The Underactive Thyroid
Table of Contents
Introduction
Hypothyroidism is almost always due to disease within your thyroid
gland that causes a decrease in the production of thyroid hormone.
The most common cause of this disorder is autoimmune thyroid disease,
which is transmitted genetically and affects women much more often
than men. Excellent tests, particularly the blood TSH level, are
available to diagnose the condition accurately, and treatment with
thyroid hormone (thyroxine ) should restore you to good health.
Because the condition runs in families, some of your relatives should
be checked for thyroid problems by their own physicians.
When the thyroid gland fails to produce a normal amount of thyroid
hormone, hypothyroidism results. Thyroid hormones act upon receptors
in tissues throughout your body controlling the rate at which various
things happen, such as the speed of chemical reactions, the rate
of tissue growth, and the rate at which electrical impulses travel
in your nerves and muscles. So when you become hypothyroid, many
of the affected bodily functions simply slow down.
As your thyroid begins to fail, you may feel perfectly well, for
often the only suggestion of a problem will be a slight enlargement
of your thyroid gland (goiter), appearing as a lump or swelling
in front of your neck. Then, as your thyroid hormone level falls
further, you may begin to feel tired and listless, perhaps chilly
when those about you are comfortably warm. As your skin, hair, and
fingernails grow more slowly, they become thickened, dry, and brittle.
Some hair loss may be noticed. Then, as your hypothyroidism becomes
more severe, changes may occur in the tissues beneath your skin
that lead to a characteristic puffy, swollen appearance known as
myxedema. This is often particularly apparent around your face and
eyes.
Your circulation is affected and your heart rate slows, but you
probably won't notice this unless someone happens to count your
pulse (it may be below 60 beats per minute). Since your intestinal
activity slows down, you may become constipated. A few pounds of
weight gain may occur due to water retention, but you are not likely
to get fat due to hypothyroidism alone because your appetite and
zest for food decrease rather than increase when you become hypothyroid.
Your muscles may become sore and you may be awakened at night with
leg cramps. Muscle swelling may occur and may make your tongue (which
is a muscle) bigger. Your nervous system may be affected in several
ways. You may notice some memory loss, decreased ability to think,
depression, and you may become more sensitive to medications, so
that weak sedatives cause prolonged sleep. Some patients experience
tingling in their fingers, or loss of balance and difficulty in
walking.
If you are a younger woman, changes in your
reproductive system may cause longer, heavier, and more frequent
menstruation. Your ovaries may stop producing an egg each month,
and, if so, it may be difficult for you to get pregnant. If pregnancy
does occur, you are a little more likely to have a miscarriage than
if you had a healthy thyroid.
What causes hypothyroidism?
The causes of hypothyroidism vary somewhat with the age when the
disorder begins. Some children born with severe hypothyroidism have
never developed enough thyroid tissue to supply adequate amounts
of thyroid hormones for their bodily needs. Other hypothyroid infants
may have an inherited defect in the production of thyroid hormones
within their thyroid gland. In some underdeveloped countries, dietary
iodine deficiency is an important added cause of serious hypothyroidism
in newborn babies.
The most common cause of thyroid gland failure in older children
and adults is a silent, ongoing inflammation of the thyroid (without
evidence of infection), known as chronic lymphocytic thyroiditis.
It is also known as Hashimoto's disease, in honor of the Japanese
physician who described it. The thyroid fails because inflammation
and scarring damage the thyroid tissue.
Thyroid failure is also very common among patients who have been
treated in the past for an overactive thyroid. Here, hypothyroidism
may occur immediately after a treatment that destroys or removes
part of the thyroid (radioactive iodine or an operation), but, in
most instances, the thyroid doesn't fail until months or years later.
Such a delayed onset of hypothyroidism suggests that the original
treatment is not the only cause of thyroid failure in such patients.
Coexistent chronic lymphocytic thyroiditis may be a factor as well.
Less commonly, the thyroid may fail temporarily after a viral infection
or because of a medication. For example, if an antithyroid drug
used to control an overactive thyroid is given in too large a dose,
hypothyroidism may result and last until the dosage of that drug
is reduced. Lithium, a psychiatric drug, can also cause hypothyroidism
in some people. Furthermore, some individuals have thyroid glands
that are very sensitive to iodine. They can develop hypothyroidism
as a side effect if they are given iodine in a medication such as
amiodarone (a heart medicine). Individuals who eat seaweed (kelp,
dulse, etc.) or take supplements which contain seaweed extracts
can take in excessive amounts of iodine.
Hypothyroidism can also develop in patients who receive large amounts
of x-rays to the neck area as part of cancer treatment.
Diagnosing hypothyroidism
If your physician suspects hypothyroidism, he or she will first
perform a medical examination to look for evidence that your thyroid
level is low. The most important test in making a certain diagnosis
of this condition is your TSH blood level. When your thyroid gland
fails, your pituitary begins to produce increased amounts of TSH,
in an effort to stimulate your thyroid more and return it to normal
function. If your thyroid is damaged, it cannot increase its activity
and your blood level of TSH rises and remains high. Detecting an
increased level of TSH in your blood also provides solid evidence
that your hypothyroidism is due to disease within your thyroid gland,
and is not a result of inadequate stimulation of your thyroid by
a diseased pituitary gland.
A word should be said about a dangerous recent trend in the lay
press to minimize the importance of thyroid blood tests in the diagnosis
and management of hypothyroidism. The thyroid tests available 20
or 30 years ago were simply not specific enough to tell if a person
was truly hypothyroid. Very accurate and relatively inexpensive
tests are available today, however, and they should always be used
by your physician to help make a diagnosis of hypothyroidism. Such
a diagnosis should never be based solely on complaints of weight
gain, fatigue, or infertility, or on such nonspecific findings as
dry skin or a low body temperature. Well-informed physicians do
not start patients on thyroid medication until blood tests, and
most importantly, the TSH level, confirming the diagnosis are obtained.
Blood tests are essential in determining the cause and severity
of the hypothyroidism, and in assessing the adequacy
of thyroid therapy. The level of the thyroid hormones (T4 and T3)
may also be checked, but these tests are less sensitive than the
TSH level in making a decision about the diagnosis of primary hypothyroidism
or the dose of medication that you might require.
The treatment of hypothyroidism
Hypothyroidism is treated with thyroid hormone tablets containing
precisely the same chemical compound that your thyroid normally
produces and therefore, you will not be allergic to it. Moreover,
the hormones are not destroyed by stomach juices, so they can be
given by mouth. Finally, if taken correctly, thyroid hormone has
no unwanted effects on any body tissues.
Today, many different thyroid hormone preparations are manufactured.
For many years, however, the only thyroid hormone medications available
were made from animal thyroid glands. These preparations were very
useful, but they contain not only thyroxine (T4), but also a second,
more rapidly acting thyroid hormone, triiodothyronine (T3). Most
doctors prefer to administer thyroid hormone tablets that do not
contain T3 for two reasons. First, the body normally makes T3 from
T4; in fact, much of our T4 is changed into T3 under normal circumstances,
as it is used by the body. Second, the blood T3 level can become
abnormally high after taking medication that contains T3. The abnormally
high T3 level can cause a rapid pulse and increase the workload
of the heart, which can be dangerous for anyone with underlying
heart disease. For these reasons, most physicians now treat hypothyroidism
with tablets of pure T4 rather than tablets that contain both T4
and T3.
Recently, there has been renewed interest in the question of whether
T4 plus a little T3 might be better for some people than T4 alone.
A study published in the February 11, 1999 issue (Vol. 340, No.
6) of the New England Journal of Medicine suggested that
some patients with hypothyroidism may feel better taking a medication
that includes a combination of triiodothyronine (T3) along with
thyroxine (T4). However, the addition of T3 could lead to complications
for certain patients. While this approach to treatment is not new,
it is not the treatment most widely accepted and prescribed by most
physicians for their hypothyroid patients.
Most physicians prefer to treat hypothyroid patients with pure
T4 because treatment with rapid-acting T3 produces abnormally high
peak levels of T3 in the blood. This could cause increased risk
for heart rhythm problems or heart attacks especially in older individuals
and anyone with heart disease. Because in most individuals the body
converts T4 to T3 as needed, pure thyroxine (T4) alone is the medication
that the vast majority of physicians prescribe for hypothyroid patients.
A change in thyroid hormone dose, or adding T3 in some form may
be a help for some patients. Some people may have health concerns
such as depression, memory problems, muscle aches, or weight gain
which may need different treatment including other medication.
There is an increasing tendency today to use generic forms of drugs
as opposed to more expensive forms sold under a trade name. Although
generic drugs are generally less expensive, generic thyroid preparations
have always presented the question of unreliable potency. Tests
conducted in the United States have shown this variability of potency
in generic T4 tablets. In fact, only three kinds of T4 tablets are
found to be consistently reliable: Synthroid, Levothroid, and Levoxyl.
These tablets have a nearly identical color code (thyroxine tablets
of different potencies come in different colors), which helps to
avoid confusion about thyroid hormone dosage. A wide variety of
dosage strengths are available to enable the physician to precisely
tailor the dose to the patient. Synthroid is available in the United
States and Canada, while Levothroid and Levoxyl are marketed only
in the United States. Other brands are available in Canada and Europe.
When changing a patient from a dessicated thyroid tablet to T4,
physicians usually start at a slightly lower dose of T4 because
many older preparations are not reliably potent. For example, someone
who has taken two or three grains of dessicated thyroid may be started
on 75 to 125 micrograms of thyroxine. In fact, few patients ever
need more than 100 to 200 micrograms of thyroxine per day.
Even when hypothyroidism is severe, a few months of thyroid treatment
should lead to complete recovery and a return to good health. At
that time, your physician will probably measure your blood levels
of T4 and TSH to be sure that your dosage
of thyroid hormone is correct. If you are taking too much T4 your
blood level of TSH will be too low and the blood level of T4 may
be above the normal range. On the other hand, if your dose of T4
is too low, your blood level of TSH will still be high and the T4
level may be low.
Don't stop taking your thyroid medication
The smooth control of thyroid hormone levels that physicians achieve
by using pure thyroxine preparations is due to the slow rate at
which thyroxine is used up by the body. In fact, if a normal person's
thyroid suddenly stopped working, it would take about seven days
for the body to use up just half of the T4 already in the blood.
Therefore, if you are hypothyroid and taking thyroxine tablets to
correct your thyroid deficiency, you will not feel suddenly sick
even if you stop your thyroxine tablets abruptly. Furthermore, because
you won't notice a sudden change in the way you feel, you may incorrectly
assume that your thyroid condition no longer exists and you may
stop taking your medication entirely. Unfortunately, when your hypothyroidism
does recur, its onset may be so gradual that you may not realize
that you are becoming ill again until your symptoms are pronounced.
If you are hypothyroid, it is important that you see your physician
periodically for checkups. Since most hypothyroidism tends to get
worse progressively over months and years, a dose of thyroid hormone
that was correct several years ago may well be inadequate as thyroid
replacement now. Therefore, your physician will probably want to
measure your serum T4 and TSH periodically to be sure that a change
in hormone dosage is not indicated. We recommend that patients should
be tested at least once each year to be sure that the control of
the thyroid condition is correct.
An exception to this rule applies if you become pregnant. Pregnancy
often increases the thyroid hormone requirement. Therefore, if you
become pregnant while taking thyroid hormone for hypothyroidism,
we recommend that you have your TSH level checked as soon as you
know you are pregnant, and again in each trimester. If the value
is high your doctor will increase your dose of medication until
you have safely delivered your child. Then your dose will be reduced
to the previous level.
A word might be said about treating certain of the less common
causes of thyroid failure. For example, subacute thyroiditis, which
may be due to a viral infection, may only temporarily decrease thyroid
function. If a patient needs any thyroxine treatment for a transient
hypothyroid condition, it should be only a matter of weeks or months
before he or she can stop the drug and remain well. And when hypothyroidism
is due to iodine ingestion or an antithyroid drug, simply stopping
or decreasing the dose of the drug may be all that is required.
In every such instance, your physician has the necessary tests available
as a guide in properly taking care of you. A very special example
of this is the hypothyroidism that occurs immediately after delivering
a child. This condition is known as postpartum thyroiditis and is
found in approximately 5 to 8 percent of all women in the postpartum
period. It usually occurs 2 to 12 months after delivery and may
be preceded by hyperthyroidism due to thyroid hormone leaching out
of the inflamed thyroid. If hypothyroidism develops it commonly
requires treatment with thyroid hormone. Surprisingly, this form
of hypothyroidism is usually not permanent and typically
goes away spontaneously after 6 to 12 months. Thus, in this type
of hypothyroidism, your doctor will discontinue the thyroid medication
to see if the problem has disappeared. However, hypothyroidism is
permanent in 10 - 25% of women with this disorder.
Other kinds of hypothyroidism
Since the pituitary gland at the base of your brain controls and
stimulates your thyroid, a tumor or other problem that involves
the pituitary can cause secondary thyroid failure. Since your pituitary
also controls other glands, including your reproductive organs and
adrenal glands, it is usually an easy matter for your physician
to tell if this is the case. For example, a woman with secondary
(pituitary) hypothyroidism will usually stop menstruating when her
diseased pituitary gland stops stimulating her ovaries properly.
Physicians have at their disposal both x-ray techniques and laboratory
tests to evaluate the function of your pituitary gland as well as
your thyroid. If there is indeed a pituitary problem, you will probably
require treatment for other hormonal deficiencies in addition to
the thyroid. If your pituitary fails because of a tumor, specific
treatment may be directed at the pituitary gland itself. Fortunately,
pituitary tumors respond to both surgical and radiation treatment.
Just as with primary hypothyroidism, you will need careful and prolonged
follow-up, for in addition to thyroid hormone requirements, the
amounts of other hormones you take may vary with time. Thus, periodic
blood tests and x-rays of the pituitary area will probably be recommended
by your physician.
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