Patient Education - Endocrine Encyclopedia
Endocrine Surgery Encyclopedia
Definition:
Primary hypothyroidism is a condition of decreased hormone production by the thyroid
gland.
Causes, incidence, and risk factors:
The thyroid gland is an important organ that regulates metabolism. It is located
in the front of the neck just below the voicebox (larynx). The thyroid gland secretes
two forms of thyroid hormone – thyroxine (T4) and triiodothyronine (T3). The
secretion of T3 and T4 by the thyroid is controlled by a feedback system involving
the pituitary gland, a small organ at the base of the brain, and the hypothalamus,
a structure in the brain.
Hypothyroidism caused by the inability of the thyroid gland to make T3 and T4
is called primary hypothyroidism. Worldwide, the most common cause of primary hypothyroidism
is deficiency of the element iodine. In the US, the most common cause is destruction
of the thyroid gland by the immune system, a condition called Hashimoto's thyroiditis.
Other causes of primary hypothyroidism include surgical removal of part or all
of the thyroid gland, radioactive iodine used for treatment of hyperthyroidism (overactive
thyroid), radiation exposure to the neck, special x-ray dyes, and certain drugs such
as lithium. Approximately 5-10% of women develop hypothyroidism after pregnancy (often
referred to as “postpartum thyroiditis"). In some cases, the cause of
hypothyroidism is unknown.
Since the thyroid gland is regulated by the pituitary gland and hypothalamus,
disorders of these organs can cause the thyroid gland to produce too little thyroid
hormone as well. This condition is called secondary hypothyroidism.
Primary hypothyroidism affects the whole body and may cause a variety of symptoms.
The body's normal rate of functioning slows, causing mental and physical sluggishness.
Symptoms may vary from mild to severe. The most severe form is called myxedema coma
and is a medical emergency. Risk factors for hypothyroidism include age (older than
age 50), female gender, obesity, thyroid surgery, and x-ray or radiation treatments
to the neck.
Symptoms:
Early symptoms:
- Weakness
- Fatigue
- Cold intolerance
- Constipation
- Weight gain
- Depression
- Muscle or joint pain
- Thin, brittle fingernails
- Thin, brittle hair
- Paleness
Late symptoms:
- Slow speech
- Dry flaky skin
- Thickening of the skin
- Puffy face, hands, and feet
- Decreased sense of taste and smell
- Thinning of eyebrows
- Hoarseness
- Menstrual disorders
Signs and tests:
Physical examination may reveal a smaller than normal gland, though sometimes the
gland is normal in size or even enlarged (goiter). Other physical findings include
pale, yellow, and dry skin; thin, brittle hair; loss of the edges of the eyebrows;
coarse facial features; firm swelling of the arms and legs; and slow muscle relaxation
when reflexes are tested. Vital signs may reveal a slow heart rate, low blood pressure,
and low temperature.
A chest x-ray sometimes reveals an enlarged heart.
Laboratory tests to determine thyroid function include:
- Free T4 test (low)
- Total T3 or free T3 (low)
- Serum TSH (high)
Additional laboratory abnormalities may include:
- Increased cholesterol levels
- Increased liver enzymes
- Increased serum prolactin
- Low serum sodium
- A complete blood count (CBC) shows anemia
Treatment:
The purpose of treatment is to replace the deficient thyroid hormone. Levothyroxine
(T4) is the most commonly used medication, but a preparation of T3 is also available.
Most people feel their best when TSH is brought into the 1 to 2 mcIU/mL range. People
get the lowest dose that is effective in relieving symptoms and normalizing blood
tests.
Life-long therapy is needed. Relapses will occur if therapy is interrupted. Medication
must be continued even when symptoms go away.
After replacement therapy has begun, report any symptoms of increased thyroid
activity (hyperthyroidism), such as restlessness, rapid weight loss, heat intolerance,
and sweating.
Myxedema coma is treated by intravenous thyroid replacement and steroid therapy.
Supportive therapy (oxygen, assisted ventilation, and fluid replacement) and intensive
care nursing may be indicated.
Support Groups:
Expectations (prognosis):
With early treatment, the condition
can be completely controlled. However, relapses will occur if the medication is not
continued. Myxedema coma can result in death.
Complications:
Myxedema coma, the most severe form of hypothyroidism, is rare. It may be caused
by an infection, illness, exposure to cold, or certain medications. Symptoms and
signs of myxedema coma include:
- Unresponsiveness
- Decreased breathing
- Low blood pressure
- Low blood sugar
- Below-normal temperature
Other complications include:
- Heart disease
- Increased risk of infection
- Infertility
- Miscarriage
- Pituitary tumors
Calling your health care provider:
Call your health care provider if signs or symptoms of hypothyroidism or myxedema
are present
Call your health care provider if restlessness, rapid weight loss, heat intolerance,
rapid heart rate, excessive sweating, or symptoms of hyperthyroidism occur after
beginning thyroid replacement.
Prevention:
Primary hypothyroidism is preventable by supplemental iodine in areas where iodine
in the food supply is low. Otherwise, the condition is not preventable. Awareness
of risk may allow early diagnosis and treatment. Some experts advocate screening
TSH testing in certain high risk groups (e.g., women older than 50 years).
Review Date: 5/12/2006
Reviewed By: Robert Hurd, MD, Department of Biology, College of Arts and Sciences,
Xavier University, Cincinnati, OH. Review provided by VeriMed Healthcare Network.
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