| To
Confirm the Clinical Diagnosis |
Index to this Health
Guide
Laboratory
Investigation of Thyroid Disease
Measurement of TSH
Measurement of Blood T3 and T4
Thyroid Hormone Binding Proteins
Radioactive Iodine Uptake and Thyroid Scan
Thyroid Antibodies
Thyroid Biopsy |
Health
Guides on Thyroid Disease
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Laboratory
Investigation of Thyroid Disease
For many patients with thyroid disease,
the gland produces excessive amounts of thyroid hormone (hyperthyroidism)
or insufficient amounts of thyroid hormone (hypothyroidism). Such
patients will usually have an associated goitre (swelling of the
thyroid gland). However, many patients with a goitre will have normal
thyroid function. Most patients who develop a lump or nodule in
the thyroid will have a normal thyroid function as well. A minority
of patients, with thyroid nodules, will have a hyperfunctioning
nodule that will make the patient hyperthyroid.
The
most important uses of laboratory tests are:
- to confirm the clinical diagnosis
of thyroid disease;
- to monitor patients with thyroid
disease who have been treated;
- to select, for removal by the
surgeon, those single nodules which may be malignant.
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Measurement
of TSH
The pituitary hormone TSH stimulates
the thyroid gland to make and release the thyroid hormone. When
thyroid hormone levels decrease, the TSH rises and vice versa. Measurement
of TSH using a sensitive assay is presently the recommended initial
screening test when thyroid disease is suspected. The TSH assay
is able to separate hypothyroid and hyperthyroid patients from normal
individuals. Basically, a normal TSH excludes primary thyroid disease.
When the TSH is elevated, this suggests hypothyroidism and when
suppressed suggests hyperthyroidism. Rarely the TSH level may be
suppressed by drugs (such as corticosteroids) or by severe psychiatric
or non-thyroidal illness. However, such circumstances are extremely
rare in the out-patient setting.
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Measurement
of Blood T3 and T4
When the TSH is abnormal, measurement
of thyroxine (T4) or triiodothyronine (T3) are performed to determine
the extent of the thyroid abnormality. An elevated T4 or T3, in
association with a low or suppressed TSH, establishes hyperthyroidism.
An elevated TSH in conjunction with a low T4, establishes hypothyroidism.
Since using the TSH assay as a primary test, doctors have identified
patients who have an isolated low or high TSH in association with
normal T4 and T3 levels. Although some of these patients will eventually
develop overt thyroid disease, it is presently difficult to predict
who they will be. The assessment and management of such patients
needs to be individualized. |
Thyroid
Hormone Binding Proteins
Thyroid hormones circulate in association
with proteins which bind thyroid hormones. It is only the free or
unbound portion which we believe to be active at the tissue level.
However, free levels represent less than 1% of the total thyroid
hormone levels. In certain circumstances, such as pregnancy or the
birth control pill, the elevated estrogen or female sex hormone,
associated with these conditions, raises the level of thyroid hormone
binding protein. The body will compensate by increasing the production
of T4 and T3 so that the free level remains normal. However, such
individuals will have a higher total T4 and T3. Because the free
level remains normal, their TSH does not change. In many circumstances,
measurement of the free T4 and free T3 is available and indeed,
many laboratories will perform only free T4 and/or free T3 tests.
Alternatively, the T3 resin uptake test can be performed and provides
an indirect measurement of the level of thyroid binding protein.
The FT4 index is the total T4 multiplied by the T3 resin uptake
and should be proportional to the true free T4 level. In pregnancy,
the total T4 is elevated, the T3 resin decreased and the free T4
index is normal. The availability of the TSH screening has largely
eliminated any confusion caused by changes in thyroid binding proteins
as the TSH will remain normal in these circumstances.
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Radioactive
Iodine Uptake and Thyroid Scan
The thyroid gland takes up iodine
and uses this to make thyroid hormone. Radioactive iodine is taken
up and metabolized by the thyroid in exactly the same way. Approximately
20% of a dose of radioactive iodine, given orally, is taken up by
the thyroid gland within 24 hours after the dose is given. This
is measured by counting the radioactivity over the thyroid gland.
The test is safe since the radiation dose is very small, although
it is usually not carried out in children or pregnant women. The
test distinguishes between permanent causes of hyperthyroidism such
as Graves' disease and temporary causes such as thyroiditis; in
Graves' disease the test is elevated but in thyroiditis the test
is supressed. Alternatively, the gland can be photographed or "imaged"
and the distribution within the gland of a radio labelled tracer,
(usually technetium) recorded. This is called a thyroid scan. The
scan can be used as an alternative to the radioactive iodine uptake
as described. In addition, the scan gives an idea of the shape and
size of the thyroid gland and can be used for patients with thyroid
nodules to determine whether the nodule is functioning.
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Thyroid
Imaging
This can be performed by ultrasound,
which is very sensitive, and provides precise information about
the size and shape of the thyroid gland and nodules, CAT scans and
MRIs also give information about the presence of nodules.
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Thyroid
Antibodies
Patients with Hashimoto's thyroiditis
have an autoimmune disease. Thyroid antibodies are blood proteins
which react against certain of the patient's own proteins (called
antigens) within the thyroid gland. In patients with Hashimoto's
thyroiditis high levels of antibodies are usually found and are
therefore markers of the autoimmune process. Low levels of antibodies
are sometimes found in older, normal women and do not necessarily
indicate clinical disease. Patients with Graves' hyperthyroidism
have circulating thyroid stimulating antibodies which act like TSH
and cause the thyroid cells to over-function.
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Thyroid
Biopsy
Thyroid biopsy is presently in common
use and is considered to be the first line of investigation for
patients with solitary thyroid nodules (by many physicians). In
this procedure, a small needle on the end of a syringe is inserted
into the abnormal part of the thyroid gland. The plunger of the
syringe is drawn out and a small number of thyroid cells is drawn
up into the base of the needle. These cells are then smeared onto
glass slides. The pathologist can then examine the smears for evidence
of thyroid disease. This procedure is simple, quick, and painless
and is equivalent to having blood taken. In patients with a thyroid
nodule due to thyroid cyst, the fluid can be evacuated using the
biopsy technique. The patients may experience mild pain at the site
and, rarely, swelling and bruising. It is almost unheard of that
the needle would damage structures outside the thyroid gland. There
have been no reports of spread of thyroid cancer. Local anaesthetic
is usually not necessary even with children.
Thyroid biopsy is not carried out
if there is no thyroid swelling or nodule to feel. However, for
patients with thyroid nodules, multinodular goitre, or possible
thyroiditis, the procedure can be extremely useful. Although only
surgery can absolutely guarantee the nature of the thyroid nodule,
the thyroid biopsy is 85-90% effective in diagnosing the nature
of the nodule and distinguishing between benign tumours and thyroid
cancer.
However, the main factor determining
the success of the thyroid biopsy is the experience of the individual
performing the biopsy and the pathologist reading the smears.
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A
printed version of this Health Guide is available to health care
professionals and the public. For more information call the National
Office or contact your local
chapter.
Production of
the printed version of this Health Guide was made possible through
partial funding assistance from Health Canada. The views expressed
herein are solely those of the authors and do not necessarily represent
the official policy of Health Canada. |
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