because there are no conspicuous signs or symptoms, the diagnosis of congenital hypothyroidism is seldom made at birth by the examining physician. If the condition goes unrecognized, which is usually the case, then signs and symptoms such as constipation, dry skin, hoarse cry, large tongue, swelling around the eyes, failure to suckle well, and prolonged periods of sleep will appear within a few weeks.
Accompanying these overt clinical changes in the infant is the less obvious damage to the brain resulting in mental retardation. Although treatment at this juncture will reverse the clinical signs and symptoms, the damage to the baby’s brain is irreversible. The longer the disorder goes unrecognized (and thus untreated) the greater the insult to the brain.
Fortunately, in recent years the burden of diagnosing congenital hypothyroidism in the newborn period has been shifted from the physician to the laboratory. In the early nineteen-seventies a French-Canadian physician, Jean Dussault, found a way to measure thyroid hormone (T4) in a tiny amount of dried blood in filter paper. The filter paper was the same type that had been used for years to collect blood from newborns before discharge from the hospital. The dried blood specimens were sent to public health laboratories where tests were done for PKU. and other metabolic diseases. Thus, this pre-existing network for collecting newborn blood specimens provided the framework upon which the same public health laboratories could not only screen for metabolic diseases but could screen for congenital hypothyroidism as well. It was not long before most industrialized nations had established newborn screening programs to identify infants born with hypothyroidism. Without question, the screening process has revolutionized the diagnosis and early treatment of congenital hypothyroidism and thereby prevented countless number of children from becoming mentally retarded.
Every child born in North America has blood collected on filter paper by heel stick before discharge from the hospital or birthing center. The dried blood specimens are forwarded to a central laboratory where a one-eighth inch paper blood spot is tested for the amount of T4 and/or thyroid stimulating hormone (TSH). A low T4 and elevated TSH indicate that an infant lacks normal thyroid function. Although an elevated TSH is a more sensitive and specific marker of hypothyroidism, the majority of North American screening programs use T4 as the initial test and confirm the diagnosis with the measurement of TSH.
Approximately one baby with congenital hypothyroidism is born out of 3,500 births. However, it is estimated that 15-20% of hypothyroid infants have a temporary form of the disorder and will only require treatment for a limited number of years. When there is a question of permanence of the disorder, the physician should discontinue treatment after 3 years of age and repeat the blood tests for TSH and T4 in a few weeks to be sure they are now normal.
There are three different types of thyroid abnormalities that are associated with congenital hypothyroidism. Approximately 40% of infants have under-developed or absent thyroid glands; 40% have thyroid glands that are in the wrong place, such as under the tongue or at the far side of the neck. The remaining 20% are unable to manufacture thyroid hormone because of defects within the thyroid gland. The latter condition is usually familial in nature.
Only a tiny number of infants with low T4 levels actually have congenital hypothyroidism. More than 90% of low T4 values (not accompanied by elevated TSH levels) in neonates are associated with other conditions such as prematurity, low birth weight, illness, a deficiency in the protein that carries T4 to various body tissues, or pituitary failure. The latter is a rare disorder in which the pituitary gland is unable to secrete TSH. This condition should be considered in an infant whose growth rate falls off its expected growth curve and has a low T4 and a low or normal TSH.
The infant with suspected hypothyroidism should be seen without delay by a physician, preferably by a pediatric endocrinologist. Blood should be obtained to confirm the diagnosis, and treatment with thyroxine should begin before the confirmatory T4 and TSH values are available. Measurements of T4 and TSH should be made 2 and 4 weeks after starting the thyroxine, 2 weeks after a dosage change, and every 1 to 2 months during the first year of life. Optimal intellectual outcome depends on maintaining the circulating level of T4 in the upper half of the normal range (10-16 µ g/dl) during the first year of life.
If the T4 value is not above 10µ g/dl and the TSH is below 20 mU/l within 2 to 4 weeks after starting therapy, the physician should consider the possibility that the baby has not been receiving the medication or that there is a problem with absorption of T4 (soy-based formula or iron supplement will interfere with absorption of T4).
After recovering from the emotional shock of learning that their baby has congenital hypothyroidism, the first question invariably asked by the parents is, "Will our baby be normal?" Many parents confuse congenital hypothyroidism with cretinism and harbor the vision of their child becoming dwarfed and mentally retarded.
It is gratifying to be able to reassure parents that their children will not be deficient in brain power and that the children are not cretins. Severe iodine deficiency, the cause of cretinism in many parts of the world, has not been a problem in the United States since the introduction of iodized salt in the 1920s. Unfortunately, mild degrees of iodine deficiency are now beginning to reappear in the United States. These changes are described in our articles about iodine elsewhere on this website.
When treatment is initiated before the appearance of clinical signs and symptoms of hypothyroidism, the intellectual outcome of the child is the same as that of children with normal thyroid function. This was shown by a group of New England hypothyroid children whose results in intelligence tests and in school performance were the same as those of their unaffected classmates.
It is prudent to remember that as children grow older they have a tendency to neglect their medication. This is especially true for adolescents. A recent study in teenagers found a significant decline in IQ that returned to normal after the appropriate blood levels of T4 were restored. Consider non-compliance with medication when school performance undergoes a change for the worse in your adolescent child who is on treatment for hypothyroidism.
There are few instances in the practice of medicine where the health and welfare of future generations can be positively affected; early treatment of congenital hypothyroidism through newborn screening is one of those instances.
Your thyroid gland tells every cell in your body the rate at which it should function?