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Hypothyroidism in Infants and Children

What Causes Hypothyroidism in Infants and Children?

Sunday, April 21, 2013 - 17:57

Contributing Author: Guy Slowik FRCS

The vast majority of cases of childhood hypothyroidism (95% or more) are caused by a problem within the thyroid gland or by lack of a thyroid gland. A much smaller number of cases (less than 5%) are caused by a problem in the brain or pituitary gland, the small gland at the base of the brain that releases the hormone that stimulates the thyroid gland to produce thyroid hormone.

Specific causes of hypothyroidism vary for different age groups.

Causes Of Hypothyroidism In Newborns

Hypothyroidism present from birth is called congenital hypothyroidism (congenital means at birth), or CH. In North America, CH is found in roughly 1 in every 4,000 newborns. Three causes of CH result in permanent hypothyroidism and require life-long treatment, and these account for about 90% of all newborns with CH:

  • Abnormal thyroid gland development includes babies born without a thyroid gland and those whose thyroid is not functioning (roughly 80% to 85% of cases).
  • Abnormal thyroid hormone production is much less common (roughly 10% to 15% of cases). It is often inherited. In the typical instance, both parents have normal thyroid function but are carriers for the defective gene. In such cases, the odds are one in four for each pregnancy that the baby will have CH.
  • Abnormal development of the brain or pituitary gland is the least common cause of permanent CH (fewer than 5% of cases). Some of these rare conditions are inherited, whereas others show no familial pattern. In all of these cases, TSH is not produced or released, and the resultant hypothyroidism is due to lack of thyroid gland stimulation.

About 10% of newborns with CH have a temporary hypothyroidism that will resolve within days or months, depending on the specific cause. In almost all of these cases, the hypothyroidism is due to antithyroid substances that crossed from the mother's bloodstream into the unborn baby's blood.

In rare cases, if the mother has Graves' disease, an autoimmune disorder affecting the thyroid gland in which too much thyroid hormone is produced, a thyroid-blocking antibody may cross from the mother's bloodstream into the baby's blood. This causes the baby's thyroid gland to temporarily stop producing thyroid hormone. More commonly, the antithyroid drug being taken by the mother for the treatment of her hyperthyroidism, affects the normal functioning of the baby's thyroid gland.

Causes Of Hypothyroidism That Develops During Childhood

Hypothyroidism that develops during childhood has a number of causes. In general, the older the child, the more likely it is that the cause will be similar to the causes of hypothyroidism in adults.

Causes of hypothyroidism that develops during childhood include:

  • Late appearance of a congenital problem - A congenital problem is a problem with which a baby is born. But sometimes these problems may only become apparent later, after the newborn period. These children may have small or poorly formed thyroid glands that could not meet the demands of the growing child.
  • Inhibition of thyroid hormone production in the thyroid gland -Inability to produce enough thyroid hormone may reflect poor function of an apparently normal thyroid gland. In many cases, a cause is eventually determined, such as too little iodine in the diet or drug side effect. Fortunately, iodine deficiency does not exist in North America, but it continues to be a major public health problem in other, less developed areas of the world.

Some drugs intended for a nonthyroid condition can cause hypothyroidism by inhibiting production of thyroid hormone. These include lithium (used for psychiatric disorders) and iodine-containing drugs such as amiodarone (used for heart disorders).

In older children especially, antithyroid drugs used to treat those who have hyperthyroidism can actually cause hypothyroidism. This happens when the drugs inhibit hormone production too much, and the child moves from having an overactive thyroid gland to having an underactive one.

In virtually all cases, drug-related hypothyroidism is reversible. If the dosage of the drug is lowered or the drug discontinued, the hypothyroidism will disappear. In some cases, however, there may not be a reasonable alternative drug, and it becomes simpler to treat the hypothyroidism with thyroid hormone and having the child remain on the original, hypothyroidism-causing drug.

  • Permanent thyroid cell loss as a consequence of a medical treatment - A permanent loss of functional thyroid tissue may develop after treatment of hyperthyroidism with either radioactive iodine or surgery (the latter is more commonly used in children with severe or unresponsive hyperthyroidism). It may also develop after radiation therapy for cancers of the neck or chest, such as lymphomas or Hodgkin's disease.
  • Autoimmune disease - Autoimmune disorders are the most common cause of thyroid problems.

The immune system is a complex network that normally defends the body against "invading" organisms and other foreign substances. When the immune system detects a foreign substance in the body, it responds to this by producing antibodies against the invaders. These antibodies will recognize and attack this foreign substance when they next encounter

An immune disorder is one in which the immune system mistakenly directs an immune "attack" against its own healthy cells. Antibodies are manufactured and misdirected against some of the body's own cells. Any condition in which the body's immune system attacks its own cells is called an autoimmune disease.

Specifically, the immune system makes antibodies (or attack proteins) that can affect the function of the thyroid. In Hashimoto's thyroiditis, the antibodies directly attack and destroy thyroid cells.

In Graves' disease, these antibodies mimic the action of TSH on thyroid cells. The antibodies act like a switch put into a permanent "on" position. Thyroid cells are continually stimulated to produce and release thyroid hormone, even after blood levels become excessive.

Need To Know:

Parents should check for a family history of autoimmune disorders and thyroid disease, especially Graves' disease or Hashimoto's thyroiditis, and tell their child's endocrinologist at the first visit. It is also critically important for mothers to tell their child's doctor about any history of thyroid trouble in themselves, especially Graves' disease or Hashomoto's thyroiditis.

  • Problems outside the thyroid gland - Hypothyroidism can develop due to a problem in a nonthyroid part of the endocrine system (namely the brain or pituitary gland). This type of hypothyroidism, which does not originate in the thyroid gland, is much less common (perhaps 5% of cases). Almost all of these cases are due to failure of the pituitary gland to produce or release TSH.

For more about underlying causes of low thyroid hormone levels, see What Causes Hypothyroidism?

 

Hypothyroidism in Infants and Children