Hypothyroidism

Hypothyroidism is a term used when someone has a lower than normal level of thyroid hormone in their body, or an underactive thyroid gland. Hypothyroidism can develop at any point in the lifespan. Infants can be born with hypothyroidism, and hypothyroidism can develop in children and adults of any age.

Thyroid hormone is produced in the thyroid gland, which is a small, butterfly-shaped gland that lies just under the Adam's apple in the neck. There are two lobes to the gland, and they lie just in front and at either side of the windpipe (trachea). The thyroid is part of the body's endocrine system, which consists of glands that secrete hormones into the bloodstream. To produce thyroid hormones, the thyroid gland needs iodine, an element contained in many foods. Almost every cell in the body, from those in the brain to those in the feet, responds to these thyroid hormones.

The thyroid gland also produces a hormone, called calcitonin, which may be involved in the metabolism of bones.

There are two different forms of thyroid hormone present in the bloodstream. The two forms of thyroid hormone differ in the number of iodine units or atoms attached to the hormone. Iodine is a very important component of thyroid hormone.

  • Thyroid hormone with four iodine units is abbreviated as T4
  • Thyroid hormone with three iodine units is abbreviated as T3
  • Most thyroid hormone in the blood is T4, however…
  • T3 is the form that is active in the body, not T4.
  • Certain cells in the body convert T4 to T3.
  • Cells respond to thyroid hormone with an increase in metabolic activity. Metabolic activity, or metabolism, is a term used to describe the processes in the body that produce energy and the chemical substances necessary for cells to grow, divide to form new cells, and perform other vital functions.If there's not enough thyroid hormone, the body slows down, like taking one's foot off the accelerator pedal.

When hypothyroidism develops in older children before growth and development are complete, they may have a shorter-than-average height or puberty may be delayed. They also may have symptoms that are more like those found in adults:

  • Slow heart rate
  • Tiredness
  • Inability to tolerate cold
  • Dry, flaky skin
  • Puffiness in the face, especially around the eyes
  • Impaired memory and difficulty in thinking (which may appear as a new learning disability)
  • Emotional depression
  • Drowsiness, even after sleeping through the night
  • Heavy or irregular menstrual periods (in girls at the age of puberty)
  • Constipation

Hypothyroidism that occurs in infancy or early childhood is called cretinism. Babies born with hypothyroidism classically show a number of symptoms in the first weeks to months of life. At birth, many symptoms are subtle, though, and can be missed. They include the following:

  • Prolonged newborn jaundice (a yellow discoloration of the skin and the whites of the eyes)
  • Poor feeding and constipation
  • Cool, mottled skin
  • Increased sleepiness
  • Decreased crying
  • Larger-than-normal soft spots on the skull
  • Umbilical hernia (a soft protrusion around the navel)
  • A large tongue

Over time, if untreated, other symptoms typically become apparent in older infants, toddlers, and young children. The most obvious symptoms observed in these youngsters reflect insufficient thyroid hormone for growth and development:

  • Short stature for age and delayed eruption of baby teeth
  • Delays in major developmental milestones
  • Puffy facial features
  • Severe mental retardation
  • Protruding abdomen and umbilical hernia (a soft protrusion around the navel)
  • Dry skin and sparse hair

For infants born with hypothyroidism, diagnosis and treatment within the first month or so of life may prevent any irreversible problems with the child's development. However, if diagnosis or treatment is delayed until after the first two or three months of life, permanent problems with the child's development, such as mental retardation, may be unavoidable.

In the United States, Canada, and much of the Western world, newborns are routinely screened for thyroid hormone deficiency. Infants with abnormal screening tests receive follow-up evaluation for hypothyroidism. Such testing commonly leads to the correct diagnosis within the first four weeks of age, and treatment can begin immediately.

Diagnosis later in childhood is usually based on information from blood tests, which check levels of thyroid hormones T4, T3, and TSH, among other related substances. Abnormally low levels of T4 and T3 indicate hypothyroidism is present.

  • If TSH is present at a higher-than-normal level, the abnormality is within the thyroid gland. It is not responding properly to TSH.
  • If TSH is low, the abnormality is within the brain or pituitary gland. The pituitary is not releasing TSH despite levels of thyroid hormone low enough that it should do so.

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.

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.

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.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.
  • 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.

The treatment for hypothyroidism is simple: Supply the body with the thyroid hormone that isn't being produced and released by the thyroid gland. Synthetic (artificially produced) thyroid hormone (T4) is manufactured in a wide range of strengths so that dosage can be individually tailored for each person. It is important that an experienced physician oversee treatment, because the body's need for thyroid hormone varies over the course of childhood and puberty. A child will usually be started on a daily dose of thyroid hormone, have it adjusted until a healthy level of hormone in the blood is reached, and then be monitored with regular blood tests. Almost all children with hypothyroidism will require thyroid hormone replacement therapy for the rest of their lives. Fortunately, thyroid hormone is simple to take, and the success of therapy is easily monitored by blood tests for T4 and TSH that can be done throughout adulthood.

Useful Links:

American Thyroid Association
(http://www.thyroid.org/)

Thyroid Federation International
(http://www.thyroid-fed.org/tfi/index.php?l=en-US#home)

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