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Hypothyroidism is a
condition characterized by abnormally low thyroid hormone production.
There are many disorders that result in hypothyroidism. These disorders
may directly or indirectly involve the thyroid gland. Because thyroid
hormone affects growth, development, and many cellular processes,
inadequate thyroid hormone has widespread consequences for the body.
Causes & Risk Factors
Hypothyroidism is a very common condition. It is estimated that 3% to 5%
of the population has some form of hypothyroidism. The condition is more
common in women than in men, and its incidence increases with age.
Below is a list of some of the common causes of hypothyroidism in adults
followed by a discussion of these conditions.
* Hashimoto's thyroiditis
* Lymphocytic thyroiditis (which may occur after hyperthyroidism)
* Thyroid destruction (from radioactive iodine or surgery)
* Pituitary or hypothalamic disease
* Medications
* Severe iodine deficiency
Hashimoto's Thyroiditis
The most common cause of hypothyroidism in the United States is an
inherited condition called Hashimoto's thyroiditis. This condition is
named after Dr. Hakaru Hashimoto who first described it in 1912. In this
condition, the thyroid gland is usually enlarged (goiter) and has a
decreased ability to make thyroid hormones. Hashimoto's is an autoimmune
disease in which the body's immune system inappropriately attacks the
thyroid tissue. In part, this condition is believed to have a genetic
basis. This means that the tendency toward developing Hashimoto's
thyroiditis can run in families. Hashimoto's is 5 to 10 times more
common in women than in men. Blood samples drawn from patients with this
disease reveal an increased number of antibodies to the enzyme, thyroid
peroxidase (anti-TPO antibodies). Since the basis for autoimmune
diseases may have a common origin, it is not unusual to find that a
patient with Hashimoto's thyroiditis has one or more other autoimmune
diseases such as diabetes or pernicious anemia ( B12 deficiency).
Hashimoto's can be identified by detecting anti-TPO antibodies in the
blood and/or by performing a thyroid scan.
Lymphocytic thyroiditis following hyperthyroidism
Thyroiditis refers to inflammation of the thyroid gland. When the
inflammation is caused by a particular type of white blood cell known as
a lymphocyte, the condition is referred to as lymphocytic thyroiditis.
This condition is particularly common after pregnancy and can actually
affect up to 8% of women after they deliver. In these cases, there is
usually a hyperthyroid phase (in which excessive amounts of thyroid
hormone leak out of the inflamed gland), which is followed by a
hypothyroid phase that can last for up to six months. The majority of
affected women eventually return to a state of normal thyroid function,
although there is a possibility of remaining hypothyroid.
Thyroid destruction secondary to radioactive iodine or surgery
Patients who have been treated for a hyperthyroid condition (such as
Graves' disease) and received radioactive iodine may be left with little
or no functioning thyroid tissue after treatment. The likelihood of this
depends on a number of factors including the dose of iodine given, along
with the size and the activity of the thyroid gland. If there is no
significant activity of the thyroid gland six months after the
radioactive iodine treatment, it is usually assumed that the thyroid
will no longer function adequately. The result is hypothyroidism.
Similarly, removal of the thyroid gland during surgery will be followed
by hypothyroidism.
Pituitary or Hypothalamic disease
If for some reason the pituitary gland or the hypothalamus are unable to
signal the thyroid and instruct it to produce thyroid hormones, a
decreased level of circulating T4 and T3 may result, even if the thyroid
gland itself is normal. If this defect is caused by pituitary disease,
the condition is called "secondary hypothyroidism." If the defect is due
to hypothalamic disease, it is called "tertiary hypothyroidism."
Pituitary injury
A pituitary injury may result after brain surgery or if there has been a
decrease of blood supply to the area. In these cases of pituitary
injury, the TSH that is produced by the pituitary gland is deficient and
blood levels of TSH are low. Hypothyroidism results because the thyroid
gland is no longer stimulated by the pituitary TSH. This form of
hypothyroidism can, therefore, be distinguished from hypothyroidism that
is caused by thyroid gland disease, in which the TSH level becomes
elevated as the pituitary gland attempts to encourage thyroid hormone
production by stimulating the thyroid gland with more TSH. Usually,
hypothyroidism from pituitary gland injury occurs in conjunction with
other hormone deficiencies, since the pituitary regulates other
processes such as growth, reproduction, and adrenal function.
Medications
Medications that are used to treat an over-active thyroid
(hyperthyroidism) may actually cause hypothyroidism. These drugs include
methimazole (Tapazole) and propylthiouracil (PTU). The psychiatric
medication, lithium (Eskalith, Lithobid), is also known to alter thyroid
function and cause hypothyroidism. Interestingly, drugs containing a
large amount of iodine such as amiodarone (Cordarone), potassium iodide
(SSKI, Pima), and Lugol's solution can cause changes in thyroid
function, which may result in low blood levels of thyroid hormone.
Severe iodine deficiency:
In areas of the world where there is an iodine deficiency in the diet,
severe hypothyroidism can be seen in 5% to 15% of the population.
Examples of these areas include Zaire, Ecuador, India, and Chile. Severe
iodine deficiency is also seen in remote mountain areas such as the
Andes and the Himalayas. Since the addition of iodine to table salt and
to bread, iodine deficiency is rarely seen in the United States.
Symptoms & Signs
Only subtle symptoms may occur early in the course of hypothyroidism.
These may include:
# fatigue
# decreased concentration
# intolerance to cold environments
# constipation
# loss of appetite
# muscle cramping and stiffness
# weight gain
Some individuals may notice hair loss, dry skin, or nail changes. If
left untreated, the symptoms of hypothyroidism will progress. This can
lead to fluid retention around the eyes or legs.
Untreated hypothyroidism can also cause congestive heart failure, a
condition in which a weakened heart is unable to pump enough blood to
body organs. In severe cases, the brain itself is affected. The person
can lose mental function and even go into a coma.
Diagnosis & Tests
A diagnosis of hypothyroidism can be suspected in patients with fatigue,
cold intolerance, constipation, and dry, flaky skin. A blood test is
needed to confirm the diagnosis.
When hypothyroidism is present, the blood levels of thyroid hormones can
be measured directly and are usually decreased. However, in early
hypothyroidism, the level of thyroid hormones (T3 and T4) may be normal.
Therefore, the main tool for the detection of hyperthyroidism is the
measurement of the TSH, the thyroid stimulating hormone. As mentioned
earlier, TSH is secreted by the pituitary gland. If a decrease of
thyroid hormone occurs, the pituitary gland reacts by producing more TSH
and the blood TSH level increases in an attempt to encourage thyroid
hormone production. This increase in TSH can actually precede the fall
in thyroid hormones by months or years (see the section on Subclinical
Hypothyroidism below). Thus, the measurement of TSH should be elevated
in cases of hypothyroidism.
However, there is one exception. If the decrease in thyroid hormone is
actually due to a defect of the pituitary or hypothalamus, then the
levels of TSH are abnormally low. As noted above, this kind of thyroid
disease is known as "secondary" or "tertiary" hypothyroidism. A special
test, known as the TRH test, can help distinguish if the disease is
caused by a defect in the pituitary or the hypothalamus. This test
requires an injection of the TRH hormone and is performed by an
endocrinologist (hormone specialist).
The blood work mentioned above confirms the diagnosis of hypothyroidism,
but does not point to an underlying cause. A combination of the
patient's clinical history, antibody screening (as mentioned above), and
a thyroid scan can help diagnose the precise underlying thyroid problem
more clearly. If a pituitary or hypothalamic cause is suspected, an MRI
of the brain and other studies may be warranted. These investigations
should be made on a case by case basis.
Hypothyroidism is not contagious and poses no risk to others.
Treatment & Monitoring
Generally, there is no way to reverse the damage done to the thyroid
gland. The healthcare provider will prescribe thyroid hormone, such as
levothyroxine or liothyronine, to be taken on a daily basis. The right
dose of medication should resolve the signs and symptoms of
hypothyroidism.
What are the side effects of the treatments?
If a person has had hypothyroidism for many years, the replacement of
thyroid hormone may be started slowly and eventually increased to normal
levels. Because the thyroid hormone medication is chemically identical
to the body's thyroid hormone, side effects or allergic reactions to the
medications are quite rare.
If too much thyroid hormone is given, the person may develop
arrhythmias, or irregular heartbeats, and osteoporosis, or thinning of
the bones.
What happens after treatment for the disease?
Treatment of hypothyroidism is lifelong.
How is the disease monitored?
The healthcare provider will use periodic thyroid function tests to
monitor the level of medication needed. These blood tests may initially
be done every 6 to 8 weeks, until a normal level of thyroid is restored.
After the right dose of medication is established, thyroid function
tests may then be done every 6 to 12 months. Any new or worsening
symptoms should be reported to the healthcare provider.
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