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e-Medical
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The thyroid
gland uses iodine (mostly available from the diet in foods
such as seafood, bread and salt) to produce thyroid
hormones. The two most important thyroid hormones are
thyroxine (T4) and triiodothyronine (T3), which account
for 99.9% and 0.1% of thyroid hormones present in the
blood respectively. However, the hormone with the most
biological activity is T3. Once released from the thyroid
gland into the blood, a large amount of T4 is converted
into T3 - the active hormone that affects the metabolism
of cells.
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The thyroid gland is
itself regulated by another gland that is located in the brain, called
the pituitary gland. In turn, the pituitary is regulated in part by the
thyroid (via a "feedback" effect of thyroid hormone on the
pituitary gland) and by another gland called the hypothalamus.
The hypothalamus releases
a hormone called thyrotropin releasing hormone (TRH) which sends
a signal to the pituitary to release thyroid stimulating hormone (TSH).
In turn, TSH sends a signal to the thyroid to release thyroid hormones.
If a disruption occurs at any of these levels, a defect in thyroid
hormone production may result in a deficiency of thyroid hormone
(hypothyroidism).
The
rate of thyroid hormone production is controlled by the
pituitary gland. If there is an insufficient amount of thyroid hormone
circulating in the body to allow for normal functioning, the release of
TSH is increased by the pituitary gland in an attempt to stimulate more
thyroid hormone production. In contrast, when there is an excessive
amount of circulating thyroid hormone, TSH levels fall as the pituitary
attempts to decrease the production of thyroid hormone. In persons with
hypothyroidism (thyroid hormone production is below normal), there is a
continuously decreased level of circulating thyroid hormones. In persons
with hyperthyroidism (thyroid hormone production is above normal), there
is a continuously elevated level of circulating thyroid hormones.
Hypothyroidism
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.
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).
Hyperthyroidism
Hyperthyroidism can be
suspected in patients with tremors, excessive sweating, smooth, velvety
skin, fine hair, a rapid heart rate and an enlarged thyroid gland. There
may be puffiness around the eyes and a characteristic stare due to the
elevation of the upper eyelids. Advanced symptoms are easily detected,
but early symptoms, especially in the elderly, may be quite
inconspicuous. In all cases, a blood test is needed to confirm the
diagnosis.
The blood levels of
thyroid hormones can be measured directly and are usually elevated
with this disease. However, the main tool for detection of
hyperthyroidism is measurement of the blood TSH level.
As mentioned earlier, TSH
is secreted by the pituitary gland. If an excess amount of thyroid
hormone is present, TSH is " down-regulated" and the level of
TSH falls in an attempt to control thyroid hormone production. Thus, the
measurement of TSH should result in low or undetectable levels in cases
of hyperthyroidism. However, there is one exception: If the
excessive amount of thyroid hormone is actually due to a TSH secreting
pituitary tumor, then the levels are abnormally high. This uncommon
disease is known as "secondary
hyperthyroidism."
The blood tests mentioned
above can confirm the presence of deficiency or an excess of thyroid
hormone and, therefore, be used to diagnose hypothyroidism or
hyperthyroidism. They do not point to a specific cause. In order to
determine a cause of the thyroid abnormality, the doctor will consider
the patient's history, physical examination, and medical condition.
Further testing might be used to isolate an underlying cause. These
tests might include more blood testing for thyroid antibodies, nuclear
medicine thyroid scanning, ultrasound of the thyroid gland, or others.
If thyroid cancer is
suspected and surgery may be required, your physician may ask for a
blood test known as thyroglobulin. Thyroglobulin is a protein made only
by thyroid cells. If the thyroglobulin level at baseline is detectable
or elevated (this means the gland does in fact make the protein) it can
be used as a tumor marker. After a total thyroidectomy for cancer
(removal of the entire thyroid gland) the level should fall to an
undetectable range since the cells that make thyroglobulin have been
removed. If the level remains detectable after surgery, there is a
possibility of thyroid tissue elsewhere in the body, and metastatic
disease should be considered. If the level is undetectable for a period
of time after surgery and then starts to climb, a recurrence of the
cancer - either at the primary site or elsewhere in the body should be
considered.
Thyroid
scanning is a nuclear medicine procedure whereby the thyroid
gland produces an image noted as it accumulates radioactive material
(technetium or iodine).
When is
thyroid scanning helpful?
Thyroid scanning is used
to determine how active thyroid tissue is in manufacturing thyroid
hormone. This feature can determine whether inflammation of the thyroid
gland (thyroiditis) is present. It can also detect the presence and
degree of overactivity of the gland (hyperthyroidism ).
Thyroid scanning is
especially helpful in evaluating thyroid nodules, particularly after a
fine needle aspiration biopsy has failed to provide a diagnosis. A scan
will reveal whether a thyroid nodule is functioning or nonfunctioning. A
functioning nodule is actively taking up iodine to produce thyroid
hormone and produces a localized "hot" area on the image of
the thyroid gland. A nonfunctioning nodule is does not take up iodine
and produces a localized "cold" area on the image of the
thyroid gland.
What is
significant about whether a nodule is "hot" or
"cold?"
Functioning or
"hot" nodules only rarely are from cancer. Nearly all thyroid
cancers are nonfunctioning or "cold" nodules. Moreover, even
among "cold" nodules, cancer is infrequent (less than 5
percent of cases).
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