Tests and Nodules
Thyroid Cancer Basics - Free 50-page Handbook: English Spanish Chinese
The following information
is from EndocrineWeb:
How Your Thyroid Works
Tests to Examine Thyroid Gland Function
(Tg) and Tg Antibody (TgAb) Testing for Patients Treated for Thyroid
Nodules and Cancer
Needle Aspiration of the Thyroid: English
Thyroid Nodules and Their Evaluation in Children and Adolescents
Understanding the Afirma Gene Expression Classifier in Testing Thyroid Nodules:
Free On-Demand Webinar with Erik K. Alexander, M.D.
Afirma Thyroid FNA Analysis: The Afirma Thyroid FNA Analysis from Veracyte combines cytopathology and the Afirma Gene Expression Classifier, for determining whether a thyroid nodule is benign or cancerous if the FNA is inconclusive.
Thyroid Stimulating Hormone (TSH) Suppression
Free Handbook - Thyroid Cancer Basics
Find a Thyroid Specialist
from American Thyroid Association: These guidelines for physicians discuss thyroid nodules and diagnosis of papillary/follicular, medullary, and anaplastic thyroid cancer, as well as their management
How Your Thyroid
Your thyroid gland is a small gland, normally
weighing less than one ounce, located in the front of the neck. It
is made up of two halves, called lobes, that lie along the windpipe
(trachea) and are joined together by a narrow band of thyroid tissue,
known as the isthmus.
thyroid is situated just below your "Adams apple" or larynx.
During development (inside the womb) the thyroid gland originates
in the back of the tongue, but it normally migrates to the front of
the neck before birth. Sometimes it fails to migrate properly and
is located high in the neck or even in the back of the tongue (lingual
thyroid) This is very rare. At other times it may migrate too far
and ends up in the chest (this is also rare).
function of the thyroid gland is to take iodine, found in many foods,
and convert it into thyroid hormones: thyroxine
(T4) and triiodothyronine (T3). Thyroid cells are the only
cells in the body which can absorb iodine. These cells combine iodine
and the amino acid tyrosine to make T3 and T4. T3 and T4 are
then released into the blood stream and are transported throughout
the body where they control metabolism (conversion of oxygen and calories
to energy). Every cell in the body depends upon thyroid hormones
for regulation of their metabolism. The normal thyroid gland produces
about 80% T4 and about 20% T3, however, T3 possesses about four times
the hormone "strength" as T4.
thyroid gland is under the control of the pituitary gland,
a small gland the size of a peanut at the base of the brain (shown
here in orange). When the level of thyroid hormones (T3 & T4)
drops too low, the pituitary gland produces Thyroid
Stimulating Hormone (TSH) which stimulates the thyroid
gland to produce more hormones. Under the influence of TSH, the thyroid
will manufacture and secrete T3 and T4 thereby raising their blood
levels. The pituitary senses this and responds by decreasing its TSH
production. One can imagine the thyroid gland as a furnace and the
pituitary gland as the thermostat. Thyroid hormones are like heat.
When the heat gets back to the thermostat, it turns the thermostat
off. As the room cools (the thyroid hormone levels drop), the thermostat
turns back on (TSH increases) and the furnace produces more heat (thyroid
The pituitary gland itself is regulated by
another gland, known as the hypothalamus (shown in our
picture in light blue). The hypothalamus is part of the brain and
produces TSH Releasing Hormone (TRH)
which tells the pituitary gland to stimulate the thyroid gland (release
TSH). One might imagine the hypothalamus as the person who regulates
the thermostat since it tells the pituitary gland at what level the
thyroid should be set.
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to Examine Thyroid Gland Function
As we have seen above, the thyroid gland
produces T4 and T3. But this production is not possible without stimulation
from the pituitary gland (TSH) which in turn is also regulated by
the hypothalamus's TSH Releasing Hormone. Now, with radioimmunoassay
techniques it is possible to measure circulating hormones in the blood
very accurately. Knowledge of this thyroid physiology is important
in knowing what thyroid test or tests are needed to diagnose different
diseases. No one single laboratory test is 100% accurate in diagnosing
all types of thyroid disease; however, a combination
of two or more tests can usually detect even the slightest abnormality
of thyroid function.
For example, a low T4 level could
mean a diseased thyroid gland ~ OR ~ a non-functioning pituitary
gland which is not stimulating the thyroid to produce T4. Since the
pituitary gland would normally release TSH if the T4 is low, a high
TSH level would confirm that the thyroid gland (not the pituitary
gland) is responsible for the hypothyroidism.
If the T4 level is low and TSH is not elevated,
the pituitary gland is more likely to be the cause for the hypothyroidism.
Of course, this would drastically effect the treatment since the pituitary
gland also regulates the body's other glands (adrenals, ovaries, and
testicles) as well as controlling growth in children and normal kidney
function. Pituitary gland failure means that the other glands may
also be failing and other treatment than just thyroid may be necessary.
The most common cause for the pituitary gland failure is a tumor of
the pituitary and this might also require surgery to remove.
- Modern measurement of thyroid hormones
is done by a new technique, radioimmunoassay (RIA), discovered by
Dr. Solomon Berson and Dr. Rosalyn Yallow. They were awarded the
1977 Nobel Prize in Medicine for this discovery which revolutionized
the study of thyroid disease as well as the entire field of endocrinology.
The following are
commonly used thyroid tests
of Pituitary Production of TSH.
Pituitary production of TSH is measured by a method referred to as
IRMA (immunoradiometric assay). Normally, low levels (less than 5
units) of TSH are sufficient to keep the normal thyroid gland functioning
properly. When the thyroid gland becomes inefficient such as in early
hypothyroidism, the TSH becomes elevated even though the T4 and T3
may still be within the "normal" range. This rise in
TSH represents the pituitary gland's response to a drop in circulating
thyroid hormone; it is usually the first indication of thyroid gland
failure. Since TSH is normally low when the thyroid gland is functioning
properly, the failure of TSH to rise when circulating thyroid hormones
are low is an indication of impaired pituitary function. The new "sensitive"
TSH test will show very low levels of TSH when the thyroid is overactive
(as a normal response of the pituitary to try to decrease thyroid
stimulation). Interpretations of the TSH level depends upon the
level of thyroid hormone; therefore, the TSH is usually used in
combination with other thyroid tests such as the T4 RIA and T3 RIA.
of Serum Thyroid Hormones: T4 by RIA. T4
by RIA (radioimmunoassay) is the most used thyroid test of all. It
is frequently referred to as a T7 which means that a resin T3 uptake
(RT3u) has been done to correct for certain medications such as birth
control pills, other hormones, seizure medication, cardiac drugs,
or even aspirin that may alter the routine T4 test. The T4 reflects
the amount of thyroxine in the blood. If the patient does not
take any type of thyroid medication, this test is usually a good
measure of thyroid function.
of Serum Thyroid Hormones: T3 by RIA.
As stated on our thyroid hormone production page, thyroxine (T4) represents
80% of the thyroid hormone produced by the normal gland and generally
represents the overall function of the gland. The other 20% is
triiodothyronine measured as T3 by RIA. Sometimes the diseased
thyroid gland will start producing very high levels of T3 but still
produce normal levels of T4. Therefore measurement of both hormones
provides an even more accurate evaluation of thyroid function.
Binding Globulin. Most of the
thyroid hormones in the blood are attached to a protein called thyroid
binding globulin (TBG). If there is an excess or deficiency of this
protein it alters the T4 or T3 measurement but does
not affect the action of the hormone. If a patient appears
to have normal thyroid function, but an unexplained high or low T4,
or T3, it may be due to an increase or decrease of TBG. Direct measurement
of TBG can be done and will explain the abnormal value. Excess TBG
or low levels of TBG are found in some families as an hereditary trait.
It causes no problem except falsely elevating or lowering the T4 level.
These people are frequently misdiagnosed as being hyperthyroid or
hypothyroid, but they have no thyroid problem and need no treatment.
Iodine Uptake Scan. A means of measuring thyroid
function is to measure how
much iodine is taken up by the thyroid gland (RAI uptake). Remember,
cells of the thyroid normally absorb iodine from our blood stream
(obtained from foods we eat) and use it to make thyroid hormone (described
on our thyroid function page). Hypothyroid patients usually take up
too little iodine and hyperthyroid patients take up too much iodine.
The test is performed by giving a dose of radioactive iodine on an
empty stomach. The iodine is concentrated in the thyroid gland or
excreted in the urine over the next few hours. The amount of iodine
that goes into the thyroid gland can be measured by a "Thyroid
Uptake". Of course, patients who are taking thyroid medication
will not take up as much iodine in their thyroid gland because their
own thyroid gland is turned off and is not functioning. At other times
the gland will concentrate iodine normally but will be unable to convert
the iodine into thyroid hormone; therefore, interpretation of the
iodine uptake is usually done in conjunction with blood tests.
Scan. Taking a "picture" of how
well the thyroid gland is functioning requires giving a radioisotope
to the patient and letting the thyroid gland concentrate the isotope
(just like the iodine uptake scan above). Therefore, it is usually
done at the same time that the iodine uptake test is performed.
Although other isotopes, such as technetium, will be concentrated
by the thyroid gland; these isotopes will not measure iodine uptake
which is what we really want to know because the production of thyroid
hormone is dependent upon absorbing iodine. It has also been found
that thyroid nodules that concentrate iodine are rarely cancerous;
this is not true if the scan is done with technetium. Therefore, all
scans are now done with radioactive iodine. Pregnant women should
not have thyroid scans performed because the iodine can cause development
troubles within the baby's thyroid gland.
- Two types of thyroid scans are available.
A camera scan is performed most commonly which uses a gamma camera
operating in a fixed position viewing the entire thyroid gland at
once. This type of scan takes only five to ten minutes. In the 1990's,
a new scanner called a Computerized Rectilinear Thyroid (CRT) scanner
was introduced. The CRT scanner utilizes computer technology to
improve the clarity of thyroid scans and enhance thyroid nodules.
It measures both thyroid function and thyroid size. A life-sized
1:1 color scan of the thyroid is obtained giving the size in square
centimeters and the weight in grams. The precise size and activity
of nodules in relation to the rest of the gland is also measured.
CTS of the normal thyroid gland In addition to making thyroid diagnosis
more accurate, the CRT scanner improves the results of thyroid biopsy.
The accurate sizing of the thyroid gland aids in the follow-up of
nodules to see if they are growing or getting smaller in size. Knowing
the weight of the thyroid gland allows more accurate radioactive
treatment in patients who have Graves' disease.
Scans are used for the following reasons:
- Identifying nodules and determining
if they are "hot" or "cold".
- Measuring the size of the goiter prior
- Follow-up of thyroid cancer patients
- Locating thyroid tissue outside the
neck, i.e. base of the tongue or in the chest.
Ultrasound. Thyroid ultrasound refers to
the use of high frequency sound waves to obtain an image of the thyroid
gland and identify nodules. It tells if a nodule is "solid"
or a fluid-filled cyst, but it will not tell if a nodule is benign
or malignant. Ultrasound allows accurate measurement of a nodule's
size and can determine if a nodule is getting smaller or is growing
larger during treatment. Ultrasound aids in performing thyroid needle
biopsy by improving accuracy if the nodule cannot be felt easily on
Antibodies. The body normally
produces antibodies to foreign substances such as bacteria; however,
some people are found to have antibodies against their own thyroid
tissue. A condition known as Hashimoto's Thyroiditis is associated
with a high level of these thyroid antibodies in the blood. Whether
the antibodies cause the disease or whether the disease causes the
antibodies is not known; however, the finding of a high level of thyroid
antibodies is strong evidence of this disease. Occasionally, low levels
of thyroid antibodies are found with other types of thyroid disease.
When Hashimoto's thyroiditis presents as a thyroid nodule rather than
a diffuse goiter, the thyroid antibodies may not be present.
Needle Biopsy. This has become
the most reliable test to differentiate the "cold" nodule
that is cancer from the "cold" nodule that is benign ("hot"
nodules are rarely cancerous). It provides information that no other
thyroid test will provide. While not perfect, it will provide definitive
information in 75% of the nodules biopsied.
Last updated: April 23, 2013
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