Hypothyroidism, or underactive thyroid, develops when the thyroid gland fails to produce or secrete as much thyroxine (T 4 ) and triiodothyonine (T 3 ) as the body needs. Because these thyroid hormones regulate such essential functions as heart rate, digestion, physical growth, and mental development, an insufficient supply of this hormone can slow metabolic processes, damage organs and tissues in every part of the body, and lead to life-threatening complications.
Hypothyroidism is one of the most common chronic diseases in the United States. Symptoms may not appear until years after the thyroid has stopped functioning and often are mistaken for signs of other illnesses. Although this condition is believed to affect up to 11 million adults and children, as many as two out of every three people with hypothyroidism may not know they have the disease.
Nicknamed "Gland Central" because it influences almost every organ, tissue, and cell in the body, the thyroid is shaped like a butterfly and located just below the larynx, or Adam's apple, and in front of the trachea, or windpipe. The thyroid stores iodine that the body obtains from food, and uses this mineral to create the thyroid hormones. Low thyroid hormone levels can alter weight, appetite, sleep patterns, body temperature, and a variety of other physical, mental, and emotional characteristics.
Although hypothyroidism is most common in women who are middle-aged or older, the disease can occur at any age. In addition, an infant can be born with congenital hypothyroidism, i.e., without a functioning thyroid. In older children, the development of hypothyroidism may progress slowly and it may be several years before the disease is diagnosed.
The most common cause of hypothyroidism in mid-to late-childhood and adolescence is Hashimoto's thyroiditis, which occurs in up to 1.2 percent of the school age population. Congenital hypothyroidism is less common. One out of every 4,000–5,000 infants is born without a properly functioning thyroid gland. Congenital hypothyroidism is twice as common in girls as in boys and about five times more common in whites than in blacks.
Causes and symptoms
Congenital hypothyroidism is a disorder that affects infants from birth, resulting from the loss of thyroid function due to the failure of the thyroid gland to develop correctly. Sometimes the thyroid gland is absent or is ectopic, i.e., in an abnormal location. This congenital defect means that the infant does not produce sufficient thyroid hormones, resulting in abnormal growth and development as well as slower mental function.
Hypothyroidism may also be caused by an abnormality of the immune system that results in damage and destruction of the thyroid gland (Hashimoto's thyroiditis). This process can result in either loss of thyroid tissue or enlargement of the thyroid. In most cases, there is no pain or tenderness associated with this disease, although sometimes persons affected complain of difficulty in swallowing, as if they had a lump in the throat.
Less often, hypothyroidism develops when the pituitary gland fails and does not release enough thyroid-stimulating hormone (TSH), which stimulates the thyroid to produce and secrete normal amounts of T 4 and T 3 . TSH may be deficient for several reasons:
- disease of the pituitary gland (occurs rarely)
- disease of the hypothalamus (located about the pituitary), which stimulates the pituitary gland
- tumor, cyst, or other abnormal structure between the hypothalamus and pituitary gland that prevents the pituitary from receiving the stimulus to secrete TSH
Other causes of hypothyroidism include:
- Radiation. Radioactive iodine used to treat hyperthyroidism (overactive thyroid) or radiation treatments for head or neck cancers can destroy the thyroid gland.
- Surgery. Removal of the thyroid gland because of cancer or other thyroid disorders can result in hypothyroidism.
- Viruses and bacteria. Infections that depress thyroid hormone production usually cause permanent hypothyroidism.
- Medication. Nitroprusside, lithium, or iodides can induce hypothyroidism. Because patients who use these medications are closely monitored by their doctors, this side effect is very rare.
- Environmental contaminants. Certain man-made chemicals such as PCBs, found in the local environment at high levels, may also cause hypothyroidism.
Often babies with congenital hypothyroidism will appear normal at birth, which is why screening is vital. However, some infants may have one of more of the following symptoms:
- large size (despite poor feeding habits) and increased birth weight
- puffy face and swollen tongue
- hoarse cry
- low muscle tone
- cold extremities
- persistent constipation , with distended abdomen
- lack of energy, sleeping most of the time and appearing tired when awake
- little or no growth
Children born with symptoms have a greater risk of developmental delay than children born without symptoms. The longer a child with hypothyroidism remains untreated, the greater is the loss of intellectual capacity, as measured by the standard intelligence testing (IQ). The ultimate IQ has been shown to be significantly higher in children whose hypothyroidism was detected and treated prior to six weeks of age, compared to those children whose hypothyroidism went untreated for six to 12 weeks.
Hypothyroidism that develops after birth is sometimes referred to as a silent disease because early symptoms may be so mild that no one realizes anything is wrong. Untreated symptoms become more noticeable and severe, and can lead to confusion and mental disorders, breathing difficulties, heart problems, fluctuations in body temperature, and death.
A child or adolescent who has hypothyroidism may have one or more of the following symptoms:
- decreased heart rate
- progressive hearing loss
- weight gain
- problems with memory and concentration
- goiter (enlarged thyroid gland)
- muscle pain or weakness
- numb, tingling hands
- dry skin
- swollen eyelids
- dryness or loss of hair
- extreme sensitivity to cold
- delayed (common) or early (rare) onset of sexual development at adolescence
- irregular menstrual periods
- elevated cholesterol levels in the blood
- hoarse voice
Although hypothyroidism usually develops gradually, when the disease results from surgery or other treatment for hyperthyroidism, symptoms may appear suddenly and include severe muscle cramps in the arms, legs, neck, shoulders, and back.
People whose hypothyroidism remains undiagnosed and untreated may eventually develop myxedema. Symptoms of this rare, but potentially deadly, complication include enlarged tongue, swollen facial features, hoarseness, and physical and mental sluggishness. Myxedema coma can cause unresponsiveness; irregular, shallow breathing; low blood sugar; and drops in blood pressure and body temperature. The onset of this medical emergency can be sudden in children with undiagnosed hypothyroidism; it can be brought on by illness, injury, surgery, use of sedatives or anti-depressants, or exposure to very cold temperatures. Without immediate medical attention, myxedema coma can be fatal.
When to call the doctor
The doctor should be called if signs of hypothyroidism or myxedema are present. Every child who has a decrease in rate of growth in height during childhood and adolescence should be tested to determine if the growth problem is caused by hypothyroidism.
In the United States, newborn infants between 24 and 72 hours old are tested for congenital thyroid deficiency (cretinism) using a test that measures the levels of thyroxine in the infant's blood. If the levels are low, the physician will likely repeat the blood test to confirm the diagnosis. The physician may take an x ray of the infant's legs. In an infant with hypothyroidism, the ends of the bones have an immature appearance. Treatment within the first few months of life can prevent mental retardation and physical abnormalities.
Older children who develop hypothyroidism may suddenly stop growing. If the child was above average height before the disease occurred, he or she may now be short compared to other children of the same age. Therefore, the most important feature of hypothyroidism in a child is a decrease in the rate of growth in height. If the disease is recognized early and adequately treated, the child will grow at an accelerated rate until reaching the same growth percentile where the child measured before the onset of hypothyroidism. Diagnosis of hypothyroidism is based on the patient's observations, medical history, physical examination, and thyroid function tests. Doctors who specialize in treating thyroid disorders (endocrinologists) are most likely to recognize subtle symptoms and physical indications of hypothyroidism. A diagnostic evaluation may include a blood test known as a thyroid-stimulating hormone (TSH) assay, thyroid nuclear medicine scan, thyroid ultrasound, or needle aspiration biopsy (which is also used to provide information on thyroid masses). All patients should be sure their doctors are aware of any recent procedures involving radioactive materials or contrast media.
The blood test is extremely accurate, but some doctors doubt its ability to detect mild hypothyroidism. They advise patients to monitor their basal (resting) body temperature for below-normal readings that could indicate the presence of hypothyroidism.
Alternative treatments are primarily aimed at strengthening the thyroid but will not eliminate the need for thyroid hormone medications. Herbal remedies to improve thyroid function and relieve symptoms of hypothyroidism include bladder wrack ( Fucus vesiculosus ), which can be taken in capsule form or as a tea. The shoulder stand yoga position (done at least once daily for 20 minutes) is believed to improve thyroid function.
Because the thyroid makes T 4 from iodine in food, an iodine-deficient diet can cause hypothyroidism. Adding iodine to table salt and other common foods has eliminated iodine deficiency in the United States. Some foods, including cabbage, rutabagas, radishes, peanuts, peaches, soybeans, and spinach, can interfere with thyroid hormone production. Anyone with hypothyroidism may want to avoid these foods. A high-fiber diet along with regular exercise is recommended to help maintain thyroid function and prevent constipation.
Thyroid hormone replacement therapy generally maintains normal thyroid hormone levels unless treatment is interrupted or discontinued.
Cretinism —Severe hypothyroidism that is present at birth and characterized by severe mental retardation.
Endocrine system —A group of ductless glands and parts of glands that secrete hormones directly into the bloodstream or lymphatic system to control metabolic activity. Pituitary, thyroid, adrenals, ovaries, and testes are all part of the endocrine system.
Goiter —Chronic enlargement of the thyroid gland.
Hyperthyroidism —A condition characterized by abnormal over-functioning of the thyroid glands. Patients are hypermetabolic, lose weight, are nervous, have muscular weakness and fatigue, sweat more, and have increased urination and bowel movements. Also called thyrotoxicosis.
Hypothalamus —A part of the forebrain that controls heartbeat, body temperature, thirst, hunger, body temperature and pressure, blood sugar levels, and other functions.
Myxedema —Severe hypothyroidism, characterized by swelling of the face, hands, and feet, an enlarged tongue, horseness, and physical and mental sluggishness.
Pituitary gland —The most important of the endocrine glands (glands that release hormones directly into the bloodstream), the pituitary is located at the base of the brain. Sometimes referred to as the "master gland," it regulates and controls the activities of other endocrine glands and many body processes including growth and reproductive function. Also called the hypophysis.
Thyroid-stimulating hormone (TSH) —A hormone produce by the pituitary gland that stimulates the thyroid gland to produce the hormones that regulate metabolism. Also called thyrotropin.
Thyroxine (T 4 ) —The thyroid hormone that regulates many essential body processes.
Triiodothyronine (T 3 ) —A thyroid hormone similar to thyroxine but more powerful. Preparations of triiodothyronine are used in treating hypothyroidism.
Hypothyroidism usually cannot be prevented, but the symptoms and effects of the disease can be controlled by prompt diagnosis and treatment.
Parents must ensure that medication is taken on a routine basis by making the process a part of the family's lifestyle. Taking the medication as prescribed helps assure the child's optimal growth and development.
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Judith Sims, M.S. Maureen Haggerty