Hypothyroidism

Definition
Hypothyroidism, also known as underactive thyroid, is a condition that occurs when the thyroid gland does not produce enough thyroid hormones.

Anatomy
The thyroid gland is located in the neck, just below the voice box and on top of the windpipe. This butterfly-shaped gland produces thyroid hormones triiodothyronine (T3) and thyroxine (T4), which help regulate several body systems, including metabolism and the conversion of calories to energy. The thyroid hormones also help control body temperature, growth and skeletal development, heart rate and function, muscle tone and function, nervous system development, mental function and body weight. When there aren't enough thyroid hormones circulating in your body, your metabolism slows down.

Causes/associated factors
Hypothyroidism may develop for many reasons, including changes in the thyroid gland itself or in other structures that affect the thyroid, such as the pituitary gland or the hypothalamus. Surgical or radioactive iodine treatment for hyperthyroidism (overactive thyroid) may ultimately cause hypothyroidism in many cases.
Other factors that may contribute to low thyroid hormone production include:

infection
inflammation or inflammatory conditions, such as amyloidosis (considered a metabolic disorder) or sarcoidosis (a disease in which abnormal collections of inflammatory cells form in various organs)
low iodine intake (rare in the United States)
congenital thyroid problems
Hashimoto's disease, also known as chronic autoimmune thyroiditis, is the most common cause of hypothyroidism. As with all autoimmune disorders, the body's immune system attacks its own normal cells. This causes inflammation and tissue destruction, which can lead to chronic disease. With Hashimoto's disease, the body's immune system produces antibodies that attack the thyroid gland. Hashimoto's disease usually causes permanent hypothyroidism.

Although hypothyroidism can affect anyone at any age, it often develops in women during middle age or later. Thyroid disorders tend to run in families, and the presence of other autoimmune diseases is also considered a risk factor.


Types of hypothyroidism
Changes in thyroid function may produce subclinical hypothyroidism, a type of hypothyroidism so slight it may not even cause symptoms. This condition is often related to Hashimoto's disease or an enlarged thyroid gland, known as a goiter. Because symptoms may not be present, lab tests may be the only way to identify subclinical hypothyroidism.
The most serious form of hypothyroidism is myxedema, which is a rare complication of severe hypothyroidism. Trauma, infection, exposure to cold and a sensitivity to certain medications for the central nervous system, such as opiates, can lead to myxedema (a progression of hypothyroidism). If left untreated, a person with myxedema can become comatose. A myxedema coma is often fatal, even with the best treatment.


Sign/symptoms
Early symptoms of hypothyroidism are often vague and easily ignored. Without diagnosis and treatment, however, symptoms will become more severe. Because the thyroid hormone affects so many bodily functions, possible symptoms may include any of the following:
feeling slow or tired
muscle weakness and delayed reflexes
muscle cramps or numb arms and legs
constipation
heavy, prolonged menstrual bleeding
unexplained weight gain
swollen hands, feet and face, especially under the eyes
dry, coarse, flaky skin that lacks elasticity
husky voice
decreased sex drive
less ability to tolerate exercise
goiter (enlarged thyroid gland)
thinning or dry hair with patches of hair loss
weak pulse and slow heart rate (less than 70 beats a minute)
elevated blood pressure
infertility
depression
decreased mental stability, difficulty concentrating or poor memory
brittle fingernails with ridges
drooping of upper eyelids
cool skin
thick, dry tongue, which may cause hoarseness and slow or slurred speech
sparse or missing eyebrows
intolerance or heightened sensitivity to cold
It's important to remember that any of these symptoms may indicate conditions other than hypothyroidism. Also, a diagnosis of hypothyroidism may be made without any of these symptoms present.


Diagnosis
The doctor will review your medical history and current symptoms and do a physical exam. You may need a variety of diagnostic tests, including the following:
Thyroid function blood tests may be used to measure your levels of thyroid stimulating hormone (TSH) and free T4 or T4 and T3 resin uptake.
Other blood tests may be done to evaluate electrolytes (substances that can transmit electrical impulses when dissolved in body fluids), which can be affected by changes in the thyroid.
A serum antithyroid antibody test may show an elevation of thyroid antibodies in your blood if the hypothyroidism is cased by Hashimoto's disease.
Imaging techniques such as computed tomography (CT scan) or magnetic resonance imaging (MRI) may be used to identify tumors in the hypothalamus or pituitary gland.

Treatment
Hypothyroidism requires lifelong replacement of thyroid hormones. Your doctor will prescribe oral hormone medications such as levothyroxine (to replace the thyroid hormone T4) or liothyronine (to replace the thyroid hormone T3). Levothyroxine remains the medication of choice, however. You'll probably begin with a low dosage and gradually build up to the proper level to avoid any side effects on your heart. This gradual approach is especially important for older adults. You'll also need follow-up blood tests to make sure your thyroid hormones are within the therapeutic range. Too much replacement medication can lead to bone loss or atrial fibrillation, which can cause a stroke.
It's essential to take your medication as directed. Avoid taking the pills at the same time as an iron or calcium supplement or antacid -- these medications may keep your body from properly absorbing the thyroid hormones. Also be careful with antihistamines or caffeine because they may cause palpitations when taken with thyroid medication. Only discontinue the thyroid medication under your doctor's supervision. It's also important to keep an eye on your weight and report any sudden increases to your doctor. Avoid excess sodium to prevent retaining water, and eat a well-balanced diet high in fiber and fluids (as directed by your physician) to prevent constipation. Schedule regular activities, but make sure you get plenty of rest to avoid fatigue.


Complications
Depending on the duration and severity, thyroid hormone deficiencies can lead to various complications. Potential complications include coronary artery disease, congestive heart failure, increased susceptibility to infection, enlarged colon (megacolon), intestinal obstruction and various types of anemia. Other complications may include swelling in the extremities, infertility and bone loss. Again, if left untreated, myxedema coma is often fatal.

Pregnancy-specific information
Serious hypothyroidism is rare in pregnancy, probably due to the fact that severe hypothyroidism often prevents pregnancy. Preeclampsia (a serious condition that causes elevated blood pressure, as well as protein in the urine), intrauterine growth retardation and fetal distress are more common in pregnant women who have hypothyroidism. Some studies indicate pregnant women who have hypothyroidism also have a greater risk of miscarriage. If you're being treated for hypothyroidism, you may require significant increases in your medications during pregnancy. After delivery, your hormone levels and additional changes in your medication will be closely monitored. Be sure to keep all medical appointments and follow your doctor's instructions carefully.

Senior-specific information
Hypothyroidism is most common in women over age 60.

Prevention
Although you can't prevent hypothyroidism, screening tests can help detect the condition. The American Association of Clinical Endocrinologists (a specialty organization of doctors who treat thyroid and other endocrine disorders) recommend routine thyroid screening tests for women age 40 and older. Discuss your need for screening with your doctor.